3.5 MW motor starting problems continue...
3.5 MW motor starting problems continue...
(OP)
A while ago, I started a thread http:/
So the issue is that a 3.5 MW motor driving a large fan has problems starting. The starting method is the soft starter. Rotational inertia is high.
We discussed several causes, such as inadequate motor, inadequate starter, week and inadequate power system, etc. But calculations show that although things are marginal this still should work, but it does not.
Today we managed for the first time to put two identical 13/20MVA transformer in parallel. Their impedance is 10.5% at 20MVA but each transformer also has a current limiting reactor 6%. However, 20+20MVA system still would no start a 4MVA motor...
System: two 13/20 MVA transformers in parallel, + about 5 MW of additional load shared between two transformers.
Motor: asynchronous, TECO-Westinghouse 3.5MW, In = 385Amp, 6kV, 50Hz, 1485 rmp, locked rotor current 2300Amp.
Fan: Large rotational inertia. The fan has no load at the present (no air, dampers closed). The Impeller weight is 15,000 lbs, and the Impeller moment of inertia (WR^2) is 66,000 lb-ft^2.)
Starter: constant current, voltage ramp ~10 sec, current limit 470%, starting time 50 sec, after which bypass contactor closes (when then speed is about 80%).
We had some luck starting and running the motor using one 13/20 MVA dedicated transformer but now with 2 transformers in parallel it would not work. We tried three times and none of the starts was successful. Each time the soft starter was manually shut down after about 20 sec since motor was creating too much vibrations and was not accelerating any more.
Problems still could be electrical, but for a change we started to suspect that there may be some mechanical problems, perhaps related to the motor rotor axial movement. If the rotor indeed does try to move, either it may not have enough space to move to the electrical center or the shaft is moving back and forth around its electrical center for some unknown reason and hitting the trust.
The attached charts provide more info. During the voltage rump time, which was about 10 sec, motor runs smooth and with two transformers in parallel the initial voltage drop is rather modest - falls from 6.4kV to 6kV. This is good news, with one transfomer we had much larger initial voltage drops. However, the bad news is that after the voltage is ramped up high enough that the current reached the preset limit (470% In), some vibrations start.
As you can see, the current and voltage outline forms on the charts start at some point having "teeth" like a saw. Once stabilized there is about 3 - 4 "teeth"/second. Just judging by the ear, the frequency of mechanical vibrations is in the same range, meaning current spikes likely correspond with frequency of mechanical vibrations.
Any idea what might be happening?






RE: 3.5 MW motor starting problems continue...
beratech had even posted some favourable numbers in that therad.
RE: 3.5 MW motor starting problems continue...
Fp = 2*s*LF where LF = line frequency
It can reduce torque capability of the motor.
Also out of curiosity (different subject), are the currents balanced?
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RE: 3.5 MW motor starting problems continue...
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If it is broken, fix it. If it isn't broken, I'll soon fix that.
RE: 3.5 MW motor starting problems continue...
Thank you all for replying.
edison123, so far we have had very poor cooperation from TECO. That outfit is located in Taiwan and we have never had any contacts directly with them. All communication went through a salesman in Canada who does provide some help but not sufficient. Generally, TECO documentation is poor, very generic instruction manual which seems to cover all motors from 5KW to 5MW... Example, instructions says that if motor is stored for a prolonged time it occasionally would need to be turned at 30 RPM. How do you turn an 8 ton rotor at 30 RPM in a storage???
We may be considering a fluid drive as my friend beratech suggested, but it is rather hard to implement. We are still hopeing that a 40 MVA capacity should be able to deal with this hard start, but probably never without difficulties.
Electricpete, currents are fairly balanced once the fan is running. During the start, they do not seem perfectly balanced, it also may have something to do with the meter display refreshment time. We did consider rotor bar problems, but so far have not done any testing to verify it. Will be working on it soon.
TurbineGen, we were closing the bypass after about 50 sec, when the speed was about 80%. We did try to close it earlier, did not seem to help much, but the issue is also that it that case it pulls too much current for a prolonged time. LRA is 2300A, so an upstream switchgear is old and the breaker has problems handling it (it is an old minimum oil 800A unit which we will be replacing with a 1250A vacuum breaker soon). Also, the bypass is not rated to FLA, but only 500Amp, so closing it on the current which is almost 5 times higher may not be the best thing to do. We also never tried a DOL start for the same reason, but maybe we could do it forcing the bypass to close first and then closing the upstream breaker after.
We just uncoupled the fan and ran the motor and it seems to be OK. Accelerates within 6-7 sec at 470% current limit. However, the rotor seem to have too much of the end play. When the full speed is almost reached, initially it moves towards the fan for about 1" (25mm), then it apparently overshoots, comes back, moves toward the fan again, etc, until in finally centers itself into magnetic center. With the coupling in place this end play is much more limited, but I'm wondering would 1" of the play when uncoupled be too much in any case? We have poor mechanical drawings of the motor and not sure how this play is limited internally, it is - there is a small nameplate on the motor saying that the end shaft play must be limited by the coupling not to exceed 4.8 mm total.
RE: 3.5 MW motor starting problems continue...
Rotor bar problems to me seem a higher probability. You have the oscillations. You have a high inertia load which is tough on the rotor during start (depending on starting method). You have many unsuccessful starts which can be traumatic for the motor as well.
If you can gather enough info to determine frequency of oscillations and speed together, you can check whether the frequency matches pole pass frequency. Typical on-line testing is steady state under load and it sounds like you will have a hard time getting there. You can also attempt some off-line tests: single phase test or PDMA-style rotor influence check. If motor were in the shop there would be some more options.
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RE: 3.5 MW motor starting problems continue...
By the way - 6 - 7 seconds to accelerate uncoupled "at the 470% current limit". What does the 470% current limit mean... can we correlate it to a % reduction in terminal voltage or locked rotor current? Unless there is a severe reduction in voltage, 6-7 seconds sounds very long for an uncoupled start.
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RE: 3.5 MW motor starting problems continue...
RE: 3.5 MW motor starting problems continue...
6 to 7 seconds for an open shaft acceleration seems a bit high. You need pull Teco up for their non-response.
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The coupling should control the axial position of the motor. As edison stated, ideally when the coupling alignment is performed, position is adjusted so that motor runs on magnetic center. But I don't think that is critical. What is more important is that the motor is coupled up in a position near enough to mechanical center that it will not contact either thrust shoulder during coupled operation. If this should occur enough to have any noticeable effect on motor torque, then you are dumping an enormous amount of heat into friction and you will surely see very large excursions in the bearing temperature (did you look at that?). In theory there can be a very small change in the effective length of the core if we for example move the rotor iron outside of the stator iron. First guess would be the max change in effective length (and therefore torque) would be 1" oveor the length of the core which will probably only be 1 or 2%. There is also the matter of vent ducts lining up or not which complicates the issue a little more. But at this point I don't see any reason to be concenred about it because I don't think we have seen anything unexpected with regard to axial rotor position other than the 1" endplay (above NEMA spec which is 0.5") which would seem to be a design condition.
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RE: 3.5 MW motor starting problems continue...
One inch of movement is just my visual observation. Could be less than that but looks a lot when one watches the motor starting.
Looks like I was wrong stating the start was 6-7 sec. The chart recorder shows that the current drops to no load running current within about 5 sec.
The way how our soft starter (as a power limiter) is set to work is that starting the voltage is about 70% and keeps ramping up until the current reaches 470%. At that time, the motor terminal voltage is anywhere between 80-90%. At some point it will start increasing the terminal voltage when the current starts dropping.
You can see some details on the attached chart. The selected scale for this chart is not very good but for the next test we will stretch the start time period on some more paper. You can see that occasionally during the 5 sec start, the terminal voltage and the motor current seem to sag shortly, creating spikes at the line voltage. Is this an indication that something may be wrong, including soft starter (thyristor) problems?
edison123, yes the axial magnetic center is scribed on the shaft with reference to a static tiny pointer and then there is a nameplate saying that the pointer (which could be accidentally bent) should be 23.6mm from a static plate on the motor end. The groove area on the shaft is marked with two parallel lines which are about 6mm apart. The nameplate also says that the coupling design is to be such to allow no more than 4.8mm of end play TOTAL. Therefore, my understanding is that + - 2.4mm allowable play means that the coupling needs to "make sure" that the pointer should never be outside the groove area, motor running or not. In the reality, though, the pointer with the coupling connected is located outside the groove area when motor is not running.
RE: 3.5 MW motor starting problems continue...
RE: 3.5 MW motor starting problems continue...
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RE: 3.5 MW motor starting problems continue...
So whether the sinusoidal values are chopped or not, it always looks the same on our charts – just like a wide strip. The soft starter changes the RMS values by chopping the middle of each half period, what we cannot see on this chart. "Strip" width is still the same, waves copped or not (unless chopped extensively).
The soft starter settings left in place were those used when the fan was coupled, starting voltage 40%, ramp time 8 sec, current limit 470%.
So in theory, the voltage starts from 40%. Ramp time is 8 seconds. After every second, the voltage ramps for abut 10% until it reaches to voltage determined by the current limit (which could be anything). In practice, with no fan, and starting at 40% voltage, the motor would reach full speed after about 5 sec (regardless what was the terminal voltage and motor current at that time).
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After about 6sec of smooth run, the motor voltage and current started to oscillate significantly, changing in amplitude up to 20% and causing severe mechanical vibrations. The speed was several hundred RPM when it stalled after about 10 sec - no more acceleration. When we shut down the soft starter manually, after about 20 sec, the motor/fan continues to turn smoothly indicating that most likely electrical oscillations were causing mechanical ones, not another way around.
We started the motor with two 13/20MVA transformers in parallel, so system capacity did not seem to be an issue. 40MVA should be able to deal with 20MVA whereas 20MVA is maximum MVA drawn during the start.
Could the terminal voltage/motor current oscillations we recorded be possibly be caused by an electrical resonance between current limiting reactors (both transformers have current limiting reactors on the 6kV side) and the soft starter circuits? We are planning to rule out this by doing a test with one transformer and its reactor shorted.
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Also look at your first attachment "motor start charts". The frequency of current oscillation decreases as motor speed increases. Why would resonance act like that. No reason I know of. I contrast there is a very logical reason that current oscillation from rotor bar defect would act EXACTLY like that. The frequency of oscillation is proportional to slip and so the oscillations slow as speed increases (the oscillation at very low speed is so fast you can't distinguish it on yoru chart). That would be many many times higher on my list of stuff to investigate than resonance.
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RE: 3.5 MW motor starting problems continue...
However, what would be a reliable test for determining rotor damage, is there is test that can positively indicate that and if yes, what equipment we need for such testing. To do a visual inspection by opening the motor is a major undertaking. It would be much easier to install a spare motor which we actually have.
Another indication that something went wrong with the motor is that we used to be able to start it with one transformer and now we cannot do it with two!
But to make things more complicated looks like we have some random problems with the soft starter too... If you look at the two attached diagrams, which are both for a start with no fan and starting voltage 80%, one start had an apparent problem – SCR's would stop firing for some reason for 0.1-0.2 sec and it happened three times for 5 sec. These sudden voltage sags could be even heard if standing by the motor as hesitation in acceleration.
So looks like we may have broken motor and broken soft starter. The only good news is we have both spares...
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RE: 3.5 MW motor starting problems continue...
Something is wrong with the starter that is causing the drop-outs.
You really need to get the manufacturer of the starter and the motor/fan together before you cause more damage with the trial-and-error approach.
Alan
----
"It's always fun to do the impossible." - Walt Disney
RE: 3.5 MW motor starting problems continue...
RE: 3.5 MW motor starting problems continue...
As you said, the strange thing is that current measured from pick to pick (positive to negative) is always the same, but the amplitudes of positive an negative half periods are not – higher amplitude of the positive half period is followed by the lower amplitude if the negative one and vice versa.
Or one could put this another way around and say that amplitudes of both cycle half period are the same, but the reference point is modulated somehow and floats up and down making so its own sinusoidal curve where it frequency is proportional to the slip...
Based on what Electricpete underlined several times here, and some other engineers did it during earlier discussions, and also some other people outside this form stated as well, the likelihood of damaged rotor is high, but then as you wondered why it would create such a phenomena that negative and positive half periods would seem to differ in amplitude from 1 ~ 1 (normal symmetrical values) to something like 0.5 ~ 1.5 the worst case which happens after every several cycles?
Edison 123, as getting manufactures involved... well.. it sends me back to square one... I started the initial thread about this problem on the forum about two months ago since we were unable to get ABB help for the soft starter at that time. After waiting forever, ABB USA sent a specialist from Egypt who after a lot of struggle made the soft starter working somehow but left blaming all problems to the weak power system... OK, maybe it was sort of weak then, but now we can provide 40MVA with reduced impedance to deal with the motor which starts at 2000A and about 5000V (average reduced voltage) so 40MVA should be enough for this...
As for getting TECO involved, as I said, their real experts may only reside in Taiwan and so far we have never had a direct contact with them, but we will try to do it this week.
That is why there is so much trial and error, it still may cheaper to just replace the soft starter then wait another forever...
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Even more effective, but more work is a single phase test. It is described on page 8/26 here.
Which figure are you guys looking at to see that the positive and negative peaks don't line up? I only see a good view of the oscillation in the very first attachment, and the scale is too scrunched to make any judgement on lining up of the peaks for me.
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RE: 3.5 MW motor starting problems continue...
http://
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RE: 3.5 MW motor starting problems continue...
350% current = 85 second start
450% current = 43 second start
The 350% current simulation is attached.
So, it appears you definately have a problem with the starter or the motor/load. It should start just fine at 450% current. There should not be an oscillation caused by limiting the current. The motor should stay smooth and just quit accelerating if you limit the current too much and it stalls. It is either a mechanical or electrical resonance. Any yes, it could be the circuit impedance causing the current limit loop in the starter to oscillate like you earlier asked.
