×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Medium voltage soft starter - commissioning

Medium voltage soft starter - commissioning

Medium voltage soft starter - commissioning

(OP)
The SSM series 6kV ABB soft starter is used to drive a 3.5MW asynchronous motor, which drives a large fan. The vendor seems to be unable to provide commissioning assistance in a timely manner, so the Client may have to commission this using its own resources.

The primary soft starter configuration starts with a disconnect switch, fuses, contactor and  thyristor modules.  There is also a bypass contactor.

On the secondary side, the equipment uses an ABB controller and an optional Multilin 469 motor protection. Optional – since the soft starter electronics seems to have a number of protective functions already built in.  

We have an idea how to set the protections, but the controller also needs all kind of settings to be applied. The process is relatively steady and the maximum number of starts is probably less than three per day.

Could anyone provide some general guidance as how to set things like start mode (there is several to choose from – dual ramp, custom curve, jog voltage, etc), starting torque, acceleration (ramp time), current limits (min and max), initial voltage, etc.  Thank you.

RE: Medium voltage soft starter - commissioning

There are a number of reasons to use a Reduced Voltage Solid State (RVSS) starter. The reason you chose an RVSS for the application will have a direct impact on the selection of starting parameters.  For example if you are using an RVSS to reduce line voltage drop during starting, you probably want limit the maximum starting current.  

For a fan application, you will not need the dual ramp (used for different starting parameters for different load conditions - chipper, crusher, etc.) Most RVSS fan applications I have started use a current ramp with three parameter settings, initial current, maximum current and ramp time.

Several manufacturers having RVSS simulation software to predict the performance of a set of starting parameters for a given application.  However several required system, motor and load characteristics required for these programs is not easily obtainable.

My recommendation would be if you can not get an ABB serice tech to commission the unit, find a qualified service engineer or commissioning agent to test and program the RVSS.  Typically the start-up should include checking SCR gate resistance, mechanical inspection, and programming the parameters. Also decide which motor protection (RVSS ofr Multilin 469) you want to provide the primary protection.  Most RVSS onboard motor protection is roughly equivalent to the Multilin 269.Since you have invested in the 469, I would use that as the primary protection

  

RE: Medium voltage soft starter - commissioning

You may be able to get some more guidance from the manufacturer of the driven load (fan).

If not, I agree with Handlin about getting a consultant or qualified power system testing company to help. You need assistance with providing (and implementing) the settings, and testing the system once it's set up.  

RE: Medium voltage soft starter - commissioning

The ABB MV soft starters are actually built by Motortronics in Clearwater Florida. ABB is, by their brand label contract, supposed to provide their own people to do commissioning. But last I knew of ABB had very few people qualified to do it, so that may be what is behind the delay. You could contact Motortronics directly, but they are going to want to be paid since their discounted brand-label price to ABB does not include field support services.

And if you are willing to pay for someone qualified so that you can get going, you can also contact Siemens Industrial Services. Siemens has brand labeled the Motortronics soft starter as well (although they recently stopped doing so), and they have a lot of trained technicians in the field.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies  

RE: Medium voltage soft starter - commissioning

(OP)
Thank you very much Handlin, DanDel and jraef for very useful comments.

We will use Multilin as a main motor protection and use the built-in one as some soft of backup.  There are actually plenty of protections... In addition to those two, there are also MV fusses in the soft starter, while the upstream protection, which is on a switchgear feeding the soft starter, is just 10 meters away and is a feeder relay REF5542plus, but it could even work as 87.

So we are unsure how to coordinate all this, what protective functions should be used at each particular device, and which functions should actually trip what – 87 and overcurrent protections will obviously be set to trip the switchgear breaker, while some other protective functions are set to trip the soft starter contactor only.

We knew that ABB makes no its own soft starter and the sell relabeled Motortrponics equipment, but we were not aware that they could not find anyone to assist with the commissioning... The equipment was supplied by ABB USA and after looking all around the Globe (and possibly wider...) they finally indentified someone who knows something about their soft starters in Egypt, however this project is in Europe, so there are now visa problems involved (plus the rate is the highest commissioning rate I have ever heard of...)

Thanks for the hint jraef, will try Siemens also. The additional difficulty here is that the likelihood of finding someone who is familiar enough (and equipped enough) to commission both, the mysterious SSM electronics and the optional Multilin 469 (which is stand alone for the most part)is probably next to none, so two testing agencies may have to be involved...       
 