We have been building soft-starters for applications >5kV for years and yet we still dump a ton of money into R&D each year to further understand these units when connected in a real world application. Building a soft-starter to reliably and properly start that motor is much more complex then it looks from the outside.
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You'd be surprized how often we do run into places that push these large motors like this and they don't seem to have a real concern about it. It's a lucky thing for this planet that 3-phase asynchronous motors are very rugged devices.
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Please refer to the attached chart called Test No6 – non-symmetrical SCR firing. This was an attempt to start the motor with the fan coupled, which failed - the motor stalled at several hundred RPM and was manually shut down after abut 30 sec as it was apparently not accelerating any more - the speed stalls after about 10 sec and just stays there. The soft starter settings were: starting voltage 40%, ramp time 8 sec, current limit 470%
What you see on the diagram is just one phase voltage (red) and current forms (green). The chart on the first page shows about 18 50Hz cycles (recorded using a multichannel scope), while the second chart captures the same values but zoomed out to show more cycles.
Each positive or negative spike on the chart is supposed to represent a half of 50Hz period, whereas all half periods should be normally chopped symmetrically, and be symmetrical by the width and amplitude.
However, you can see that the amplitude of voltage and current half periods are very uneven – some spikes are 2-3 times higher than the other. Also please look at the wide negative half periods which look like a letter W. This cannot be a normal voltage form since a positive half period in the middle of "W"is completely in the negative area – below the mid reference line.
Similar for the current - higher amplitude of the positive half period is followed by the lower amplitude if the negative one and vice versa.
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RE: 3.5 MW motor starting problems continue...
You mentioned about 400 meters of cable. Did you consider the possibility of a major voltage drop across the cable, especially when starting ?
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Edison 123, sorry I mistyped, should say 300m of cable. No, there is no big voltage drop on the cable. The cable is oversized, we have much more of the voltage drop before the cable. The system is: 13/20MA transformer + (or two of them, if in parallel) at 10.5% impedance at 20MVA (or less at 13MVA) plus a 6% reactor (or two of them in parallel if transformers are paralleled) + 300 m of cable. Most impedance is on the transformer(s) and reactor(s), but we tried everything including shorting the reactor, made no difference. Line voltage measured just before the soft starter, which is only 10m from the motor, when the motor starts, and if two transformers are paralleled, is 5.8kV. Not a big drop from the initial 6.3kV.
RE: 3.5 MW motor starting problems continue...
RE: 3.5 MW motor starting problems continue...
RE: 3.5 MW motor starting problems continue...
Also, the voltage measured on the motor terminals during the start is usually not higher than 5000V (with the fan coupled), so the soft starter itself, as presently set(to let no more than 470% of the current) is actually a major "system impedance" where the voltage drops from 5.8kV (before the soft starter) to about 5000 kV (after the soft starter) - if one can trust to the RMS measurement of that funny voltage form on the motor terminals.
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RE: 3.5 MW motor starting problems continue...
Best wishes. You seem to need them. :)
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If you are hitting the 470% current limit then I don't see the voltage drop as being an issue. Your data says the motor will start just fine at 450% current so 470% should work just as well. Heck, we've gone to 20% plus before and been fine because we were still able to give the motor enough current to start.
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RE: 3.5 MW motor starting problems continue...
I am worker in local repair electric shop and we are trying to help Mr Ters with this big challenge.
Pete, I am familiar with motor current signature analysis because I have worked more then 5 years with SKF instrument CMVA 60 and I found more then 30 motors with rotor problems but, unfortunately, it was out of the service few days while motor worked loaded. We were also concerned about rotor because motor has a lot of unsuccessful starts last two months and I have watched two ampermeters in 6 kV station and I didnt see swinging needles of ampermetrers which is usually sign of broken rotor bars.
We have worked in the past single phase rotor test on smaller motors but this rotor is problem to rotate because it has 8 tons weight!!
I noticed that strange sound, vibration, ect. motor have had from start of commissioning and speed of the motor where problem starting depends of starting current limits. I have attached oscilloscope recording one of start, when the motor runs rough and it looks like that motor receive double frequency from soft starter.
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We have a 7000hp 2-pole horizontal motor with sleeve bearings that I assume is similar size/construction to this one. We have manually rotated with a strap wrench - the toughest part was breaking free but then easy to keep rotating once started. As a conservative measure to avoid sleeve bearing damage when rotating motors by hand, some people recommend to prelubricate the bearing by spinning the oil ring a few times by hand before you rotate the shaft.
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RE: 3.5 MW motor starting problems continue...
We did not record the current form with such details -zooming into several cycles - the only current form we have is on the charts I posted, which is more crude but still indicative. Also, the chart Panter posted was recorded about two months ago and at that time we occasionally managed to make a successful start with the fan, but now that is not the case any more (last 4 starts failed), perhaps suggesting that some problems with the soft started developed meanwhile or worsened if always existed.
About 2 month ago we sent Panter's chart to the manufacturer (Motortronics) and they commented it looked OK. Also some people here on the forum said the same.
However, I'm still puzzled with the created voltage form. When, say, the positive half period is chopped, then the voltage goes to the negative before it recovers. The same applied for the chooping of the negative half period. So if you count all positive and negative picks in period (regardless of their amplitude and duration) you will actually count 6 picks instead 2, which means we have voltage of some sort of 150Hz frequency... Why that would be still considered normal? I know that the current fills the gap and still remains 50Hz, but would not it be more natural to chop the wave form at the beginning and or end, rather then in the middle?
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To comment on the voltage waveform. It looks fine. When the SCR is off the voltage on the off phase will go to the voltage as dictated by the motor and the voltage applied to the other 2 phases. The off period is in the middle of the waveform becuase the current is lagging the voltage. This means, for example, that the negative SCR will turn off after the voltage has crossed 0 and become positive.
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We received an interesting comment from the motor manufacturer (TECO). They seem to believe that the problem can be solved by "fine tuning" the soft starter settings and offered their commissioning engineer to do it.
And while enything is possible, I don't quite believe that so far we were unable to figure out what would be close to optimal soft starter settings. There were a large group of engineers involved in this, starting from the Client, who has some very good engineers like Mr. Panter, then we also had ABB rep on site for 10 days who is also a good engineer except he had English problems and finally we had even three (3) consulting companies involved in this each having an average of 2 engineers working on this, plus, a great help from several member of this forum, plus some other engineers I personally know worldwide were sending me some comments by e-mail. And so far we all failed to "fine tune" it...
I'm not a hiring authority here, but if I were one, the deal I would be willing to offer to TECO would be: come and fine tune it, and I will pay you $1000/hour; but if you fail, you hourly rate will be $5/hour...
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Your line voltage oscillating during the start is also odd. You should be drawing a steady continuous current which should cause a steady voltage drop. Are there tap changers switching?
Messing around with different transformers and switchgear is most likely not going to fix the problem. You have been using 450% current for many start attempts. You should have had many good starts.
I remember reading about spare motors. Maybe it is time to try another one? I'm not convinced the softstarter is working correctly though. You have enough line capacity according to the voltages you measured during starting, assuming your motor and load data is correct.
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We wanted to try different power system configuration as there were arguments (by soft starter manufacturer) that one 13/20MVA transformer with somewhat higher impedance then typical was barely enough for 2000Amp start. Maybe it was, but two units in parallel should be just enough and impedance goes down as well...
In summary we had 3 scenarios:
1. Initially no start at all (spins but does not reach full speed)
2. Then, occasional successful starts
3. Finally, no start at all again, after the motor have been running for a month or so, ON and OFF
I would say that difference between 1. and 2. was related to the soft starter settings and tuning. But the same is unlikely to apply for difference 2. vs. 3. - the soft starter settings are still the same as they were when starts were possible.
So based on the fact that the motor was able to start the fan (admittedly with difficulties, and randomly successful starts), but now we cannot start it at all any more, one can still make a number of reasonable guesses and among other things say that the difference between 2. and 3. above may be because:
A) We have always had some problems with the soft starter which just got worse
B) We have always had some problems with the motor, such as rotor problems, which just got worse
C) We have always had problems with the soft starter but meanwhile a motor problem has developed as well
D) We have always had problems with the motor, but a soft starter problem occurred meanwhile as well.
Probability that any of the above statements is true still exists. However, soft starter manufacturer says they believe there is nothing wrong with the soft starter while motor manufacturer says there is nothing wrong with the motor...
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System capacitance was also mentioned by the soft starter manufacturer. But I could not identify much of it. Cable between the motor and the soft starter is hardly existing - just abut 15 meters... Cable from the soft starter to the upstream switchgear (which is to feed 3 soft starters) is also very short - maybe 10-20 meters. There is a longer cable, about 300m from the upstream switchgear to upstream-upstream switchgear, which is 3x240mm sq per phase (in the range of 3x500MCM per phase), but that is much before the SCRs, not sure if it can create some sort of resonance phenomena?
RE: 3.5 MW motor starting problems continue...
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RE: 3.5 MW motor starting problems continue...
RE: 3.5 MW motor starting problems continue...
Few requests before I can ofer any useful suggestions:
1:What is the system fault level (Xd") at the 13MVA transformer secondary side with single transformer?
2:Do you have recorded Toque Vs. Speed Curves? I mean the failed motor+fan starting case?
Thx!
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Unfortunately, we have not recorded torque vs speed curves for any of the failed cases. Nor I'm sure how to do it, we can record speed, voltages and currents, but how do we measure and record the torque in our situation? Could you provide some guidance so that we can try recording it next time when we are in position to start, provided we can find adequate test equipment. We do have standard torque vs. speed curves as provided by the manufactures for both, the motor and the fan, if that can be of some help?
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Are you try shunt this reactors for 50sec?
Are you think about something like to Clip or Is-Limeter for the shunt those reactors?
Best Regards.
Slava
RE: 3.5 MW motor starting problems continue...
RE: 3.5 MW motor starting problems continue...
RE: 3.5 MW motor starting problems continue...
Examining the motor and load data says it will start with the 470% current you have been trying. You recorded voltages on the input and output that showed the soft-starter is reaching the programmed current limit without the SCR's being full-on, meaning you have enough power.
The only issue the system could be causing is that the control voltage dip is messing up the soft-starter controls. The soft-starter should have protection to trip before this happens though. Maybe try a good sinewave output UPS on the soft-starter control circuit? I doubt it would help but you never know unless you try.
Problems not accounting for cable capacitance tends to cause SCR failures, not an unstable start. The capacitors tend to affect the SCR's as they are switching.
Basically, I see these possible problems.
1. The motor manufacturer gave you bad motor data.
2. The load is worse than the data you were given.
3. The soft-starter is bad.
4. The motor is bad.
I'm tending to think it's the starter. The rough starting and line voltage oscillations are pointing to it not being stable. Something in the control loop or SCR firing is not working like it should.
My company could help you figure out how to start the motor for you, but it would cost you new soft-starters because we won't support those units. I doubt you want to go that far.
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Thanks!
More questions, the fault level you provided is for Xd".
Do you have the fault level for Xd' and Xd. If you have them it would be helpful.
LionelHutz:
Am I missing sth here since it has a long chain.
1. The motor manufacturer gave you bad motor data.- Could be
2. The load is worse than the data you were given.-I am confused since the fan was not loaded up and Ters mentioned once a time it only has estimated 500kw.
3. The soft-starter is bad.-could be
4. The motor is bad.-Interesting point but what does it mean "bad"
I looked the cases you did for Ters. I don't know which kind of software you were using to simulate the motor starting. But the software you used must be a very rough one with no motor electrical data input. Therefore, it may not correctly representing what really happened in reality.
I agree with you for your analysis:
1: System is too weak to start. Could be and that is the reason I request fault level under different time zones.
2: Soft starter has problems.Could be But I recalled by reading the long chain, it can start the motor without the fan.
3: Motor design problems
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"QBplaner, on the motor switchgear, which is just 15 m from the soft starter, the short circuit level is about 175MVA with one transformer, or ~ 350MVA with two transformers in parallel."
I guess even with two transformers in parallel, you may not get 350MVA. you may maximum get 200-250MVA depends on the Trx Leakage impedance and the rest of the system impedance.
More questions: I recalled when you were trying to start this motor. you shut down the other loads in the same plant.Is that right?
Do you have the station one line of your plant or at least a drawing shows how the motor connects to the system and the other loads in the plant? It would be helpful
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RE: 3.5 MW motor starting problems continue...
I like to understand what happened first before I can offer any helps.
First of all, using IEEE standard induction machine model-simplified one. The opertaing characteristic of the induction motor is such that the internal torque developed by the machine is a function of the rotor resistance and slip at which it occurs.
When starting the motor, R2'/s is small because the s = 1. So the current is high like a short circuit on Trx secondary side.
When trying the increase the voltage and build up the flux and torque(I recalled you use lower voltage starts the motor and increase it step by step), the motor could not speed up to operating one which is about 95%-99% of Synchronous speed but stayed at 15%-30%. It can explain why your upstream CB got tripped. Because if the motor can not speed up, the rotor resistance will still be very small. Ex. if the speed stays at 15% only, then the equivalent motor circuit resistance only increases to 117% compare to the motor with zero speed. Therefore, when the voltage kept increasing, high current will be created until the upstream CB tripped. I estimated the current will be around 2kA.
Let me know if I missunderstood or misinterpreted anything.
Regards,
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Edison123, I have to agree with you. Using a soft starter with 500% In vs 600% In for DOL start is pure waste of time... What the hell is the difference... But I'm not one driving the show here. Rather one who is trying to plant some common sense here and there whenever I can...
QBPlaner, unfortunately that is all I have, Xd", but I can possibly get info on Xd' or Xd or whatever else. If it helps, the fault level on the high voltage side (110kV) is 2600MVA
"More questions: I recalled when you were trying to start this motor. you shut down the other loads in the same plant. Is that right?"
Generally no. The voltage used to drop, but we were experimenting with one transformer with no other load, so nothing else could be shut down. There was one instance when the voltage drop was somehow reflected on the HV side (110kV) as well, but I would ignore it, who knows what was on the grid at that time. I'm not about to speculate (again) that a national grid has problems starting a 3.5MW motor
"Do you have the station one line of your plant or at least a drawing shows how the motor connects to the system and the other loads in the plant? It would be helpful."