RE: Medium voltage soft starter - commissioning

In general, when I commissioned them I would recommend to people that if they have a Multilin and their technicians are familiar with programming the Multilin, then allow that to be the only protective device. Troubleshooting after an incident can be extremely difficult when you have multiple protective devices that have different error tolerances. Besides, although the printed specs on the Motortronics / ABB RVSS micro shows a lot of functions that appear to duplicate what is in the Multilin, it is not as sophisticated as a 469 relay (I would know, I wrote the specs for the literature they both use). Just let the RVSS micro handle the ramp control and sequencing of the contactors.

Just be sure to make allowances in the Multilin programming for the fact that the RVSS will extend the acceleration. And if you use Deceleration, the Multilin can get "confused" because the current profile is a mirror image of the Accel profile. You usually have to watch it a fews times and make adjustments.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies  

RE: Medium voltage soft starter - commissioning

Hi.
Im not familiar with this softstarter, but if it is include protection functions..from my point of view you don't need additional protective relay.
In additional, you have line protection with REF542plus.
Best Regards.
Slava

RE: Medium voltage soft starter - commissioning

I agree with jraef.  The Multilin has better fault reporting.  Set the protective elements on the soft start as backup to the Multilin.  Also, the Multilin is a better device to communicate with for your SCADA.

old field guy

RE: Medium voltage soft starter - commissioning

(OP)
Here is an update on this, hoping that some of you may have put more light on the problems we are experiencing.

We have gone through various stages of the commissioning the soft starter, had an ABB specialist, also had some other consultants involved but still have problems. The soft starter does not behave as we expected.

The motor:  TECO-Westinghouse  3.5MW, In = 385Amp, 6kV 50Hz, 1485 rmp,  locked rotor current 2300Amp, driving a big fan with a large rotational inertia. The fan has no load at the present (no air, dampers closed). To drive the fan with no load (air),the motor need to develop some 500kW.

The soft starter was set to starting voltage to 50% or around. Ramp time varied anywhere between 10 and 30 seconds (various trials). The current limited to 330% In (limit by some upstream protection).

The motor starts moving smoothly, but it never reaches the full speed before one of several protections trips on an overcurrent or overload (1500 Amp and up).  The speed may be reaching about 30% and then it just stays there(no more acceleration)until the protection trips. During that time (running at flat low speed) the motor runs rough, strange sound, starts even to de-accelerate slightly and something finally trips.

We installed a test PT to monitor the motor voltage during the start, and it appeared that the motor (soft starter output) voltage never recovers to 6kV from some 4kV, which it reaches several seconds after the start.   This is the voltage after SCR's. The voltage before SCRs is OK, dips from initial 6.3kV to about 5.8kV.

To develop enough torque and reach the full speed, looks like the soft starter would need to deliver about 5 x motor nominal current for a few seconds, which would not be a particularly soft start.

Our expectations were that the soft starter would accelerate the motor (and the fan) at about 3 In and a prolonged time, which may be in the range of 40 sec. And then once at the full speed and the normal operating current, a bypass contactor would close and bring the motor directly online.  If the fan is not moving any air, the current would be in the range of 200 Amp or less.

Since bringing the motor to the full speed and then closing the bypass contactor proved to be impossible so far, a decision was made to try to close the bypass contactor much earlier, when the motor is still far away from the nominal speed and when the current is still 3 x In or higher. This may not be a particularly healthy condition for the contactor since it would closing on a current which is much higher than its nominal rating.    

Any comments would be greately appreciated. Thank you.

RE: Medium voltage soft starter - commissioning

Compare your actual time-voltage-current with the manufacturer's provided curve.  Then compare them with the parameters for soft start by your facility.  This may include limitations to the facility by the supplying utility company.

You may find that with your rotational load, you cannot do a successful start within the parameters allowed.  More likely, though, you can tighten up your overload and acceleration time protection to squeeze a start in.