Yes here is something, attached. Assume that two 110/6kV 13/20MVA transformers connect to the national grid via two short line. SC level before and after lines 3500 and 2600 MVA. The attached sketch does not show any other load, but there is some at Bus 2 and more at Bus 1, but most of the time during testing, there was not much if any other loads connected – one transformer was dedicated just for this motor.
"I guess even with two transformers in parallel, you may not get 350MVA. you may maximum get 200-250MVA depends on the Trx Leakage impedance and the rest of the system impedance."
Could be. Lets assume that I'm not getting more than 200MVA, although I do not see where all other 150MVA will vanish. I asked somebody else who does calculations for us to calculate, will confirm. So if I'm getting only 200MVA, is the something what will prevent me to start the motor with such load we have?
"Because if the motor can not speed up, the rotor resistance will still be very small. Ex. if the speed stays at 15% only, then the equivalent motor circuit resistance only increases to 117% compare to the motor with zero speed. Therefore, when the voltage kept increasing, high current will be created until the upstream CB tripped. I estimated the current will be around 2kA.
Let me know if I missunderstood or misinterpreted anything."
Here is how this starter seems to work. Voltage ramps up quickly until we reach 470% In. Then the voltage floats and can be anything – if the current tend to increase the voltage will drop to bring the current back to 470% and vice versa. Therefore motor may want whatever current it wants, but it will get only what the settings allow, which in our case is 470% In. If motor want more current, the soft starter will simply reduce the voltage what in turn surely will reduce the torque. Forgot some my previous posts, we have all kind of nuisance tripping, but now in general, if the start is say 50 sec, we have say 5 sec to ramp up the voltage to whichever level will push 470% of current and then it just stays there until the current starts dropping (if the start is successful) in which case voltage starts raising. Typically, during the start voltage masured on termainal was not much more than 5kV which is 80% while the current sticks to about 1800Amp or below.
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It's almost there, the general plan right now seems to be to just keep adding system capacity until the soft-starter can be set to start across-the-line in the hope the motor starts.
I'm not sure why you conclude that. All the required motor data has been entered. The simulations almost always prove to be conservative compared to the actual motor starting.
The motor and/or the starter is bad. Since I believe that, I have nothing else useful I can add so I'll just be checking back to see if the problem is found.
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Thanks for the information and drawing you have provided and you don't need to provided Xd' and Xd fault levels.
I can understand a little bite of your frustration but thing can only be done step by step.
Forgive my different understanding of soft starter. I understand of how soft starter works is it will starting at a lower voltage 50%-70% in order to avoid full in rush current because of the small rotor resistance at zero speed and ramp up later to build up the motor flux and finally stays at its operating point.
The reason of asking the fault level was that I was trying to dismiss the possibility that the failed starting was caused by the lack of enough system strength.
For a single Trx with current limiting reactor (0.21 Ohm mentioned in your previous post) it is about 0.52p.u 100MVA base 6.4kV nominal voltage), you will have approximate 90MVA fault level at the terminal of motor or slightly less considering the cable impedance, and with the 470% current limit, you will have about 18% maximum voltage dips.
If you parallel two Trx and two reactors, it will give you the same fault level-170MVA at the motor terminal as you run a single Trx without current limiting reactor. 170MVA should have no problem to start a 3.5MW motor with 6 X FLC with a direct cross line start without the soft starter.
Therefore, the problem should not be on the system side.
Now, I am not sure the motor status, I saw some posts you mentioned the noise and vibrations happened. I am not sure if the motor has any internal damages.
You may try followings steps:
When you parallel the two transformers, you may try a cross line start without soft starter. It will cause approximate 12% voltage dips. You may have to coordinate P&C settings. If the problem is still there which means the motor can not be started and still stays at a lower speed. Then it means it is not the soft starter problems.
Finally, it is the problems of the motor which is the tough part.
Assuming your motor has no internal damages, then what I can contribute to you from my narrow mind is that, the motor may have 5th or 7th harmonic torque components when starting the motor. (The other people already mentioned lower voltage, lack of starting current, lack of Torques and lack of system strength which I won't mentioned again). With theses torque components combined with the fundamental component, it may be has following scenario:
The sum of the load torque including the load (Fan) and motor loss torques (T Load +T loss) is smaller than the required starting Torque Tst but larger than the minimum Torque T min (During starting period motor will have a minimum Torque), the motor will stay at a lower speed 15-30% and never reach its operating point. If it never reaches the operating point with the slip value of 0.01-0.05, then the motor current will always stay high like 4 or 5 times FLC until it tripped out.
Hopefully it helps.
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http://boo
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RE: 3.5 MW motor starting problems continue...
ters
If you can post the no. of stator & rotor slots and the motor speed & frequency, I could that check out for you in a trice.
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Just my thoughts.
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I always wanted to try a DOL start. But there are two reasons that it has never happened yet:
1. It is a bit cumbersome, would require a new cable from the switchgear to the motor, to eliminate the soft starter, as the DOL start through the soft starter may not be the smartest thing to do doe to rating of bypass contactor which is much lower that the LRA. And even if we want to do it, honestly, I'm unsure if we can set the bypass to close at zero sec, maybe we can.
2. More importantly, the Client kept refusing a DOL start being concerned about aged equipment upstream - a switchgear with minimum oil breakers rated to only 800Amp and transformers which are 40 years old. But we are getting there... Within one month, we will have one more (new) transformers available, slightly larger, 20/27MVA and old minimum oil breakers of 800A will be replaced with new 1250A units... At that time we will probably be in a position to try it, unless someone finds another reason to reject it...
Speaking of inadequate motor design, anything is possible, I'm not an expert for motors, but will try to find some info and post.
However, interestingly, the motor manufacturer commented that there could actually be a problem with inadequate soft starter design... We have a 400Amp unit, which is capable of 600% short term overload. They think that if we had a 500Amp unit, that might make a difference implying that a 400Amp unit, as used (almost 500% current) cannot cope with such torture causing some instability which creates electrical oscillations and mechanical vibrations. They believe that upgrading it to 500Amp may help.
So LionelHutz , may I ask you to comment on this. I know you said you had not much to add, but you are directly in the business of making soft starters, so would you agree with the comments provided by TECO?
QBplanner, glad that you confirmed that the problem is not likely on the system side (unless we have some sort of strange resonance). About your comment that the motor may have 5th or 7th harmonic torque components when starting, where such distortion may be coming from and how to get rid off it? We have nothing else in the system that is known for generating much of harmonics except the soft starter.
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No, I would not just blindly agree with them.
Practically, the motor requires >400% current to start. The SCR's should be failing or the soft-starter trip on over-temperature if the soft-starter can not handle the current. I know Motortronics claims 500% current for 60 seconds but that rating claim appears to be a marketing number at some starter sizes after looking at some of the SCR stack assemblies. So, it would be a good idea if you checked for failed SCRs.
You have to understand that there is only a limited number of 6.5kV SCR sizes available. So, if you design a stack assembly to make full use of each SCR, you will end up with 3 or 4 stack designs. So, some starter sizes end up complete overkill and other starter sizes are at the limits of the stack used.
The bypass contactor is a HP rated vacuum contactor though, which would be fine using across the line. I believe it will be a Joslyn Clark contactor, likely the rating plate says 600A which is really a 500A UL listed contactor.
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Also - Where does the control power for the starter come from? Is it constant during starting??
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5th and 7th Harmonics torque are bascially motor design issues - the tough part. I hope it is not. THe only reason made me thought about the harmonic Torque is because you mentioned about Noise and Vibration. Either you have large harmonic components or the motor got damaged.
I was trying to believe it is the soft starter problem since the system with two Trx in Parallel should have no problem to start a 3.5 MW motor with 6X FLC inrush even with a cross line direct start. You won't get worse voltage dip than using Single Trx and current limiting reactor with a 470% current limiting soft starter.(I believe you mentioned you even get 15% voltage dip once with single Trx and reactor.)
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I don't know much about the transmission system the plant connecting to. I can only tell it based on the piece meal information mentioned in your previous posts.
Here are the reasons:
You have ?/110kV sub with 150MVA transformer(s) I would believe the fault level at high side of the sub must be higher than 1500MVA minimum (later on you confirmed more than 2600MVA) other wise people may have difficulties to energize the 150MVA Trx. Two short 110kV lines (Within 10km ??) from that 110kV sub to your plant with current limiting reactors on the feeders. The main reason people put in the reactor is to limit the fault current if it is too high for the CBs and others switches. Here in North America, every utility has its own rules to put in the current limiting reactors. In the place I work it is around 300-350MVA on 25kV side. In your case, the p.u. impedance of the current limiting reactor is almost the same as the 13/20MVA transformer. Therefore, at the motor terminal you only get approximate 90MVA or even less fault level. Even with soft starter let 's say 300% limiting, you will still get 12% voltage dip on your 6.4kV side which is huge if constantly starting the motor.
I am glad to hear that the up stream system will be upgraded CB or cables. It is a good start.
The system is quite a strong system at the point of interconnection but the plant equipment as you mentioned 800A CB may not be suitable to coordinate the new 3.5MW motor.
Before doing the cross line start without soft starter, make sure some people can do a quick hand calculation (Should be enough) to make sure the maximum voltage dip caused by the 6 X FLC with two transformer in parallel with the reactors is within accepted range on utility side as well as close by loads.
It can not tell if the soft starter is good or bad but at least it can tell if the motor is a good one or not.
The 5th and 7th torque components are motor design issues and it is the last thing you want to think about it.
Vibration and noise of the induction machine are not normal unless with harmonic torques or internal damages.
These are to my best knowledge and I 'd like to hear some good news fro you soon.
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Here I am again... We opened the motor (removed one of the cover plates). Not much of the rotor can be seen, but the steel ring plate around the end of rotor (oposite side from the cooling fan) seems to be a subject to some extensive heat - as you can see on the attached picture, the paint around is burnt. Almost looks like one used a heat gun to peal it off... Does anyone have any suggestions how (ab)normal is this? Thank you.
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I haven't seen that exact construction but I assume this assembly at the end of the rotor is just a fan and should have no current going through it. Something is wrong to make current flow through there. Whether it might be a broken end ring something else I'm not sure.
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Look at the place where the hole is. Imagine current flowing circumferentially. The current crowds as it goes past the hole. That is why the heat damage is worse there.
I was predisposed to think you had a rotor problem even before this photo. I can't say the photo is conclusive as to what's going on, but to me it certainly fits with the faulty rotor scenario.
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1 - as said before - there is more heating radially adjacent to the hole which is consistent with circumferential current flow.
2 - there is less heating circumferentially adjacent to the hole which is consistent with circumferential current flow.
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As per the attached picture, the way how the heat dissipation seems to works could possibly be explained like this:
1. The paint is still intact (not damaged) around any of the impeller blades. This could be an indication that blades are working as "coolers" conducting the heat away for the impeller plate.
2. Plate areas between the blades are apparently burnt as there is no blade to conduct the heat away.
3. Area between the blades where holes happen to exist too (whose purpose is unclear to me) are more badly overheated, perhaps due to heat conducting surface being reduced due to the hole?
Or maybe I'm completely misunderstanding this and some of you experts can make much more sense of this?
Even if things are as I tried to explain, it is still not obvious to me where that heat was coming from, and why. As electricpete explained, it more seems that the heat is generated due to induced currents, but why they are induced? Rotor damage is a strong possibility, but could it be as simple as due to such (possibly bad) design the stator windings which are partly around end rotor fans are inducing currents in the impeller in a "normal" way - same like currents are being induced in the rotor winding?
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Another interesting thing is that only the outer flange of the fan has burn marks but not the inner flange (one that is fixed to the rotor shorting ring), which kinda disses the circulating current thinking. Also, the fan blades themselves have no burn marks.
Can you unbolt the fans and remove them from the rotor shorting rings to take a gander at the bars and the rings ?
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I agree it is hard to explain how/why rotor damage would cause this particular pattern.
Less damage on inner flange than outer flange might possible be due to heat sink effect ters mentioned but I don't know... both the shorting ring and fan appear to "stand off" from the core/spider which is the main heat sink. We also notice the heating on outer flange is less directl above the fan blade... do we attribute that to heat sink effect or current pattern?
Another aspect was metnioned that the damage is symmetric (except for where the holes are). If it were rotor problem, I would have expected assymetric damage on the fan (more heating in areas of bad bars or damaged ring), but we don't see that.
(By the way the holes I would expect have something to do wtih balancing. Either they were drilled during trim balance at the factory. Or more likely they are provided so the client can easily bolt on balance weights if he chooses during subsequent field balancing of the rotor in situ.)
So the alternate scenario edison is suggesting is that these are currents created in the fan due to direct electrical contact with end ring which is said to be a bad design. What features in a "good design" prevent this? Maybe painted surfaces are supposed to serve as an insulator? I guess even if it is not a design problem, the fact that this motor has experienced many severe starts may make this type of heating more likely than some other motor with similar design that doesn't see those severe starts.
Final note - regardless of the source of this particular damage we still have lots of reasons to suspect rotor damage - namely the apparent slip frequency oscillations in current during start.
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Possibly:-
Due to current sharing with the shorting ring.
Due to induced magnetism from the end turns. Yes, I know that the field is supposed to be much weaker with no iron, but what about the magnetic material of the fan?
End play. The end play may be letting the fan move part way into the stator core.
The heating damage may be an indication of severe duty as a result of failed starts.
Is the end play in such a direction as to allow the fan to enter the stator core?
Bill
--------------------
"Why not the best?"