I had the same problem a few years back at a facility here.  The client had NO parameters from the utility other than it had to be a 'soft start'.  We ended up with a very short, fast ramp-up to full voltage, keeping an eye on the voltage on the supplying bus to make sure we didn't drop out other loads during startup.

old field guy

RE: Medium voltage soft starter - commissioning

Quote:

The motor starts moving smoothly, but it never reaches the full speed before one of several protections trips on an overcurrent or overload (1500 Amp and up).  The speed may be reaching about 30% and then it just stays there(no more acceleration)until the protection trips. During that time (running at flat low speed) the motor runs rough, strange sound, starts even to de-accelerate slightly and something finally trips.

Indicative of insufficient torque and not building up of the volage. Torques is the function of voltage. Try higher voltage and/or faster ramping of voltage from starting voltage to full voltage.




 

RE: Medium voltage soft starter - commissioning

(OP)
oldfiled guy and rbulsara thank you very much for replying. Looks like we have several problems here.

Seems that we exhausted most of the combinations of starting voltages, currents, times, etc. with no success. Yes, we are limited with the configuration of an upstream substation and that seems to be a good part of the problem.

I finally talked to someone from Motortronics who knows something about this soft starter and he indicated that with the fan we have (The Impeller weight is 15,000 lbs, and the Impeller moment of inertia (WR^2) is 66,000 lb-ft^2.)  a likely minimum starting current may need to be about 450% of the nominal motor current.  

Well, this is not far off from the LRC which is 600%, so use of a soft starter does not seem to provide a big advantage. Except, there is still one. With the DOL start, 600% In is instant while with the soft starter, it will start with a much lower current (voltage) but needs to develop some 450% In to overcome to torque hump and finally bring the motor to the speed.

So if a high starting current cannot be avoided, then we seem to have more problems. The initial system design was based on the assumption that the motor + fan can be started at about 300-350% In with a prolonged start time.   Now, to get 450% In from the upstream switchgear and the transformer seem to be problematic, both have limitations. The transformer is only 20MVA and is a high impedance one, so getting 4.5 In while there are other loads on the same transformer will probably cause a large voltage drop. The switchgear is also problematic since both, the feeder breaker and protections seem to be inadequate for 450% motor current.  

Any ideas how we can make our life less miserable?
 

RE: Medium voltage soft starter - commissioning

I suspected weak source as well. If that turns out to be your real problem...well it is a problem. I am not sure how many starting parameter combinations have been tried so far.

Even if the starting current goes up but drops off quickly, they system may like it better.

The fact that it does start at 50%V, I think ramping up voltage little faster would help. There is a sweet point somewhere between how fast you ramp up the voltage and how long you want to keep accelerating (trying to limit the current). One thing you cannot avoid is providing adequate torque to acceplerate, which is keeping up the voltage.

 

RE: Medium voltage soft starter - commissioning

(OP)
We tried various acceleration times, from say 25s to 40s. We also tried various voltage rump up periods from 10s to 20s. All with limiting the current in the range of 300-330%.

We then made the upstream 110/6kV transformer fully available to feed only this load, but that does not seem to help either.  Maybe we still have not hit that magic combination of numbers to bring us over the hump, although we have obvious problem not being able to pull more than 330% of the FLA due to upstream breaker and protection limitations. With this current limit, and the voltage rump time of 10 sec, the line voltage (before SCRs) during the start drops for about 10%.  

With the starting current of 400% FLA, which we have no luxury of using yet since it trips the upstream breaker as the associated protection cannot be set that high (but we tried it anyway), the line voltage (before SCRs) drop is substantial. It drops from some 6.4kV to about 5.6kV. This is too much of a drop since the transformer is feeding only this load. The transformer rating is about 13MVA ONAN, 20 MVA ONAF, impedance 11%.

 One time we closed the bypass after some 25s,  but then the voltage drop was even more harsh, from 6.4kV to 5kV, but the condition was cleared quickly the overcurrent  protection which was set at about 2000Amp.

We are unsure what to do next. The motor accelerates smoothly up to about 250 rmp, which is some 15% of the speed and then the speed stalls and even slightly de-accelerates before something trips or we shut down it intentionally after about 30 sec realizing that it will not accelerate any further.

Various parties involved in this suspect various causes of problem. Some believe that the week source may be a the most contributing problem, some speculate that there could be some mechanical problems making the high turning resistance at higher speeds, while some still believe that the soft starter is defective since, when the speed stalls, the motor behaves like it lost one phase (vibrations, abnormal noise and slight de-acceleration).

Tomorrow we will be connecting 3 PTs on the motor side to monitor the starting voltage and the wave forms, hoping that we can make some things more conclusive and eliminate some of the potential causes.
 