Jimmy Carter
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I don't understand how this fan would work. Does it suck air from the rotor through the the big ventilating holes in the rotor core (in which the case there must be a baffle in the axial middle of that core and there isn't any) or does it suck from the winding overhang end (in which case it just seems to recirculating air in the winding over hangs (since it is delivering the air back to the winding overhangs) ? Why those big holes in the outer flange when they have a series of threaded holes in the inner flange for trim balancing ? And the million dollar question, why the burn marks ?
bill
Given the welded blades, it is a steel fan and hence magnetic. And given the thickness of the rotor shorting ring (at least 3 inches ?), there is no way the rotor would have that kinda end play.
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After taking a closer look it seems that holes around the outer fan plate are actually access holes to stick through a 13mm socket to unbolt the fan from the connecting ring, or whatever it may be attached to. But unbolting the fan(s) is a bit challenging. For one at the front, it is probably possible, there is enough access, however bolts are secured by welding touch and I'm not sure how easy would they would break lose. At the back of the motor, not much access since there is a large stator cooling fan at the shaft end working a a part of the heat exchanger. There is some room between the stator cooling fan and the motor body at the back where the inspection plate is, but not enough to get inside (sufficient to stick in you hand with a camera).
When coupled, there is no that much end play so the rotor does not really move more than 5mm away from the magnetic center. Whichever direction it moves, it brings one fan closer to the stator core, and the other fan further away. I'm also not sure what these two fans are exactly doing, but looks like one is puling some air through the endwindings from the heat exchanger area sitting on the top of the motor, and then the air passes through the hollowed rotor and being pushed out by another fan through the other end winding?
This motor was probably started about 60 times so far, each time the start (successful or not) lasted 10- 50sec. Therefore, the total time when the stator current was in the range of 1500-2000Amp is about 30 minutes. If both fans worked as some soft of "extension" of rotor windings carrying high induced current, the time there were exposed to such torture was likely long enough to develop some burns...
Another picture of the ring area is attached. Seems that there was some overheating there too, but hard to say, the picture has a lot of "noise", so not sure what is a real discoloration and what the noise...
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But, tack welding the bolts kinda defeats the very purpose of these holes. Guys at Teco must be wonderful engineers.
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A lot of people claim that Eng-tips forum is great place and this treat is one of the good demonstrations.
My office is less then 1 km away from the facilities but I ,through Eng –tips, first time heard that Mr. Ters opened the motor and looked into!
Unfortunately I can't open the photos at my home and I will see rotor tomorrow morning.
Like Mr. Ters said, I am, temporary, just the Client but, in the future, I vill be, one of the people, who care about motor, 6kV netvork, protective releys, ECT.
In this project, people who work on were not generous. Recently I have come across the fan curve and I have seen that it need BHP of 4650 HP or 3500kW with the dampers 100% open and I saw, when system worked that motor draw nominal current.
On the other hand, many information and recommendation for heavy application like this call for minimum 10% - 15 % more motor power above BHP required for the fan.
With bigger motor and better rotor design I and Mr. Ters would be more peaceful person, but it is not worth crying over the spilled milk....
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But this is rather an academic question... Even if I'm right, this provides no explanation why the motor does not start :)
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You could dedicate each start to those people around the motor. :)
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Did you mean name each start after someone hanging around :)
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Perhaps I can join in very late in the discussion with a few questions.
1. I understand that the cables between the starter and the motor are quite long. Are these screened or armoured cables?
2. What speed does the motor/fan get to when the vibrations begin? Is it always the same speed?
3. Is there any natural mechanical resonance in the fan? If so, at what frequency?
4. What is the susceptibility of the motor to harmonics?
Looking at one of your earlier graphs, the oscillation in the current is very sinusoidal suggesting that there is some form of resonance going on.
I have found a situation many years ago, starting a large crusher where the motor would accelerate to part speed and then would vibrate significantly with the shaft appearing to move axially. The motor was dismantled and the rotor was found to be slightly off round. A few thou were skimmed off one side of the rotor, it was reassembled and worked perfectly from then on.
I have also recently been involved with a large fan on a VFD which had a natural resonant frequency within the operating range of the fan and this interacted with the open loop vector control algorithm to cause torsional vibrations which destroyed the fan.
I have also seen similar results when the motor and load are not perfectly aligned, and also when the mounting bed for the motor and/or the load are able to flex slightly.
As this is occuring at a low speed, you should not have a torque issue. What happens if you attempt a start at a much lower start current? does the motor still vibrate at the same speed, or does it pas through this speed?
I would suggest that you try at say 250% start current.
I would also have a series of video cameras and strobes etc to see if you can get a good picture of the mode of vibration and see where it is occuring.
Best regards,
Mark Empson
L M Photonics Ltd
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Thank you very much for joining again. I will try to answer rest of your questions tomorrow, it is already late here in Europe. Re cables, there are three cables in the circuit:
1. The cable from the soft starter to the motor is actually very short, only about 15m, 2x120mmsq per phase, screened.
2. The cable from the process switchgear (which is to feed two identical 3.5MW motors some day...) to the soft starter is also very short, only about 20m, 2x120mmsq per phase, screened.
3. The cable from the 110/6kV substation switchgear to the process switchgear is long, ~ 400m. This cable is a bit strange mix of cable types. In the past, there was one three core armored cable 3x240mmsq, which was sufficient then. With new big motors, the power needs changed, so to meet it we added more cables to the same feeder. That feeder now included the original cable of 3x240mmsq three core + new cables 2x240mmsq per phase single core. Could this mix of long three core + single core cables be causing or contributing to some resonance phenomena?
Our only attempt so far to eliminate something which might theoretically contribute to electrical resonance phenomena was to eliminate current limiting reactor (at the substation swithgear - before long cable), but that made no difference.
However, it is now very hard to draw any conclusion what does or does not make a difference since meanwhile our problem progressed from occasional successful starts to no successful start... So, if something failed meanwhile, the problems is probably the same with or without reactor. We have not started motor for about 3 weeks as the substation is not available - due to a major upgrade.
Unfortunately, we do not measure the speed religiously during each start (only manual measurement is available), but it does seem that the motor stalls at about the same speed all the time. That speed may be in the range of 300 RPM. We have not attempted to change the current limit for some time, so we can try lowering the limit and see what is going on next time. We will be in position to do it about one week down the road when the substation should be back online.
Best Regards
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While you are at it, could you note down the no. of rotor bars and no. of stator coils. ?
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The top chart is the line voltage; the mid chart represents the voltage on the motor terminals; while the bottom chart is the motor current.
If you take a closer look at the motor voltage, you can see that SCRs, for some reason, for about 8 cycles are chopping only positive half periods, while let negative half period pass through NON-chopped, and then it switches around, so next 8 cycles SCRs are chopping only negative half periods while positive ones are let through NON-chopped, etc.
I think that non-chopped sinusoidal half periods are those which are transparent all the way to their tip, while those which are chopped have black shadowed tips.
Consequently, the whole current chart (or one could say the reference for the current form) oscillates up and down – goes up when SCRs are chopping only negative half periods and goes down when SCRs are chopping positive.
This seems to indicate that something may be wrong with the soft starter, unless the soft starter behaves so due to some severe motor or system asymmetry.
This chart captures the time from 21 to 24 seconds after that start. It was long after the motor stopped accelerating and just stalled at an estimated speed of 150 RPM.
Rest of the chart, which I will post below as well but at a reduced resolution (too long, about 2 meters) suggest the following:
1.The motor accelerated normally and smoothly during first 6 seconds. This approximately matches voltage ramp up time. After 6 sec, the speed was about 90 RPM. During this time SCRs were chopping the voltage (firing) symmetrically - all half periods seems to be equally chopped, either positive or negative ones.
2.Then, from 6th to about 12th sec SCRs would seem to decide NOT to chop some of the sinusoidal half periods. Initially, around 7th second, it started with one positive half period being non-chopped, followed by one negative half period non-chopped, etc, then about 9th second two of each would seem to be non-chopped, then 3 half periods were not chopped, etc, etc. It seemed that during this time the motor was still accelerating but at a much slower pace than during first 6 seconds and it started to develop severe mechanical vibrations.
3. Finally, at about 13th second, the motor completely stalled and an estimated speed of 150 RPM. After that, the frequency at which the thing alternated chopping only positive or only negative half periods remained steady - about three "periods" per second, whereas the "period" is defined by about 16 normal 50 Hz cycles. Mechanical vibrations continued at the constant frequency.
Does this provide any more light to explain what is going on and why?
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Some axial fans have strange problems if started into a closed systems. The air coulumn in front of the fan compresses and unloads. You may need to find a smart fan person to explain it. I have seen fans shed blades because of it.
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http://www.sendspace.com/file/c3wfu5
We are starting a centrifugal fan with the moment of inertia of about 70,000 lb-ft^2 (3000 kg-m^2), Double Width, Double Inlet arrangement.
Edison, I will try to count the bars some other day.
Mark, I could not get info today on natural mechanical resonance in the fan and susceptibility of the motor to harmonics, will keep trying...
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From your latest graph, there certainly seems to be a sinusoidal oscillation that is apparently affecting the firing of the SCRs.
It is difficult to determine what is happening from the graph as it is too compressed. I would like to see the graph showing just half a dozen cycles so that we can see the actual conduction and commutation angles with some detail of the motor current which is definitely not sinusoidal as implied by this graph.
Is it possible that the problem is occurring at 136RPM?
This is one eleventh of the speed and is one of the speeds where cogging can occur due to harmonics. The seventh is usually another difficult spot.
If the motor has a harmonic problem at this speed, then I would expect to see a sinusoidal modulation of the commutation angle of the SCRs which could explain what you are seeing. If you study the approach to this speed, you will probably see the modulating frequency reduce and amplitude increase as it approaches this speed.
I believe that this characteristic is altered by the design of the motor with the skew of the slots and the ratio of stator slots to rotor slots.
If there is a harmonic interaction going on with the motor locking on to a harmonic, then I would expect the problem to get worse as the supply impedance is reduced. At a higher supply impedance and a given current, the conduction angle of the SCRs would be greater and the harmonic content would reduce, reducing the problem.
Best regards,
Mark Empson
L M Photonics Ltd
RE: 3.5 MW motor starting problems continue...
From that latest graph, you can see the oscillations starting at a high frequency and low amplitude and the beat frequency reducing and amplitude increasing as you approach the critical speed.
I am pretty sure that you have a critical speed that is either due to mechanical resonance, or harmonic interaction in the motor. Check the shaft speed and see if it is almost 136.36 RPM (1/11 synchronous speed.)
Best regards,
Mark Empson
L M Photonics Ltd
RE: 3.5 MW motor starting problems continue...
Our shaft does tend to move axially towards the fan during the start, but it does not have much room to go - the play in the coupling is small, just enough to allow the shaft to move a bit to the magnetic center depending where it landed during the previous stop.
But, again, why a slightly elliptical rotor would cause the soft starter to, as it seems to be doing when the speed stalls, chop 8 or so positive half periods but conduct the entire time during corresponding negative half periods. and than do another way around...? This phenomena is like superposing normal 50Hz voltage with something like 3Hz voltage of lesser amplitude... so it is probably not due to rotor shape.
It is possible. For the last chart I posted, we unfortunately did not measure the speed, but we modeled the motor start and it seems that the model, which tells us what is the speed each second during the start is OK - matches the reality when we have a successful start. So based on that, we can be more or less sure that before the problem starts to develop when the motor reaches at least 90 RPM. It does probably catch a bit more speed in the next second or so, so maybe it is really around 136.
Next time we start we will measure the speed along with voltage and current using a better recorder.
If we did not do some damage (or more damage) to the motor and/or the soft starter due the repetitive harsh starts, than your above explanation may be exactly what we are going through...! We were able to start the motor before (more frequently than not), but once we either decided toput two transformers in parallel or reduce the impedance of one transformer circuit by eliminating its current limiting reactor, than there is no more successful start.
So assuming that you really nailed it down, what would be our next step, what measurements we should do to confirm that this is the exact problem, and if yes, how to deal with harmonics in our situation, i.e, what would be a cost effective and practical solution which would not be as demanding as getting a new motor of different design?
RE: 3.5 MW motor starting problems continue...
If there is a resonance occurring, and the graphs strongly suggest that this is the case, then any feedback system can amplify that resonance if the poles occur at the wrong frequencies and this can make a marginally stable system very unstable. Modifying the gain and frequency response of the feedback system can often shift the poles sufficiently to soften the problem and allow the system to pas through the resonance.
In this case, there is a closed loop, the current control loop and it is possible that this is amplifying the "resonance".
If you revert back to a voltage ramp start with a start voltage at 60% and a ramp time of 120 seconds, you should have an approximation of a current limit at around 400%, slowly increasing.
If the motor accelerates through the "bad" patch, then you have a potential solution. If the motor still vibrates at around the same speed, you have a problem and I would suggest talking to the suppliers about the harmonic susceptibilty of the motor and investigating resonant frequencies of the mechanicals.
Best regards,
Mark Empson
L M Photonics Ltd
RE: 3.5 MW motor starting problems continue...
Resonance means a single frequency where the system gives a large response. Why would the frequency of oscillation change like this for a resonance? For any type of frequency I am familiar with the frequency is relatively constant, not smoothly varying.
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RE: 3.5 MW motor starting problems continue...
"For any type of resonance I am familiar with the frequency is relatively constant, not smoothly varying."
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Eng-tips forums: The best place on the web for engineering discussions.
RE: 3.5 MW motor starting problems continue...
Doesn't resonance require a capacitance ? Or you think the cable and stator winding capacitance is enough for a resonance ? Given the cable length, it is a possibility ?
RE: 3.5 MW motor starting problems continue...
Gents, maybe "resonance" was not the most descriptive word for what is going on, nor I think even "oscillations" is the best qualifier for what we see on the chart since the RMS measurement does not show much of the current oscillations. It is only the chart recorder graph showing that most of the time SCRs tend to chop the voltage unequally in one single period, making so the amplitude of the negative vs. positive half period different. Mark did explain in a way which I understand as 50Hz being like a carrier and it is then being modulated by the amplitude due to certain harmonics.
But we would need to prove this "beyond reasonable doubt" :) before we can search for an appropriate solution.
Mark, I do not seem to understand another part of your expanation. If the motor stalls at say 136 RPM, which is one eleventh of the synchronous speed, how do we know which harmonic is causing this? Or to rephase the question in the most general terms, what is the relationship between N fraction of the speed with Nth harmonic, if any?. Or in our case, which harmonic would cause such modulation which makes approximately 8 periods "more positive" and then next 8 periods "more negative"? What I see from the graph is like 50Hz being modulated by something close to 3 Hz...
RE: 3.5 MW motor starting problems continue...
Whatever you want to call the effect, I would agree with this. It's very likely the problem is an unstable current control loop in the soft-starter. Some unstability in the connected motor system is being amplified by the current control loop. Fix the current control loop and the motor will likely start.
Unfortunately, if you use an open loop voltage ramp and starve the motor for voltage then it can also exibit a similar unstablility.
The number of smart people who understand the feedback theory yet still don't properly impliment a digital feedback loop might surprise you.
RE: 3.5 MW motor starting problems continue...
Naturally, as long as there is anything else that may contribute to this phenomena, equipment manufacturers (either motor or soft starter) will continue pointing to that as the root cause. Their explanation is more in line with Marks explanation that the root cause are harmonics but they are pointing to a bit different stall speed/harmonic and it also goes beyond that explaining that the power system is the main culprit which happens to be just tuned to oscillate. I will take a liberty to copy/paste what they said:
Quote:
In reviewing the waveforms and information you have supplied, the soft starter actually is firing at all times, as evidenced by the continuous output seen on the current waveform at the bottom of the traces. This holds true because the soft starter is a current-controlled device, which means it uses current feedback to monitor the output of power sent to the motor, so current is a good indication of our firing control.
Now, that being said, it can be seen that as the motor is accelerating, it eventually reaches the stall point as you have mentioned. In calculating the period of the oscillation against the source frequency ("about 16" cycles as you stated in Item #3 below), it comes out to a "Triplen Harmonic" of 16.67Hz in relation to the 50Hz source frequency. This means that the resonant point of the motor RPM is quite likely to be near 150 RPM, which is the extended triplen frequency of the RPM versus the source frequency (16.67Hz x 3 squared).
If you look at the 7 to 8-second point of all three waveforms, you can see where the resonance of the motor is beginning to impress itself on the source voltage waveform, the motor voltage waveform and the motor current waveform. As the resonance builds up along the timeline within the motor voltage waveform, it reinforces itself, gaining amplitude and frequency, until it remains at the same values from 13 seconds to 24 seconds. Interestingly though, the resonance is superimposed within the motor voltage, yet can be seen to only affect the p-p values of the source voltage and motor current waveforms. This resonance will create an off-frequency, counter-rotating torque in the motor thereby causing the mechanical vibrations you describe. Also, as you reach the triplen harmonic, you may find that there are high currents in the neutral return conductor due to the high return currents being generated by this issue.
To summarize, the overall power system is acting like a tuned circuit and as you slowly build up RPMs in the motor during starting, the motor will stall as it becomes loaded down within the resonant waveform.
From my standpoint, there may be a couple of ways to deal with this issue. One, is to change the reactance in the circuit, which may be something as simple as jumpering out the 6% line reactors at the feeder. The second possibility is to accelerate the motor as hard and as quickly as possible to try to break past the triplen harmonic point in the acceleration curve of the motor before it loses so much torque it can no longer accelerate the load. Or perhaps, a combination of both will do.
We have seen this issue once before and even after starting the motor across-the-line, the motor still had an oscillation like the one you show on the charts. The fix for that one was that they had to change the feeder system in order to break the harmonic tendency of the overall system to oscillate. I can't say exactly what needs to be done to fix your situation though, as harmonics are never an easy thing to deal with and the conditions vary from site-to-site.
End of quote
RE: 3.5 MW motor starting problems continue...
Our 6kV system has no natural conductor. And even if it had one, what it would had to do with a 3 phase motor?
The 110/6kV transformers are WYE-WYE, secondary not grounded.
RE: 3.5 MW motor starting problems continue...
Hope you don't mind some rambling questions. That post about triplen harmonic completely lost me. 16.67 is not a triplen harmonic of 50hz, it is 1/3 of 50hz. 16.67*2^2 is on the other hand third harmonic.
If all three phases are identical shifted by 120 degrees of fundamental, then the triplen harmonics of the three phases are shifted by 2*120 = 360 degrees from each other, in other words in phase - zero sequence currents. Those would flow in a neutral connected back to the source but as you point out there is no neutral connection on wye connected motor. On the other hand if connected in a delta winding connection (maybe that's what y9ou have?) you could have zero sequence third harmonic currents flowing in the three phases leading to the motor.
And how did he start with an observation about 16 cycles length of envelope and conclude there was something at 16.667 or a triplen harmonic?
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RE: 3.5 MW motor starting problems continue...
Aren't the wye/wye transformers with unconnected neutral supposed to have unstable neutral potentials ? Would that cause the soft starter to misbehave ?
I agree with pete 16.67 Hz is not a triplen harmonic.
If the motor is stalling all the time at a specific speed, I would say the stator/rotor slot combination is more of a problem at this point.
RE: 3.5 MW motor starting problems continue...
A second transformer was added and the motor would not start.
The series reactor was jumpered and the motor would not start.
This lends weight to the suggestion that the system is incompatible with the motor/soft starter combination.
Why not try a start as it used to be, one transformer and the series reactor in the circuit? How about adding capacitors to shift the resonant point?
If you can start with one transformer the strip chart recording may give some clues.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: 3.5 MW motor starting problems continue...
Electricpete, I have the same dilemma. As you said, 16.67x3^2 is 150, that is thirds harmonic. But the modulation I see on the chart has a frequency of about 3 times in one second, not 150 times, so I don't understand it yet either.
Edison, we have an option to ground the 6kV neutral through a reactor - there is disconnect swith for that. The Client keeps it usually ungrounded as the capacitance in the system is not (or was not) high enough. So we can try with grounded natural and see what happens. However, the soft starter measures the line voltage with two PTs in an open delta configuration, and uses that information for firing circuitry so why it would care about floating neutral?
Waross, we will go back to one transformer scenario when we can - at the present it is no transformer scenario as the substation is being rebuilt. However, as it behaved then, that mode is not sustainable as a long term solution due to long starts and dip and unstable voltage drops. We were able to do it only having one transformer dedicated for the test. However, as you pointed out, maybe adding capacitance would help in either case (one transformer or two) but I think we have to know what the exact problem is in order to add something which will compensate for it.
RE: 3.5 MW motor starting problems continue...
Induction motors have a series of slots in the stator and in the rotor. These slots should not be equal in number because if they are, there is a good chance that the motor will not start at all due to a characteristic known as cogging. The slots will align like a stepper motor.
For this reason, there are an unequal number of slots in the rotor and in the stator, but there can still be situations where the slot frequencies coincide with harmonic frequencies and this can cause torque modulations. The slots are skewed to keep an overlap on all slots to reduce this problem.
Another characteristic of induction motors, is crawling. There are harmonic fluxes developed in the gap due to the magnetics of the motor. These harmonics create additional torque fields. A common problem is with the seventh harmonic where the seventh harmonic creates a forward rotating torque field at one seventh of the synchronous speed. There will be a maximum torque just below 1/7 Ns and if this is high enough, the net torque can be higher than the torque due to the line frequency where at 1/7 Ns, the slip is high. This can cause the motor to crawl at just below 1/7 synchronous speed.
There is another crawl speed at 1/13 Ns.
When a motor is started with a soft starter, the gap harmonics are increased by the harmonic currents produced by the phase controlled SCRs of the soft starter.
A motor that has a tendency (small) to crawl when operated with a clean supply, will have a much greater tendency to crawl when controlled by SCRs and a chopped waveform.
The subject of harmonic torques and harmonic fluxes can get very complex when you try to analyse the effects of all the electrical harmonics, plus the magnetic harmonics plus the slot noise.
The seventh harmonic is the best known and documented crawl speed, but others can exist due to the total interaction of all harmonic sources.
The two critical speeds are 1/13 Ns and 1/7 Ns. If the motor crawls just below either of these speed, there is a definite interaction between the motor and the harmonics produced by the soft starter. At other odd order non triplen harmonic speeds, there can also be more complex interactions. - I have seen reference to interactions at 1/11 Ns but I can not recall the mechanism.
The design of the motor, no of slots in rotor and no of slots in stator, and the skew of the slots are all selected to minimise these problems, but all design is a compromise.
A motor crawling at say 1/7 Ns will produce maximum torque at a low slip (relative to 1/7 Ns) in the same way that the maximum torque is produced at just below Ns. There will therefore be a "slip" frequency at the maximum torque. This will cause a modulation in the current flow in the motor and this in turn can modulate the commutation angle of the SCRs.
Where the SCRs are triggered relative to the voltage wave form, there will be a modulation of the conduction angle of the SCRs as observed. This will tend to accentuate the crawl torque by effectively amplitude modulating the line frequency applied to the motor. Where the SCRs are triggered relative to the current waveform, the phase modulation will not affect the conduction angle of the SCRs and the effect will not affect the crawl torque.
Where the phase modulation of the commutation angle causes an amplitude modulation of the motor current, and there is a current control loop, it is possible that the response time of the loop can further amplify the modulation.
The greater the level of modulation, the lower the probability that the motor will accelerate through the crawl speed.
In the same way that the torque just below crawl speed is a maximum, the torque just above crawl speed is a minimum. There must be sufficient synchronous torque at the minimum to accelerate the load to full speed.
The major influence of the supply on this scenario, is that an increased supply impedance will result in increased conduction angles and reduced harmonics which will reduce the crawl torque. A reduced supply impedance will cause a reduced conduction angle for the same current and this will increase the harmonic content and result in an increased crawl torque.
At this stage, we have no positive proof that you are having crawl torque issues, but the characteristics that you have described suggest to me that this could be the issue.
The best indication is to determine the actual shaft speed where the motor acceleration stops. If this is at just below 1/13 Ns (115RPM) or 1/7 Ns (214RPM) then I would suggest that this is a very likely scenario.
In my opinion, the low frequency modulation of the current waveform does not indicate a rotor bar problem. If the motor was operating close to synchronous speed, then I would certainly be concerned about a damaged bar when presented with a low frequency modulation of the current, but the slip frequency in this case is very high, so bar modulation would also be at a high frequency.
Best regards,
Mark Empson
L M Photonics Ltd
RE: 3.5 MW motor starting problems continue...
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: 3.5 MW motor starting problems continue...
I don't have much new to report, we cannot run the machine until about mid next week. At that time we will be able to try a couple of new things as we have not tried yet, and also do some more measurements.
If the increased system impedance does help to dampen oscillations, unfortunately by increasing it we will be encountering another problem, the voltage drop too big and lasting too long. With one TR (=large impedance), the system capacity becomes marginal with only 90 MVA SC level at the process switchgear (next to the soft starter). So these two things are rather conflicting. During successful starts with one transformer, in addition to voltage drop, we also had voltage oscillations (on the line side – before the soft starter). The transformer we used is a 13/20MVA, with 10.5% @ 20MVA, and we have 2 such units. Its capacity is rather barely adeqate for the start as the motor draws something like 20MVA during 50sec (at 470% current limit).
We hope to have some more luck with the new third transformer we are just installing which is 20/27MVA with 14% at 27MVA, it will not have a reactor, but will have some longer cable before it connects to the same bus, so the overall impedance will actually be somewhere between one old TR + reactor and two old TRs in parallel (with reactors). But it might behave differently to worse or better – the total cable length with the new transformers before it reaches the motor with be substantial – some 700m.
Once we get out of the puzzle, we are supposed to have three such 3.5MW motors running... Just wondering is one is already running, would starting the second one on the same bus make its start more difficult? Or maybe for some reason the first one which is running might actually help the second one to start?
Electricpete, we have a six-lead motor connected in WYE where the neutral in former in the termination box for differential protection CTs. So it is WYE (transformer) – WYE (motor), ungrounded, with the soft starter in-between. If the transformer neutral is not grounded though a Petersen's reactor (which has been the case for years), then the only sort of reference to ground are switchgear PTs, there are 3 of them, a WYE connection.
RE: 3.5 MW motor starting problems continue...
As you can see, the impact on the line voltage is much severe with one transformer only, oscillations/amplitude modulation/resonance/disturbance, whatever you prefer to call it, is much more noticeable. These oscillations are even noticeable on the 110kV side.
The equivalent impedance of two transformers in parallel with their reactors in the circuit (not shorted) is not much different than having only one transformer with reactor shorted (transformers are 10.5% @20MVA and reactors are 6% @ 1000A). However, one transformer with no reactor still has slightly more impedance than two transformers in parallel with reactors. So this case may not support the explanation that less impedance makes it worse, although I think that this difference is oscillations is more due to marginal MVA capacity of one transformer vs. adequate capacity of two in parallel, since using a 13/20MVA transformer for a start which draws about 20MVA seems to be rather marginal.
If we dedicate new 20/27 MVA (14% @27MVA + 300 of cable) for those three large 3.5MW motors only, which we could do, that might prove to be some sort of optimization between the system impedance and capacity, which we just happen to have by a chance, but we will know if it works only when we are in a position try it, a couple of weeks down the road.
RE: 3.5 MW motor starting problems continue...
The voltage drop was not that bad, from 6.4kV to 5.8kV and the voltage remained firm and steady during the entire start, without swings as we used to have with the soft starter. The LRA current of 2300Amp existed for the first 200ms or so, and then stabilized at 2000Amp and remained there very constant until about sec 25 when it dropped to something like 130Amp at full speed.
This DOL start was actually noticeably softer than with the soft starter...