RE: Medium voltage soft starter - commissioning

There is something wrong with the picture here.

Are you saying a 13 MW transformer cannot start a 3.5MW motor? Even a transformer close to motor kVA rating should be able to start a motor with acceptable voltage dip.

4XFLA of the motor is still close to 1XFLA of the transformer!. Why would anything trip at or near full load rating? What is the upstream protection?

5.6 kV from 6.4 is still only 12.5% VD, why is that an issue? Perhaps you are sitting there too long. Key is to recover from the dip quickly and only a higher voltage would do that. You are just not allowing motor to accelerate that will help it drop the current.




 

RE: Medium voltage soft starter - commissioning

(OP)
Yes, I'm saying that a 13MVA transformer has problems starting a 3.5MW motor. I do not know why yet, but as I mentioned above, at 400% FLA starting current, the voltage dip was big, at least 15%. At 300% FLA, the voltage drop is about 10%.

A contributing factor is surely an unusually high transformer impendence, but there is likely more to this. Either the soft starter problems, or motor problems, or fan/bearing problems. Or maybe a mismatch between the fan characteristics and/or motor and/or soft starter in some ways.

Why anything would trip... Heh... Because the soft starter is fed from a new switchgear where the protections can be set as high as necessary and we do not have problem there.

 However, that new switchgear is fed from an old switchgear where that old feeder breaker is only 800 Amp, and an old protection cannot be set any higher than 1440Amp and 5sec, that is the max, cannot be changed.

During the design of the system, it was assumed that the soft starter will limit the current to about 300-350%. That is now quite questionable, and is we don't find some magic soft starter settings, this all may lead to necessity to change that 800Amp breaker and the associated CTs and protection before we can apply 400% or even 500% FLA as the soft starter manufacturer is now recommending. 500% is not much different than LRC which is 600%...

And even if we change the upstream protection and breaker, that is not going to solve the problem of the voltage drop, for which we still don't have a sound explanation...
 

RE: Medium voltage soft starter - commissioning

(OP)
And yes, forgot to add, that is true that we are sitting there too long.

But we don't seem to be able to avoid it. The voltage dip I was referring to is before SCRs, not on the motor.  The motor voltage appears to be about 1kV lover than the dipped line voltage. So when we adjust the voltage ramp time to 10 sec, and start the motor, here is what happens:

1. The current quickly (3-4 sec) develops to the set limit, in our case about 1300Amp, and stays there.

2. The line voltage (before SCRs) for about 10-12% and never recovers before something trips off or we turn the soft starter off after 30 seconds.

3. The  motor voltage (after SCRs) builds to about 4.2kV and never moves up.

4. The speed reaches 15% and stays there.

So there seems to be a consensus that we need much more current in order to build the motor voltage to a much higher value, close to the line voltage, to go over the torque hump.

However, we cannot get more current for now, until we deal with the upstream limitations...
 

RE: Medium voltage soft starter - commissioning

I understand your frustration.. I hope some other people would chime in here if I am beating the dead horse or the wrong horse..

Quote:


So there seems to be a consensus that we need much more current in order to build the motor voltage to a much higher value,
Above statement has fundamental issues..current does not build the voltage! To raise the speed from 15% you need higher 'Voltage' which may increase some current but will also drop off..

If you can sit for 30s with 1300A, you sure should be able to take say 1600A for 5 sec. What does the TCC of the over current protection looks like? At 1300A for 30s, motor will trip out on thermal protection even.

You may not keep it that way, but you still need to find out if this motor-load combination even works at higher or full voltage with shorter ramp time. Then worry about voltage drops.

Who knows the motor is mismatched for the load, but that seem less likely with the info you are providing.



 

RE: Medium voltage soft starter - commissioning

(OP)
I knew that the sentence "so there seems to be a consensus that we need much more current in order to build the motor voltage to a much higher value" may be confusing or maybe conflicting some basic principles, but when I asked the soft starter people why the motor voltage never seems to get higher than some 4.5kV at 300% FLA, the answer was that the SCRs are basically limiting the current by controlling (chopping) the voltage, so unless we let more current through, the motor won't see much higher voltage and will not develop sufficient torque.

The newest input from them, and looks like I'm talking to someone who knows his stuff is: let 400-500% FLA and rump the voltage very quickly within 2-3 sec.  

RE: Medium voltage soft starter - commissioning

That sounds much more promising..to try anyway.

RE: Medium voltage soft starter - commissioning

Hello, Ters!!
I have attached picture of   oscilloscope recording one of many unsuccessful start   and it looks very strange. It looks like that tiristor starts in the middle of the cycle and I think that we have problem with bad synchronization   between control voltage and 6kV network.
Greetings from Elektrika USSSerbia.
 

RE: Medium voltage soft starter - commissioning

OK, how do you do the quote thingy?

RE: Medium voltage soft starter - commissioning

zogzog:

Look up "Progress TGML" below the reply window.

RE: Medium voltage soft starter - commissioning

Hello panter

Those waveforms look perfect to me. That is exactly what I would expect a soft start waveform to look like.

ters

Your description is certainly one of insuffient torque, but that does not preclude another problem as well.

When you are attempting a start, are all three current balanced and stable. There should not be a big difference between the currents and they should not be flickering.

To start a motor and load, you must develop sufficient torque to overcome the work output and to provide sufficient surplus torque to accelerate the load inertia to full speed in a reasonable time.

The minimum torque requirement is a function of the load characteristics and the current to develop that torque is a function of the motor characteristics. For more information have a look at my web site at http://www.LMPhotonics.com/m_control.htm

I would expect that for a motor of this size, you are going to need a start current of 400 - 450% to start a fan.
I expect that if you do the calculations, you will find that the torque available from this motor at 300% is very low and will not start many types of loads.

I would try for a 400% current limit and a 3 second ramp time. I would expect that at this setting, the acceleration time could be in the order of 30 seconds, but that is very dependent on the motor characteristics.
You may need to increase the current limit if the motor does not reach full speed.

Best regards,

Mark Empson
L M Photonics Ltd

RE: Medium voltage soft starter - commissioning

Quote:

Look up "Progress TGML" below the reply window.

Like this? Sorry not trying to hijack or anything, you can all ignore me.

RE: Medium voltage soft starter - commissioning

Yup! You can see by yourself! Now you can also use the "preview post" feature to see what you will get, rather than this method.

RE: Medium voltage soft starter - commissioning

(OP)
Mark, thank you very much for a detailed reply. Mr. Panter is on the same project and equally desperate to find a solution, except he is the Owner and I'm a consultant involved in this since recent times.  

It seems to me that the issue is exactly what you and some other folks above said, insufficient torque, due to insufficient voltage.

The motor is fed from a high impedance 110/6kV transformer, which is fairly big, 13/20MVA ONAN/ONAF but the impendence may go as high as 20%, depending on the tap changer position, and the best we can hope for is 11%.

In addition to the TR high impedance, there is also a short circuit current limiting reactor on the 6kV side + some 300m cable to the motor (which is oversized).

So today we started the motor at 470% FLA, and it accelerated much better (reached 1000 rpm), but the soft starter start time elapsed after 40s and it tripped off.

During the start, the voltage dipped initially for about 15%, then after a few sec it recovered back for some 5%, dipped again where it was before, and so on, recovering after each swing slightly higher.

It seemed that the voltage dips/recoveries were initially following SCRs' voltage ramp steps, although swings continued after the ramp time elapsed. After each voltage upward swing (partial recovery) the motor would seem to catch some more speed, etc.      

It also seemed that during the dips, the transformer voltage was starting to collapse, perhaps not only delivering insufficient and unstable energy to the motor, what made the motor to slow accelerating, but possibly having somewhat distorted voltage forms as well, what contributed to increased motor growling sound.

We used longer ramp time (20s) as the voltage seems to collapse less severely with a prolonged ramp.

We then repeated start with a shorter ramp time (10 sec), but even though the current was theoretically limited to 470%, what is about 1800Amp, we tripped off the upstream protection, which Mr. Panter set to 2000Amp. Not sure why it tripped, unless what the SSM measures as 1800Amp is not exactly what the relay sees.

I also have to note that Mr. Panter is not particularly generous when it comes to setting upstream protections higher :).  He agreed to change the current setting higher, but he then shortened the time, so I2T is still about the same... :)

So, if the issue is the system impendence, we are unsure what the solution is going to be, but we are still hoping that some of you, experts, may post that magic resolution here!
 