So we seem to have a defective soft starter, especially having in mind our last attempt to use it when SCRs would conduct for say 0.4 sec and then make a pause for 0.2-0.4 sec and kept doing it for about 50 sec, at which time it was tripped. A sample of such intermittent soft starter operation is attached. From the top to the bottom, values are: line voltage B-C, motor voltage B-C, phase motor voltage A and line current A. This was a start with one transformer, which in the past was not intermittent, but now it is all time. A start with two transformers behaved differently, the soft starter did not work intermediately but the motor halted at about 150RPM as previously discussed. So we will be replacing the soft starter with the spare one, and even though I'm sure the new one will behave differently, my felling is that using a soft starter for this particular application is rather waste of time, or at least it is waste of time trying to use a particular make and model which we have,unless the manufacturer is willing to get involved and do some modifications on it.
RE: 3.5 MW motor starting problems continue...
Thanks for the closure post. Good to know you have singled-out the defective part!
RE: 3.5 MW motor starting problems continue...
RE: 3.5 MW motor starting problems continue...
The problem may be a faulty soft starter, or a compatibility between the starter and the motor, or the motor may have a tendency to crawl which is exaggerated enough by the soft starter harmonics to make the motor crawl.
It is easy to blame one component when it is the combination that does not work.
Best regards,
Mark.
Mark Empson
L M Photonics Ltd
RE: 3.5 MW motor starting problems continue...
However, if the soft starter is defective, then the new one may still offer some advantages vs. DOL start, if it works say at 400% In, in which case the start with only one transformer will be possible what would simplify our life eliminating time consuming procedures for reconfiguring the power system for each start (paralleling transformers and reverting back to normal).
The soft starter we have been struggling with for 3 moths needed at least 470% In, drawing so the current in the range of 1800Amp with unstable voltage and current conditions.
The DOL start, with the voltage drop to 5.8kV drew 2000Amp steady, so even if the soft starter was working fine all the time (which was not the case) and was not causing voltage and current swings and offered shoot and steady start like it did most of the time, still the difference 1800 Amp vs. 2000 Amp is obviously not very dramatic, only ~ 10%.
I'm attaching the info (chart) of the DOL start for reference for those that one day may struggle with similar issues and will report back what happens when we replace the soft starter.
RE: 3.5 MW motor starting problems continue...
RE: 3.5 MW motor starting problems continue...
I'd like to comment on your post. A soft-starter will work just as well with a high inertia load as it would work with any other load. In your case, you should have been able to achieve a current reduction down to about 1350A, but more realistic about 1600A = 20% current reduction. If that is enough to allow a start with a single transformer then the soft-starter is worthwhile. If it's not enough of a reduction to allow a single transformer start, then there is little point in using a soft-starter.
I'd like to expand on something I posted before. Digital control loops, in this case the current control loop, are not nearly as simple as they seem. I'd bet, conservatively, that we spent over $50k on the current control loop alone for our newest soft-starter model. I know who was involved and how long it was worked on. And, it does work much better than any of our previous starter models. With our older model, it was possible to pick some unrealistic settings and create an instability similar to what you are recording.
RE: 3.5 MW motor starting problems continue...
DOL starts may be even possible with one transformer with plenty of impedance in the circuit to work as a soft starter, but the disadvantage of using DOL start with one trafo would be that no other loads could be on the same bus. With two transformers in parallel, and initial voltage of 6400V, the voltage drop is not severe, so other loads on the same bus may be OK to live through a 25-sec modest voltage reduction. However, paralleling and un-paralleling is a bit time consuming.
We also have a new transformer rated to give 2600A, not commissioned yet, so there seems to be some other options on the horizon, but temp unavailability of one of the transformers may be an obstacle in any case.
Your last comment on the quality of the current stability loop does likely explain our case if not to the full extent, then at least partly.
edison, we have online tap changers on all transformers, but they aren't fast enough to replace the soft starter in varying the voltage during some 30 sec :)
RE: 3.5 MW motor starting problems continue...
Meanwhile, we added one 110/6kV transformer, 20/27 MVA, 10% @20MVA. And looks like that due to its higher impedance and 700m of cable in the 6kV circuit, it works as a "natural soft starter"...
So we now start directly online with this new trafo and encounter a very smooth start lasting only 25-30 sec, voltage drops from 6.3kV to 5.6kV but it is stable and does not swing, starting current is average 1800 Amp expect for the inrush, when it may be reaching about 2200Amp, but it drops down with the voltage drop quickly and stabilizers around 1800Amp in ~ 1 sec, before it finally drops to about 130Amp at full speed (dampers closed fan has no load except some air leaks)...
So it appears that the soft starter provided no advantage, since it also needed 1800 Amp as minimum current (470%), which is the same as for direct online starts... Plus, with the soft starter, the start was two times longer then with direct online start (55 sec vs. 25 sec). Also voltage swings were huge with the soft starter, but using direct online, the voltage remains rock stable after the initial drop of 10%...
So, in summary, the start without the soft starter is actually much softer then with the soft starter...
However, we are still planning to bring ABB in to try to determine why their soft starter is not performing any better (theoretically it should start the fan at 1400 Amp or lower), and if they find a fix and if it can start the motor with significantly less than 1800Amp, maybe we will still use it... But I'm not very optimistic. For this sort of rotational inertia, using a soft starter seems to be a rather suboptimal solution... The fan cannot be started without some 400% In as a minimum, while the direct online start is theoretically 600%, but due to system impedance it never gets higher that 500%. So 400% or 500%, the difference may not be significant and worth the efforts...
For a change, we now have major fan vibration problems. All sort of rework has been done on bearings, pedestals, foundations, alignments, etc, still the same and gradually getting worse... Only the fan has such problem, motor bearings vibrations are within limits...
RE: 3.5 MW motor starting problems continue...
Do not let this experience convince you that soft starters do not work with fans etc. Soft starters do usually work under these conditions and give a very worthwhile result. The current reduction is a function of the load torque/speed curve and the motor speed torque curve, but in most cases there are worthwhile improvements.
In this installation, there appears to be a problem of some form that is causing the system to go unstable. This is not necessarily the soft starter, rather the whole system.
I note with interest . Perhaps the real gremlin is starting to show it's head??
Best regards,
Mark Empson
L M Photonics Ltd
RE: 3.5 MW motor starting problems continue...
The vibrations cease swiftly upon the motor is shut down, within about 2 sec they drop to half and continue progressively dropping, while the fan speed does not drop that quickly due to inertia.
Several skilled experts measured vibrations, but nobody came up with a positive conclusion what is going on. The motor shaft does not vibrate that much, in average 4x less than the fan.
RE: 3.5 MW motor starting problems continue...
It sounds like you have some resonance somewhere, and it is possible that the harmonics were being excited by the action of the soft starter. Once resonance occurs, the whole system loop can begin to chase the resonance and exaggerate it.
Best regards,
Mark Empson
L M Photonics Ltd
RE: 3.5 MW motor starting problems continue...
Very encouraging words :). So what can we possibly do to avoid resonance being chased and exaggerated?
RE: 3.5 MW motor starting problems continue...
Have not got on this site for almost a month.
It seems the voltage drop is 11% which is close to what I estimated 12% voltage drop in my previous post with cross line direct start. Not sure how many transformers you use single or double. At least you can start the motor. congratulations!
Fan vibrations problems is another area which may triggered by mechnical or electrical or both. I disagree with some words like "chased and exaggerated" there are no such things. There are always the causes. people should provide some professional comments.
In your case, soft starter is not useful since the system the motor connected is pretty strong with 220kV substation closeby.
Good luck with your 3.5MVA motor. Keep us posted once you solve the fan problems. AT least you ruled out the soft starter.
RE: 3.5 MW motor starting problems continue...
RE: 3.5 MW motor starting problems continue...
Good to know you fixed the problems. As I said problem has its causes.
Know I can conclude that you are the expert of motor starting now after this 3.5MVA motor case.
Best Wishes to you.
RE: 3.5 MW motor starting problems continue...
We meanwhile commissioned two units, but none still works in the soft start mode. Both have some hardware problems and we use DOL start. The manufacturer's rep was here, but looks like he will have to pay another visit.
The question I would like to ask you experts is related to MV fuses this time. They are in the incoming cell of the soft starter. Bussmann type JCR-A-24R 7.2kV, double arrangement. Several have already blown. We have all kind protections before fuses which should trip much faster in case of a fault, but since the protection is not operating, then the only explanation is that fuses go because of extensive heat.
Fuses blow on motor restart, after motor has been running for some extended hours. Starting current 1800Amp and fuses tend to blow after 10-15 ec.
We just did thermal imaging and the results are: fuse temperature about 120 DEG C (~250F), while fuse cabinet air temperature is about 65-70 DEG C (~ 150-155F). The ambient temperature at the time of measurement was 32 DEG C (~90F).
The question is: do these temperatures appear to you being well outside normal operating range for similar applications? The fuse compartment dissipates like a furnace of several kW.
RE: 3.5 MW motor starting problems continue...
You indicated an approximate 30 second start earlier.
30 seconds minimum melt =~ 2200A
2200A x 0.75 = 1650A
So, the starting current should be under 1650A for the duration of the start. The fuses might be acceptable if the start time is shorter and/or they were kept cooler.
R fuses should not open on motor starting. The motor overload relay should trip before the fuses open. R fuse are for short circuit protection. 24R fuses should never operate if the current is below 2000A.
I'd have to look up the UL file to see what the maximum temperature allowed is. The fuse may be within it's ratings but 120C is still hot. The fuse is allowed to run with a continuous 450A in a 40C ambient but the ambient is getting up to 70C. The fuses need some cooling or need to be replaced with larger fuses.
RE: 3.5 MW motor starting problems continue...
You mention that "fuse cabinet air temperature is about 65-70 DEG C" and yet you claim "The fuse compartment dissipates like a furnace of several kW."
The whole system sounds screwy and the data integrity seems suspect.
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RE: 3.5 MW motor starting problems continue...
As for other protections, we have plenty. Feeder protection before the soft starter, ABB 542Plus, which is set to trip much faster, something like I>> = 100 ms at 2500 Amp, and I> = 500 Amp for 50 sec (to allow motor start). I also set I>>> to something like several ms at 3000 Amp. Then, we have Multilin 469 motor protection just one meter before fuses (and some 10 meters after 542 Plus) which also has adequate setting for both, I>> and overload set so that is coordinates with the upstream protection, 87 works fine, etc. All protection elements are set and tested and definitely operating if and when needed.
But since none of the protection elements ever operated, the fuses simply blow for now reason, just due to extensive heat.
So I was suggesting to eliminate fuses as they seem to be rather redundant and unnecessary and just maintenance nightmare. The manufacturer elected not to comment on this proposal, silent like fish, while the Client is for now too scared to agree with such modification...
As for the fuses, they are double arrangement, and according to manufacturer's curves, minimum melt time @ 2000 Amp is about 70 sec, while total clearing time @ 2000 Amp is in theory indefinitely. Since manufacturer's curves are declared for a normal ambient temperature range, I suppose that fuse characteristic is severely altered by very elevated fuse and fuse cabinet temperature.
As Edison indicated the system seems to be screwy, but not the whole system, just the soft starter part...
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Yes, the fuses seem to be inadequate for the enclosure type (NEMA 12) and size and cooling is very poor, none. However, the manufacturer blames it all on the way how we use the soft starter and said that we need to cool down fuses for one hour after each motor restart... But, sometimes we have no way of doing it as the process must work, so there is some cooling time always but may not be longer than 30 minutes.
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The fuse is rated;
450A maximum continuous at 40C
1910Arms minimum interrupting rating
This is not a full range fuse. There is a range of currents where it is not safe for the fuse to operate. If you allow the fuse to operate in this range the fuse might actually burn or rupture. The fuse should never be allowed to clear an over current or fault current that is above 450A and below 1910A. Above this, I'd believe the 450A rating would be lower when the fuse is in an ambient above 40C.
I believe the fuses should be 36R rated. However, depending on the other components in the starter you could lose your fault rating with a 36R fuse. Still, the 36R fuses is your best bet to keep the fault rating of the starter.
Without the fuses a vacuum contactor could rupture. You'd have to do a co-ordination study between the failure curve of the contactor and your protection to ensure that fuseless operation is safe.
Also, it sounds like the fuse compartment should be ventilated. I'd expect that a few 4" grills top and bottom of that compartment would be all it would require.
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RE: 3.5 MW motor starting problems continue...
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RE: 3.5 MW motor starting problems continue...
Why is this so hard? The soft-starter is shipped with class R fuses and called, say, 50kA fault rated. If you put jumper bars in place of the fuses then you have modified the origional package and it will no longer have a 50kA fault rating.
If you decide to remove the fuses, then you would need to look at the fault withstand ratings of the components in the soft-starter and "take ownership" of ensuring there won't be a catastropic explosion the first time there is a failure after the fuses (likely a motor or wiring failure).
RE: 3.5 MW motor starting problems continue...
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RE: 3.5 MW motor starting problems continue...
Contribute to this never-end story, the power fuses are there to give an extra protection for the switchgear (contactor type...). For our case, normally,the motor control center (contactor type)is provided with (51 and 51N) protection relays.
Depends on the types, so we decide the power fuses. If you dont like power fuses, you can easily provide (50,51, 51N...) in the protection scheme. This case, power fuses of course are eliminated.
Due to extremely fact acting with instantaneous over current (naturally), so power fuse still can be an option. I think.
Furthermore, you can see in low voltage motors, some of them are also equiped with fuse above starters over MCCBs... Anyway, it really depends on which type of startes used.
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Speaking of interrupting capacity, yes, it appears that the soft starter package was designed to provide its own high interrupting capacity since the manufacturer cannot normally anticipate for every single case what current interrupting device exists upstream. In our case, however, we have a brand new switchgear with a vacuum breaker feeding the soft starter (rated 40Ka 1 sec) and an adequate protection just 10 meters away from the soft starter.
The protection is settable to operate at very low time delay, such as 30ms (has 4 current elements,I>, I>>, I>>> and IDMT). So 30ms + breaker opening time, that all should not exceed 120ms - is this far inferir to the fuse clearing time at say 20kA? 20kA is the short circuit level on the swithgear bus.
HienN, thank you for participating.
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If you want a factory supported solution that works then you're more than welcome to purchase a replacement starter from us. We compete with Motortronics in the North American marketplace.
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Power Converters may develop short circuits or faults and the resultant fault currents must be cleared quickly. Fast acting fuses are normally used to protect the semiconductor devices. As the fault current increases, the fuse opens and clears the fault current in a few milliseconds, (less than 1/4 cycle). Fuse manufacturers recommend placing a fuse in series with each semiconductor device. The individual protection allows better co-ordination between a device and its fuse and protects from shoot-through-faults. In selecting a fuse, it is necessary to estimate the fault current and then to satisfy the following requirements:
1) The fuse must carry continuously the semiconductor device rated current.
2) The I^2t let-through value of the fuse before the fault current is cleared must be less than than the rated I^2t of the device to be protected.
3) The fuse must be able to withstand the voltage after arc extinction.
4)The peak arc voltage must be less than the peak rating of the semiconductor device.
Thyristors have more overcurrent capability than transistors, as a result it is more difficult to protect transistors than thyristors. I don't know what your SoftStarter is fitted with.
From previous entries it appears that you may degrade the fuses, either through using too long a starting current, or by not giving the fuses time to cool before attempting subsequent restarts. They thus become degraded but not blown. If you need to perform a series of tests or starts on an already well established circuit, remove the fuses, replace with shorts and hopefully rely on your upstream fast high-set O/C protection. You can change/replace fuses after test completion.
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Re taking the ownership of that decision (to eliminate fuses), we don't have problems with that, however, we believe that this equipment simply has some design flaws. As I said, the fuse compartment typical air temperature (when the motor is operating) is 70C (155F). This is high. Also, motor protection (Multilin 469) panel temperature is about the same – the protection panel is somehow indented (sunken) in the fuse compartment, it is very close to fuses, and its temperature is no different. We are going to consult GE about this, but it is unlikely that they will answer that Multlin 469 normal operating temperature can be 70C. It should be much less, it is a sensitive electronic device.
As for the new soft starter, I'm not sure that is practical to replace it now, but who knows, if we face other problems, we may have to research that option too. This is a 50Hz world, however, not N. America.
Oldnotbold, thank you very much for reply. Our soft starters use SCRs (thyristor). The fuse characteristic is attached (page 8, JCR type, exact model JCR-A-24R). This appears to be motor circuit protection fuse, and does not seem to be also an ultra fast SCR protection type. From its characteristic, it seems that total clearing time at 15 kA is about 30 msec. These fuse in in the incmoning cell, one per phase.
However, the soft starter is not being uses as a soft starter yet, SCRs are still in the circuit but turned off and the motor is being started through the bypass contactor only (DOL).
Instantaneous protections are set at low current and short time (3000Amp; 0.1 sec) but none ever operated. We wiull shorten the time furher but there appears to be no shorts on the motor or the soft starter circuits since the same phenomena (fuses blow on motor restart) occurred on two different units under the same circumstances. It is just that during normal operation, fuses operate at high temperature with no ventilation and cooling and occasionally blow on the motor restart after a time delay of 10-15 sec.
And while we could "buy" the explanation offered by the manufacturer that fuses need to be cooled down before each motor restart, there are two problems with that: the process sometimes cannot wait an hour or two just for that trivial reason (to cool down fuses) and the manufacturer never took an exception that such cooling would be required, and, as I mentioned above, in no way the fuse compartment and protection compartment normal operating air temperature should be 70C (155F).
According to the curves, fuses should never blow during the motor restart, 1800 Amp, 30 sec. But curves are not given for the temperature we measured (fuse temperature 120C), so we have to determine what is the total clearing time at 1800Amp if fuses are already preheated to 120C (250F).
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Yes, there does appear to be some issues.
We do sell some products in South America, China, Middle East, Africa, etc, etc. 50Hz or 60Hz makes no difference.
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Although coordination is carefully studied, application and actual condition are very different.
Old problem, it happened one a rainy day, there was "unlucky" rat found his shelter by hiding himself inside 6.6kV Switchgear supplying power to 640kW motor. He touched one leg to one of the phase. "BANG", the whole system shut down, 6.6kV power fuse, 51, 51N Protection relays, 22kV switchgear at incoming of 22/6.6kV transformer, and the recloser at 22kV incoming power pole...
HienN
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RE: 3.5 MW motor starting problems continue...
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RE: 3.5 MW motor starting problems continue...
I agree with you for this case. I did mean by saying that all large motors need fuses. But would like add up by saying a little bit more that: For big motors (Medium voltage motors) that use contactor as switching equipment.
Fused vacuum contactor is the cheapest one in comparison with any other switching equipment. Let's take an example: We provide 50,51, and 51N protection relays for the switching device (Circuit beaker type, this will get a faster tripping as a result in comparison with contactor). However, there is always the mater that should be though about: Price, and while circuit breaker type seems to be not fit with normally ON and OFF switching. That is why the switchgear manufacturer prefers fused contactor type with 51,51N protection relay for overload and ground fault current.
Otherwise, in most of brochure and manual, you will see that many manufacturer mentions fused contactors as the most convenient device (easy operation, replacing, and coordination).
Could you describe a little bit about your motor starter (protection and switching type).
I am working on a project that has motor more than 800kW. I will also discuss this issue with the manufacturer and see how they answer.
Regards,
HienN
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RE: 3.5 MW motor starting problems continue...
I still tend to believe those fuses are there being part of the standard package and with adequate protections can be omitted. The inline contactor is rated 4.8kA without fuses and 36kA with fuses. Our short circuit level will never exceed 20kA. It is more like 17kA most of the time.
Our protections are redundant, there is even 3 devices fed from different power sources, so the likelihood of a malfunction is not that high. As electricpete underlined, most large MV motor starting circuits don't seem to employ fuses.
But fuses apparently go due to extensive heat in the cabinet and we are trying to settle the issue down with the manufacturer. Whatever, the source of the heat is, it is too extensive, a design flaw, and they should deal with it.
Two major sources of the heat in the NEMA12/IP54 non ventilated fuse compartment are fuses (24R) and inline contactor (800 Amp, vacuum), operating at ~ 400Amp 6kV. Could someone comment what would be typical heat dissipation for those two items? Various sources indicate various numbers, one of them being that a 600Amp vacuum contactor at the mentioned current could be dissipating ~ 1kW. Is this realistic?
Oldnotbold... You deserve several stars. :) Well, the ABB you are talking about might still exist. Possibly somewhere in Europe. But this is ABB USA selling equipment which they actually don't manufacture, to a country where they actually weren't allowed to travel until very recently and moreover not using any help of the local ABB office, even though a sizeable one exists... So does it get any worse than this ? :) Or maybe we are both just a bit too old to understand some new trends and business practices...
I do admit that using the soft starter for this application is suboptimal (load too large), and using this particular make and model is rather waste of time since the only way they can operate seems that starting current has to be more or less the same as DOL (start in bypass mode), which is 470%. Lower current, 450% or lower results in the above discussed modulation phenomena, making starts much more harsh than DOL...
But OK, the system is strong enough, can take the DOL start abuse, while workable alternatives would be ~ 10 times more expensive (VFD) or ~ 4 times more expensive (fluidrive). So you get what you pay for. However, the problem is that these units have overheating problems so working as ordinary starters in the DOL mode is rather unreliable too, and also using them as soft starters even at that high current limit setting (470%) is not working yet. ABB tried to commission them, but both units had some hardware problems and they have to come back again.
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As for the heat - have you ductored (low resistance ohm meter) the bus and fuse clips? Have you performed infrared thermography? Have you taken apart the burnt fuses to see if all links are melted indicating short circuit, or some links are melted indicating thermal failure? Have you verified adequate bus bar cross sectional area for the rated current?
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RE: 3.5 MW motor starting problems continue...
I'm not sure what you mean by some other protection devices (other than fuse). If that means overcurrent protection devices, we have several multifunction relays in the same circuit.
I agree that at 17kA the fuse might blow faster than breaker opening time, but at the same time, if breakers are too slow, then fuses would be used for just about anything which is not the case. Why, for example, fuses would be a must in motor circuits but not for transformers of the same size?
I have no desire to argue, I'm just looking for help and solutions, but this starter came with the incoming switch + fuse cell, however , some other soft starter manufacturers seem to offers the incoming switch + fuse cell as an option:
http:
ht
Does this mean that in these cases fuses would still have to be somewhere else in the circuit, even though there is a switchgear with motor protection before the starter, and if yes, where that somewhere else would be? I worked on some 15MW motor starters which had no fuses.
We did some thermal imaging and found that fuses operate at about 120C, but we will do some more to confirm (hard to do it when the unit is running, cannot access, door cannot be opened).
No, we have not opened fuses to see what is inside yet. But since this is happening on two different units, and only on motor restart and only occasionally and on hot days and none of other protections even ever pick up, I think it is waste of time to look for short circuits. There isn't any, it is just poor design with no cooling and ventilation and fuses are hot (cannot touch them)
The busbar appears to be adequate for 400Amp, actually the unit is rated 600Amp, (contactor 800Amp) and it is a standard desing, not a prototype, supposedly they made many of these.
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For most of contactor switching, equipped with fuses may be standardized. The manufacturer offers you fused compartment as an option means this switchgear is already protected from short circuit protection.
For transformer, in most ANSI books, transformer protected by fuse with disconnector is available. The important thing, I think, depends on the cost.
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So seems fuses could be eliminated, but that is not what we are recommending. Since there is no spare any more, fuses might be replaced with bus jumpers, not all, but one by one as they go, only as some sort of emergency solution until a solution for their failures is found, i.e., some sort of panel ventilation. The supplier just sent a small fan, something like 50W, which will circulate air from one sealed cabinet (where the fuses and the inline contactor are) to another very small adjacent sealed cabinet where Multilin protection is, but we do not seem to understand how is that going to help. Will try...
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Some more words, for big motors (hundreds of kW up), "starting regime" is required by manufacturer. (Like 01 hot, 02 cold...) For example, it can only be restarted after several minutes for cooling down (cooling the motors down only). At the same time, power fuses could also have time to rest. After an unsuccessful starting, it normally takes longer than the required starting time of the motor. Heat then accumulated, inside motor and inside the power fuses.
We go on with another starting. It draws a longer time. As a result, the fuse is melted up because of heat accumulation. It is not like short-circuit problem, because if it is a short circuit case, the fuse not only melt but also shatter the tube. Of course, this depends on the type of fuse (with strike pin or not).
For safety, should better check the cable and busbar... for sure of any abnormal conditions. Recheck the starting regime of the motor (this can only be sent by its own manufacturer and seems it is impossible for you in this case).
No, it does not seem. It is actually that if you are provided with 51 relay. The power fuse of course can be eliminated. I am not sure about your protection relay (it should be better confirmed ABB directly). I just guess that, it just has time-delay overcurrent protection device and not instantaneous protection. That is why the fuses are there. (That is why we need to confirm).
For my air compressor, after starting and operation, then shutdown, we have to wait for next 5 minutes for restarting. Note: At this, my motor does not take any load. Load will be applied gradually on its shaft when it is in full operation or something like that...
Hope it helps.
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Re motor thermal capacity, our motor is a 400Amp unit, but it has an 800Amp frame, so it is a bit more massive and we can squeeze two starts without waiting that much. However, minimum waiting time is still ~ 20 min, that is how long motor/fan takes to get to the full stop once de-energized. Thermal capacity is monitored by Multilin motor protection, which is a fairly advanced unit. Typical waiting time is usually longer for other reasons.
As for the ABB protection, we know its characteristics in details. It is a general trend among newer digital protection devices than traditional definition of 50 and 51 is getting obsolete. For example, this relay (ABB REF 542PLUS) has one IDMT + three definite time current elements, called OC low, OC high and OC instantaneous. But time ranges are:
OC low, 20 ms – 300 sec
OC high, 20ms – 300 sec
OC instantaneous, 15 ms – 30 sec
So the difference between so called instantaneous element and those which are like not is 5ms (15 vs. 20 ms minimum operating time). Practically all three definite time elements are about the same, it just depends how you want to use them.
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Have you ever seen a major vacuum contactor bottle failure. That is what will occur without properly protecting it.
Why can't you understand that the sytems without fuses go like this;
Power Buss -> Vacuum Breaker -> motor
The vacuum breaker can handle opening a fault without using fuses.
If you have a circuit with a vacuum contactor then it would begin to open when a fault occurs - it's coil will be powered by a transformer connected to the main power which has just been shorted out due to the fault - and if the bottles can't handle the energy it's trying to open then the bottles will fail. This isn't rocket science.
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It could be understood. However, I think, just base on the safety aspect. The main coil as well as protection relays shall be energized from the external sources (like batteries or UPS).
So, there will be time to cut off the main contacts during the short circuit problem.
For reference, some other times, the manufacture produces the switchgear withstanding a very high current. That is to bear the short circuit case if it is available downward the outlet of the switchgear. These times, during switch on, the main contacts are closed with the current much higher than the actual short circuit current in the system.
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LioneHutz, although I write a lot, it is hard to mention all details. One which I probably haven't mentioned before is that both contactors (inline and bypass) and other soft starter controls were originally powered from an internal CPT. This, however, has been modified and it is now fed from an external UPS. So a collapsing motor voltage will not collapse the control voltage.
Also, motor protection (any current element, including motor differential) is wired or set in that way that it trips the upstream breaker, not the contactor.
Therefore, neither inline or bypass contactor should open on a fault. We recognize that a difference still exists and that instantaneous protection operating time + breaker operating time is significantly longer that the fuse blowing time at 17kA, so we will eliminate fuses only if we have to, until we find the solution for cabinet overheating.
The equipment is still under warranty and we are not doing anything contrary to normal practice to make it operating at abnormally high temperatures. The manufacturer have finally realized that mentioned operating temperatures are too high and decided that we should install a small fan which blows air from the adjacent very small protection control cabinet (sealed, IP54) to the fuse cabinet (also sealed, IP54), claiming that this will make fuses much cooler. They say: "the fan will increase skin effect of the enclosures and stop the issue." That remains to be seen. I don't see how is that possible based on only internal air circulation.