RE: Medium voltage soft starter - commissioning

Hello ters

Ok, the time has come.
I think that to progress, there should be some engineering done. I prefer to do this before the equipment is purchased.
It is possible that the motor is just not suitable for the application. I have found plenty of those!!
Try to get the speed / torque curve of the fan with the dampers closed, and the speed torque curve of the motor, and the speed current curve of the motor.
Determine what the full speed of the fan is and what the full speed of the motor is. The last thing we need is the inertia of the fan and motor.

With this data, it is possible to calculate what the starting current and time should be.

Best regards,
Mark.

Mark Empson
L M Photonics Ltd

RE: Medium voltage soft starter - commissioning

Better Marke than me!! Beat me to the punch.

Also a motor transient analysis using a software can be done, provided necessary data is available.

One thing worth doing, if there is a means to do it is to try a full voltage (DOL)start (or close to it) and just see what happens. If still motor does not come to full speed in reasonable time, it is time to do some re-engineering.

The last data still does not say what was the maximum voltage reached at the motor terminal at 1000rpm and what is the rated rpm?


 

RE: Medium voltage soft starter - commissioning

(OP)
Thank you very much gents. Mark, I will post all curves we have, hoping that you can provide some comments. We did some calculations during last few days, and it shows that the acceleration time at 500%FLA should be 36s. We did not achieve that, possibly because the calculation didn't factor in limitations related to system impendences and voltage instability (swings) during the start, or we possibly made some other errors.

Rbulsara, at the moment we cannot do a DOL start. It would require about 2300Amp and for now we are not is position to try it yet due to upstream limitations, but eventually we will probably try that too.  

I posted nominal RPMs as 1485 somewhere far above. I'm not sure what was the voltage at the motor terminals when it reached 1000rmp since at that moment I was monitoring what was going on with voltage before SCRs, but it if I remember correctly, it was about 5.2kV, according to others.
 

RE: Medium voltage soft starter - commissioning

(OP)
Here is some more info on the motor and fan. Hope that Mark or anyone esle could provide some comments.

The fan impeller weight is 15,000 lbs, and the Impeller moment of inertia (WR^2) is 66,000 lb-ft^2. The motor rotor weight is about 7000kg and the diameter is about 0.9m.  

RE: Medium voltage soft starter - commissioning

I have worked off the curves and it would appear that provided that everything matches the curves, then the minimum start current is in the order of 280%, but at 300% current, the start time will be 180 seconds.
At 400% start current, the start time will be 60 seconds, at 450% start current, the start time will be 44 seconds and at 500% start current the start time will be 34 seconds.

So from the information, it would appear that provided the equipment matches the data, it is possible to start this fan with the dampers closed at 500% current in around 34 seconds.
Note : This is right on the specified limit for that motor.

From your descriptions, it sounds as though the torque required is higher than is shown on the curve supplied and if this is the case, you will probably need to be around 450% to start the load and the time could be several times longer than the calculations.

Best regards,

Mark Empson
L M Photonics Ltd

RE: Medium voltage soft starter - commissioning

Mark,

Thank you very very much.  Looks like results of your calculations are about the same as ours, we also came up with similar numbers, such as close to 3 minutes for 300%FLA and 36sec for 500%.

However, tests show that we are not getting there, i.e, today we repeated three tests @ 470%FLA with different ramp times, all resulted with the same outcome, it took about 50 sec to accelerate to 1200rmp, which is close to the "hump" and then the soft starter times out.

This (50 sec) is probably a severe abuse for the motor (rotor), since we reach 100% thermal capacity (according to Multilin settings) after one start.  

One of the reasons why there is a discrepancy is between calculations and the reality may be that that calculations don't factor in for random voltage swings and dips. Every 3-4 seconds voltage dips and recovers partly and so on, what I believe reduces the torque and extends the acceleration time. The calculations also may not factor in for the distorted wave form.

Also, people running the test insist on a long ramp time (they started with 20 sec but shrinking it down to 10), in order to avoid the initial larger voltage drop (if ramp is only 2-3 sec) which creates subsequent swings with larger amplitude and may trip on O/C, but the consequence of this is that for about 15-20 sec almost nothing happens, the motor starts moving but hardly reaches 100rmp before 20sec.

They also insist on current ramp rather than voltage although the manufacturer did not recommend it nor there seems to be a big difference in essence, you control the current controlling the voltage or another way around.