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However, some other manufacturers appear offering switch + fuse cell as an option. Since you stated that the only way how an instantaneous overcurrent motor protection can operate without fuses is to use the combination: Power Buss -> Vacuum Breaker -> motor (no MV contactors in the circuit), does this mean that that those offering medium voltage soft starters and similar equipment (such as VFDs) without fuses, assume that the upstream switchgear is already equipped with fuses, rather than breaker? If so, where in such a system I will wire my motor differential protection, which, if exists, should trip a breaker somewhere?
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Even the switchgear comes with breaker (if not equiped with relays like the ones you have). It is no longer a breakers. It is common sense.
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RE: 3.5 MW motor starting problems continue...
We have a bit more redundancy there. UPS feeds only the soft starter circuits, including motor protection, Multilin (but its tripping circuits are DC). Feeder protection, where instantaneous is located, is fed from a 220V DC battery. Feeder breaker is some 15 meters from the soft starter. Incoming breaker instantaneous can be set similarly. We hoped then two upstream breakers + two 50 elements set to minimum would be something adequate to deal with available fault currents, if we have to use it that way. We hope not.
HeinN, I' m not sure I understand your comment about motorized and mechanical breakers. You seem to be referring to a different situation. Could you reword it please. There could be many theoretical cases out there, but we have a specific case to deal with. Our switchgear is ABB UniGear (ABB says it is a toughest switchgear in the world :)), normal motorized vacuum breakers VD4, with multifunction protective devices.
I'm still puzzled with the fact that some other manufacturers appear to offer soft starters which also have contactors and where fuse are an option, unless it is trictly for a case where fuses are upstream.
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1. For the switchgear: Normally, the control power is VDC or 110VAC fed from the external source. For example, the UPS, which is energized from another transformer, let's say, even your factory use medium voltage for big motor. It also must have another transformer for lighting and general purpose. 22/0.4kV transformer and it is separated from the medium voltage one. When the short circuit occurs all over the medium voltage system, the protection devices are still alive and able to isolate the faults.
2. You mean that, when using a breaker instead of vacuum contactor, how you could wire a motor differential protection? You mean is about power circuit or control circuit?
3. Why don't you choose the fuse with a higher thermal withstanding capability?
4. The fuse was melted during sort start starting or via by-pass? Just look for the another fuses which have the higher operating current, not the one that used to protect motor. This fuse, use for only one purpose, protect the system from short circuit only.
5. For soft starter, the by-pass contactor does not need fuses (even the manufacturer offers you or not, you are customer, you will be responsible for what you choose). If the inline is a breaker, do not need fuse. Simple...
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I our case, everything is already there, installed. Fully featured ABB UniGear switchgear fed by 220VDC (battery) and and fully featured ABB soft starter with optional GE motor protection fed by 120AC (UPS). Nothing to choose, everything is there, just to optimize as much as possible given the circumstances.
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Fuses go on the restart in bypass mode. The soft starter mode is not working, but it does not really matter, the current would be about the same (maybe 5% less in the soft start mode, as lower currents cause modulation phenomena which was initial subject of this thread).
Is inline a breaker? I would say yes. The entire 6kV circuit is like this:
motor switchgear incoming breaker- > bus-> motor feeder breaker -> 15m of cable -> soft starter switch -> fuses-> inline contactor -> bypass contactor -> 15 of cable -> motor
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http://us
I know you don't have Ferraz fuses but they are still 'R' rated fuses.
The point I was trying to make is that you can't just say "I've got a breaker with a protection relay and lot's of equipment is supplied with a breaker and protection relay so this application should not require fuses". You'd better ensure that the fault energy allowed by the breaker and protection relay doesn't exceed the fault energy that the downstream equipment can handle.
I did not state the only way to not have fuses is to use Power Buss -> Vacuum Breaker -> motor. There are posts here stating that many motors don't use fuses. Those motors operate as Power Buss -> Vacuum Breaker -> motor with no vacuum contactor (or any other power component in the circuit for that matter). Arguing that motors are connected Power Buss -> Vacuum Breaker -> Motor as proof that the fuses can be eliminated in this application is an invalid arguement.
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Based on the link you left above (Ferraz), 24R fuses appear inadequate by far for our motor (FLA 385 Amp). They would seem to be good for 330 Amp FLA and full voltage starts lasting 10 sec or less. With a large inertia load and using a reduced voltage start (and we have a reduced voltage start anyway, even in bypass mode – motor voltage is about 90%), the start lasts much longer in which case 24R would need to be de-rated to 75%, making it good for only 250 Amp FLA, according to Ferraz. Even 36R would seem to be marginally adequate for our case.
But as I said, we did not select fuses, the soft starter manufacturer did and they still claim they are adequate. Or, the problem could be that a large fuse, 36R, would properly coordinate with the motor load but would be too big to provide adequate protection for the soft starter contactors and other internal elements...
Speaking of the interrupting capacity fuse vs. breaker + instantaneous, in our case it is not so easy to conclude at what kA exactly the fuse becomes clearly more advantageous since the Bussmann fuse curves are a bit crude. VD4 breaker opening time is, ABB says 40-60 ms. Minimum protection operating time is 15ms. Adding other delays, seems that the fault would be cleared by the breaker at no more than 100 ms. Fuse 24R, on the other hand, based on availabe curves appears faster than 100ms for fault currents above ~ 12kA.
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Honesly, my concern is that a bad decision to remove the fuses could end up killing some poor innocent operator or site electrician.
The soft-starter problems seem to have dragged well into the it's never going to work territory so at this point I'd say your best bet is to move the motor leads and motor protection relay to the circuit breaker and use it as a full-voltage starter.
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May still be simple if we fix the heat issue so that fuses stop blowing and we continue using the soft starter as a regular starter. However, we also have dust problems, so we would have to pressurize existing IP54 enclosures with something like Vortex cooler if we want to use soft starters long time, as sensitive components inside don't seem to like dust, so all together, as you said, not have these soft starters at all (except for the motor protection which is part of it) would be the best.
As for removing fuses, you are surely making a valid warning, nor we are looking forward to remove them, but I think that we deal with a border line case here. The SC level of 17kA on the motor terminals I stated above is the maximum one. Depending on the operational scheme of the upstream utility the range of SC level varies significantly. With a lower SC level than maximum, the fuse will be just marginally faster than breaker. I know that we have to factor in the worst case, but probability is still somewhat reduced.
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... which is a possibility, if some of the multitude of control/ monitor/ supervisory circuits attached to the soft starter are interacting with each other or with the soft starter in some unintended way.
Since you have apparently tried everything else already, may I humbly suggest carefully labeling and disconnecting _everything_ from the soft starter except that which is absolutely necessary for its basic operation, and attempting a start. If the system then works as intended, you know where to look.
Mike Halloran
Pembroke Pines, FL, USA
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But we don't worry about it that much any more. The soft starter enters into that modulation phenomena if currents are below 450%. I would think that it is designed to have fairly good measurement accuracy and stable control loops if currents are in the area which still can be called a soft start. Say up to 400% at most. Beyond that, its accuracy and stability is questionable.
However, if the load characteristic is such that you need more than 400% LRA, what sort of soft start is that anyway? It then becomes a marginal case where use of a soft starter is not worth the hassle. One should then use direct online starts and live with marginally higher currents, or if that is unacceptable, invest in something which is way much more expensive, but does provide a real soft start, such as VFDs or mechanical devices like fluidrives.
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TECO-Westinghouse I thought was based out of Round Rock, Texas versus just TECO being based out of Taiwan. I also had the impression the big custom stuff like your dealing with was Round Rock's bread and butter whereas smaller higher production runs were TECO's. See here:
ht
http://www.teco.com.tw/fa/about.htm
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RE: 3.5 MW motor starting problems continue...
RE: 3.5 MW motor starting problems continue...
Since April we were using soft starters as just regular direct online starters (DOL) in the bypass mode as the soft start mode was rather much more harsh and came with severe current modulations, where the 50Hz was a "carrier" being modulated with something like 2-5 Hz depending on the speed during acceleration.
Soft start mode caused voltage swings and other oscillations, so it was basically unusable. We tried many things to tune it up, but nothing helped, so we used DOL mode.
At the time when reduced voltage soft starters were delivered, they came with two possible soft start modes:
- Voltage ramp/current limit mode. Meaning, it initially ramps the voltage from the starting preset level using preset steps until the motor current reaches a preset limit. Then keeps the current constant until the motor accelerates or until the bypass contactor closes, if set so.
- Current ramp/current limit mode. Similar as above, except initially ramps current until it reaches preset limit.
Neither of them worked in our case without electrical modulations and oscillations or occasionally complete motor stalling. Commissopning and tune ups were done with manufacturer's acceptance.
Recently, the soft starter manufacturer finally decided that they should do something about this and came up with a new control option, which they call kW mode, whereas during both, ramp and reduced voltage run, the soft starter is attempting to control power delivered to the motor, rather than current.
In essence, all three ramp/control modes do a similar thing - they all increase/control voltage on motor terminals. The difference between the three seems to be linearity of voltage increase during the ramp and what feedback information is used in the control loop, only current (I), or both voltage and current, being power (kW).
So one would hope that selection of a particular ramp/control mode shouldn't mean it does not work at all, but rather to help with tune up and optimization. But in our case, only kW mode, which became available 8 months after the equipment was delivered, does the trick and the motor accelerates with no current modulation and voltage swings.
The manufacturer indicated that two original control modes were purely hardware controlled, while the new kW mode was a firmware built in feature.
OK, that solves one of several design deficiencies these soft starters seem to have, while remaining, namely problems with equipment overheating during normal motor operating (fuse and protection cabinet overheating), frequent MV fuse blow on restart (fuses appear to be undersized) and dust issues are still to be dealt with.
Once all is solved, these RVSS units will be a sort of what they were expected to be when specified two years ago.
But we still cannot change laws of physics; RVSS are clearly a suboptimal starting method for such high rotational inertia fans, offering, compared with DOL starts, marginally softer but much longer starts. Our present options are:
- DOL start 26 sec, current in the range of 1800 Amp, or
Some sort of semi-soft start:
Ramp kW portion with average current 1300 Amp during 10 sec
Reduced voltage portion at constant kW with current in the range of 1600 Amp for next 10 sec
Full voltage portion (bypass closed) current in the range of 1700 Amp for ~ 17 sec.
Total start time ~ 37 sec.
RE: 3.5 MW motor starting problems continue...
Great demonstration for the need of shutters. I would guess that shutters would have allowed the SS to work with any mode without instability.
They'd also allow things like a pony motor to get it all moving first.
I have a hard time seeing why kW would improve any dynamics associated with starting over the original modes. Perhaps they actually mess with the dynamics of the system with a control algorithm now. Or, the delays in the control loop provided new stability.
Thanks for the report.
Keith Cress
kcress - http://www.flaminsystems.com
RE: 3.5 MW motor starting problems continue...
I cannot explain any specifics as what makes the big difference between original hardware controlled control loop and new hybrid controlled loop, which is a combination of hardware and software control (part of firmware).
On the modulation subject, I could publish a novel with initial correspondence with the manufacturer, and where they were insisting that the modulation phenomena was caused by 110/6kV transformers, impedances, cables, motors, and the like. We then did a major 110/6kV substation upgrade, substantially reconfigured 6kV system, etc, but the modulation phenomena did not go away. Actually it got worse -:). So if we were to seriously take their explanations of the modulation problem, our net step in solving that phenomena was probably supposed to be to upgrade the entire national power generation and transmission system only to suite RVSS original control modes :); even though there is a 1000 MW power plant next door to this facility.
At the end, the modulation problem was solved just by revising RVSS controller firmware and introducing an improved more stable control loop. Normally, one would expect a stable control loop to be part of our RVSS from day one, however, the fix arrived about 8 months after the equipment was delivered.
Who knows why it all went like this. These are fairly large units, in theory rated to 7.2kV 2400 Amp for 30 sec, and since it is a USA manufacturer, but the equipment is installed in the 50 Hz world, we are unsure how many of similar applications they had experience with before, if any. They also indicated that their new control mode, kW, was not available before and was employed for the very first time in our case :(
RE: 3.5 MW motor starting problems continue...
I'm just going to be completely blunt. This is still a completely useless piece of garbage acceleration profile. Motortronics has still not fixed the issue and your soft-starters are not working as soft-starters. If you were my customer, the head engineer who designed the soft-starter controller would be on your site until the controller worked as intended.
The fact that the bypass closes before the motor reaches full speed makes the soft-starter completely useless. The soft-starter should not ramp the motor for a time and then just bypass and full-voltage it. Completely useless....
Just don't judge future soft-starter performace based on the crappy operation of that piece of junk. It deserves to be pulled out and smashed or used as target practice for giving you so many issues and such a bad impression of soft-starters.
It appears to me that a properly working soft-starter would work well on this application. You use about 470% current and 26 sec. with a full-voltage start and, from what I calculated before, a 450% current would require 43 seconds to start. This indicates that your data which I used was very conservative and, most likely, the 350% current limit start would work very well.
RE: 3.5 MW motor starting problems continue...
Strange situation and a strange business practice. The Client is basically at Motortronics mercy when it comes to solving any issues as the equipment was paid long ago, so remaining options are perhaps some legal actions, while Motortronics apparently have enough work and do not seem to care that this case might possibly be damaging to their commercial reputation.
Maybe their approach is such since this is all happening in a Balkans country, a market, both of them, ABB and Motortronics appear totally uninterested in when it comes to the RVSS equipment. However, still strange, as this industrial facility is owned by a USA company, one of the largest steel manufactures in the world, a prime customer for many big equipment suppliers, while, we, owners engineers on the project, also come from a large Canadian consulting company (8000 staff), so we are occasionally in the business of recommending someone's equipment or not... Apparently, the fact that equipment was made and serviced by Motortronics, but sold with an ABB label doesn't seem to help too.