Finally, we cannot be sure about accuracy of the supplied curves, especially for the fan, nor the dampers can be considered fully closed, there is some leak.

Speaking of the motor, I'm not an expert on motors, but it seems to me that this unit is of very modest starting efficiency – relatively low torque @ high current. Do you concur with this?.  

I'm not sure how it affects the starting torque (does it make it worse?), but the motor is of an oversized frame, it is 800Amp frame, while the actual current rated is below 400Amp.  

Just to make the first successful start, we may try to jog the motor before the start or kick the bypass in after about 30-40 sec, but for normal operations it doesn't seem we will get out of the puzzle without supporting the voltage in some way, which would be possible paralleling two transformers during the start (provisions exist)or supporting the voltage from a local plant generator, which would also be possible with some system reconfiguration. Plant people for now are not agreeing with any of this, but eventually there may not be much other options left.
 

RE: Medium voltage soft starter - commissioning

(OP)
I wrote the above, but since Panter was logged on this machine once, since then, for some reason, his name keeps remaining logged in by default any time I visit this page (even though he was logged out), unless I log him out again and log myself in, what I forgot to do this time...

RE: Medium voltage soft starter - commissioning

Hello ters

I would recommend that for this application, you use a fast ramp, not a slow ramp. Usually around 2 seconds is fine for allowing monitoring equipment to track the voltage dip. If you use a slower ramp, you are prolonging the start time and dissipating more power in the rotor.

The sagging voltage should not make any difference to the performance during start as the voltage applied to the motor is being regulated by the soft starter. A drop in the supply voltage just means that the soft starter causes the SCRs to conduct for longer.

A start current of 470% is going to lengthen the start time. If the start time at 450% is 44 seconds and the start time at 500% is 34 seconds, then the start time at 470% will be between the two, say around 40 seconds. If you have a ten second ramp, then it should take around 50 seconds provided all things are as quoted.
Looks to me like things are very marginal all round!!

I would suggest that you have a good look at the mechanicals and see if there is any way that you can further reduce the start torque. Belt tensions, motor alignment are all factors that can have an influence.

The start characteristics of the motor are determined by the rotor design. It is possible to change the rotor to get very different results, but in a motor of this size, that is not easy.

Quote:

kick the bypass in after about 30-40 sec
Make sure that the bypass contactor is rated to DOL start the motor. Manufactures often use AC1 rated bypass contactors and these can not carry the DOL current.If you bring the bypass contactor in early, the voltage will go to full voltage and the current will go to full voltage current. This will increase the voltage drop and at that point you may as well just start the motor DOL unless you bring the bypass contactor in around 90% speed.

Quote:

this unit is of very modest starting efficiency – relatively low torque @ high current.
This is not uncommon on large motors. I would not suggest that this motor is bad, probably more average.
Best regards,

Mark Empson
L M Photonics Ltd

RE: Medium voltage soft starter - commissioning

(OP)
Mark,

Thank you again for clearing some of my confusion. We will keep trying. However, as I said, I'm not particularly optimistic. At the present, we have a 13/20MVA transformer reserved just to do this testing. In normal operations, however, that transformer will have also other things to do (feed other loads) and given its high impedance, things don't look particularly promising - voltage sags will be higher on a transformer already carrying some load. But we have a new larger transformer coming, which should make a difference, but that one will not be commissioned that soon, will take several months.
 

RE: Medium voltage soft starter - commissioning

Quote:

voltage sags will be higher on a transformer already carrying some load
Don't think so, the change in voltage during start is a function of the transformer impedance and the change in current. If you have a start current of 2000 amps and it drops the voltage by 7% during start with the transformer unloaded, it will drop by the same amount when the transformer is partially loaded. The end point voltage will be lower due to the additional load, but so will the start point voltage.
Best regards,

Mark Empson
L M Photonics Ltd

RE: Medium voltage soft starter - commissioning

(OP)
My thinking was about voltage collapse in case of the overload. Say if the transformer is already working at 30% capacity and then we start the motor and pull additional 100% of its capacity. Since this is an overload condition, I imagine that the voltage collapse may not be linear any more? Even if it is still linear, I'm not sure that other loads on the same bus will be able to easily cope with substantial voltage swings which would last up to one minute.   

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources