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Surge test?
5

Surge test?

Surge test?

(OP)
I have a 700HP 460V induction motor that I have been asked to perform a surge test on. The motor megs good with a regular hand crank megger set to 500V but I am unfamiliar with the term "surge test". Would someone describe exactly what this entails? Thanks

RE: Surge test?

A surge test is described in IEEE522.  The purpose is generally to check the turn insulation condition.  You might find info on the baker site.  Or PJ Electronics.

It is a potentially destructive test and should only be done by qualified personnel.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: Surge test?

As pete says, you need a surge comparison tester (baker, pj electronics make them) to do the test. While the megger, hipot etc. test the coil-to-ground insulation, surge test checks the turn-to-turn insulation of the coils at a higher voltage and a higher frequency.

* I would go green if only I were not yellow *

RE: Surge test?

(OP)
Thanks for the information. Here is a short description of the problem. It was a OEM Maintenance Support Manager that suggested the surge test as a comparison test. Both terms I was not familiar with.

700HP motor soft start shorted a B phase SCR (cathode to anode) for no apparent reason. Checked the motor, replaced the SCR but after 5 seconds into ramp up the exact same SCR shorted again. After extensive checking I found the insulator to be electrically bad between the failed SCR and the heat sink but I could not  find any visual. I have an identical soft start that I can install but was concerned about the potential damage it could receive if the motor caused the fault. I cannot picture a way that the motor can cause one particular SCR to fault. Wires look good and megs open to ground at 500V. I did a DOL the best I could but it tripped the main CB immediatly like it should have because it is sized too small for DOL. When I looked at the uncoupled motor shaft it was silently rotating a at fast clip indicating to me it is ready to run. With these conditions would it be risky to install a new soft start without a surge test done by qualified people. With what I have described it seems a good bet that the whole problem originated from the soft start. A little history, two years ago there a major phase to phase short in the motor peckerhead. The only thing done was repair the short damage in the peckerhead and it has run good since.

RE: Surge test?

Could also be the circuitry that controls the SCR has a problem.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Surge test?

Yes, very probably as Ketih says.

If one SCR doesn't fire, you will get a very heavy DC component (no more AC reactance, only DC resistance) and that will certainly melt down the SCR if you have standard (not thyristor) fuses - or no fuses at all. The fact that DOL tripped the CB instead of blowing fuses points to that.

A new soft-starter wouldn't have that problem. So, I think that you should change it. Also, if there is a by-pass contactor, check it so it isn't stuck closed in one or more phases.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Surge test?

(OP)
I only wish to prove that it is the soft start that caused the recent SCR fault. I can buy a new soft start but it needs to be running Monday morning so I want to make the swap. The main CB of the circuit is 1200A and the fuses in the switchgear are 1500A so I expected the CB to trip before a fuse blew when doing the short sine wave test. I am going ahead with the new soft start installation but it would cause great distress if I install it and the same exact SCR shorts again after already shorting anode to cathode twice. Just trying to cover all my bases, the dang thing already tried to kill me when someone applied control power at a remote location that was unknown to me. When this happened the main control transformer (25,000KVA) had 115VAC on the secondary and that created 480VAC on two phases of the secondary even though the circuit was locked out at two different locations. I got away with my life and this has become a quest that I must solve.

RE: Surge test?

Take care! Use rubber gloves if you expect that to happen again and don't have the time to find out the source of the back-feed.

I feel rather confident that you have a problem with the soft-start. Turn-turn shorts in motors do not take out thyristors after five seconds. But missing gating pulses on one thyristor does. Can you check the gate current on all six thyristors? Easy to do with a current clamp if you can get at it. Reading shall be same on all six thyristor gates. If one deviates much, you have a problem. A DC clamp and a scope are the best tools for that, but an ordinary current clamp can also be used. Expect 10 - 100 mA readings due to the pulse character of the gate current.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Surge test?

(OP)
Mr. Skogsgurra, I thought about gating early on but do not have a way to test it. I came up with the idea of after making the swap and repairing the original soft star to power a 5HP motor with it and check current on all three motor taps. With a good motor they must be less than 10% deviation and if I get a current imbalance then the gating is bad.

Question: Will a 700HP motor soft start run a 5HP motor correctly? It seems to me that it does not make any difference to the thyristors as to the size of the motor but I could be wrong.

RE: Surge test?

I think that it should work. At least, if it works with the 5 HP motor, you can assume that the soft-starter is OK.

Connecting incandescent lamps to the output is also an easy test. But you need to string several lamps in series so they don't get serious overvoltage. Connect strings in wye for least voltage.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Surge test?

(OP)
Dear Mr. Skogsgurra, I never wear goves, that would be like taking a shower with a rain coat on!

RE: Surge test?

(OP)
Dear Mr. Skoggs, the 5HP motor will be used as the sacrifial lamb. If it survives then we will boot up.

RE: Surge test?

Awaiting results with breath held...

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Surge test?

Making the sign of the cross!  LOL

Hey Gunnar, would you just put the clamps on the gate wires to the thyristors?

I presume they are actual flying leads on something this size?

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Surge test?

Yes. For puck thyristors, there is one auxiliary cathode connection and one gate connection. Bolt thyristors usually has the gate lead coming up close to the catode wire. They are sometimes hard to cet a clamp around (just one of them, of course). I use a Fluke 510 for that. It is a lot smaller and also faster than ordinary clamps. Not as fast as the TEK clamps, though. But needn't be that fast.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Surge test?

Something that I commonly do if I suspect the soft starter control, is to connect three lamps on the output of the soft starter. The lamps need to be rated with at least the phase - neutral voltage and greater than 100 watts. Connect the lamps in star with the star point floating.

When the soft starter is given the start command, all three lamps should light up together. Watch for the lamp that comes up early, or the lamp that comes up slow, or remains at reduced brightness. Triggering and SCR problems are usually very obvious.
Best regards,

Mark Empson
http://www.lmphotonics.com

RE: Surge test?

Ok I have to ask.

Skogs stated that if one SCR fails to fire there will be a

"very heavy DC component (no more AC reactance, only DC resistance) and that will certainly melt down the SCR"

Can you elaborate?  I would have thought that not firing one SCR would only result in a single phasing type situation, not damage to the SCR.

My experience is in LCI so if soft starters are not a similar breed of animal, my apologies

RE: Surge test?

'tis like this:

A motor that big has very little resistance in its windings. As you know, it is electrically equivalent to a (rather bad) transformer.

So, the current is roughly U/X, where X is the winding's reactance (omega*L).

Now, if there is more magnetization than the system can take, i.e. saturation, the X goes to zero and the current rises beyond normal values. A lot beyond. Also, there is a lot of 0 rad/s (DC, that is) and that is only limited by the near zero R in the circuit.

The very high current heats thyristors and fuses and starts to trip breakers. If the thyristor i^2*t is less than that of the fuse - which is alewys the case if standard fuses are used - then the thyristor either melts down or evaporates before fuse cuts out. Usually the first. Breaker usually takes longer time, so it trips a little while after.

I had a similar experience a couple of years ago and the recordings from that event are interesting. See attached file.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Surge test?

Thanks for the graphics..

Clean hookups!

I'm starting to get the feeling Ginnar never met a starter/VFD he didn't record.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Surge test?

Hey buckeye!

Any success?

Did you sacrifice the 5 HP to make the 700 HP run?

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Surge test?

If he doesn't come back soon we'll know he should've used gloves...

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Surge test?

(OP)
I have installed a good soft start from another unit and will probably do the smoke test in the next few hours. I was going to rebuild the old soft start and use the 5 HP motor to test it. I know that even at locked rotor it cannot short the SCR.

Instead of rebuilding the old soft start I thought about installing a new soft start, one rated at 500HP across the line but 700HP inside the delta (no need to go into the pros and cons about inside the delta)to save money, time and embarrassment. I have 12 350KCMIL cables between the location of the soft start and the motor peckerhead, cable length is 30'. This will give me two 350KCMIL cables per phase, is this enough?

The shop gloves are still in their bag and have never been used. My new tool is a length of 14 gauge green wire that I use to check after I check with the meter.

RE: Surge test?

buckeye3; Your cowboy attitude may get you someday.  I think you should reconsider it.  Electrocution or third-degree burns are not broncos you can get back up from.    
   
Your 14 gauge wire may become just a source for a ball of 20,000F plasma in your hands.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Surge test?

(OP)
Motor passed smoke test and will be coupled to the load next. Current on the B phase (where SCR short was occuring) ramped up to 2863A and then dropped extremely fast to 144A once up to speed.

itsmoked, I apologize for sounding cavalier with respect to safety. I am very safety conscious but when I got locked up in it I had been working on it for two weeks and it was locked out at the source and everybody knew that but did not think about the back feed when they did the remote wiring to the wall socket. My 14 gauge wire is just my added protection after I am sure the circuit is dead by turning off power and checking with my meter. I would not use it on anything over 460VAC.

RE: Surge test?

Good to hear.

Wow that's 20X.  Seems like a bit much especially with a SStarter.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Surge test?

Smoked,

A little French lesson for you: "Au corsair - corsair et demi"

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Surge test?

(OP)
I have just been thinking the same thing. I think the thyristors are rated in the area of 2800A. I did not monitor the other two phases but only the B phase during start. After thinking about it, somewhere in the area of 1500A is what it should have peaked at during start. Would you agree that this is abnormally high warranting further testing before placing the motor under load?

RE: Surge test?

Agree. Should be somewhere around 1500 A. What does the motor nameplate say about FLA?

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Surge test?

(OP)
Motor FLA is 754A. I checked the thyristor data chart and the first amps listed is "I TRMSM" and it is 2800A. The motor is also uncoupled and 2863A during start is way too high. I have already done two start and run tests but I watched B phase only during start. After start the current fell to 156A-144A-126A and those readings are good to me. I am leary about trying another but would another test monitoring either A or C phase give me any more information about the problem?

RE: Surge test?

Would a longer start ramp take the current down? I think it would. Do you need the fast start?

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Surge test?

(OP)
Ramp is OEM set to 6 seconds. I will lengthen that value to say 10 seconds and do a test. I will try this and see what happens but suspect that the current will remain large because I now think there is a motor fault.

RE: Surge test?

(OP)
The ramp is controlled by a nine position switch with position 1 meaning six seconds which is factory default. I do not have information as what the different steps mean on the switch but I set it to 3 and used a stop watch. The ramp was 7 seconds and the highest current reading was 2450A. I then set the ramp pot to 5 and current peaked at 2190A but time stayed about the same. These values still seem to high to me. Would this high current cause you enough concern to do the surge test?

After the motor stops spinning I will turn the pot to step nine and see what happens.

RE: Surge test?

Sorry. One of those pesky customers called.

If the motor runs OK, once started, I don't think you have a motor problem. The current may seem to be high, but not high enough to be worried about.

Especially not if the SS obeys commands. Which it seems to do.

There is always a more or less uncontrollable current peak when starting with an SCR SS.

I guess you are right, Smoked, there's one more recording. From yet another SS. The current peaks as the speed gets up to peak torque. Thar part is hard to control in most SS.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Surge test?

(OP)
I got called away by a whining and whimpering one horse motor.

Thanks to everyone for their inputs. The voltage of the switchgear that powers the motor is 500V and this would be adding some to the current but to be on the safe side I am calling in a professional to do the surge comparison test. If the motor is declared good then I am re-pulling the soft start and installing it back where it was and make current measurements during start. From that point I will have a good footing of what a working system looks like. I should have done this before I moved it but hindsight is 20-20 and I was sure the motor was in good shape.

I am having trouble accepting that when using the factory default settings I am pulling over 2800A when the thyristors are rated 2800A. No electrical engineer, IMHO, would design a system with the current of the system having the value of the major components current rating.

RE: Surge test?

Can you possibly roll the phases over (R to Y, Y to B and B to R) and check whether the high current goes with the supply lead or stays at the motor pecker head ?

* I would go green if only I were not yellow *

RE: Surge test?

If the motor starts smoothly and sounds good when starting then there is likely nothing wrong with it. It's very seldom a motor has an intermittent winding failure, especially when running on a grounded utility system. The fault currents during the initial failure typically cause more damage and it's permanent.

You already said you found an insulation failure in the soft-starter so why don't you think that caused the SCR problem?

One other thing that could be the problem is the snubber circuit on the soft-starter. I have seen that before. Replace the snubber and the SCR quits failing. It was an RC snubber but the R was shorted putting the capacitor directly across the SCR.

Thyristors are rugged devices. You are somewhat misreading the current rating. It's possible you could push 8000A through those SCR's for a motor start assuming good clamping, gate drive, and heatsinking. And, you could do that every motor start without degrading the devices.

One other comment - in this day and age it's a very poor softstarter if it does not protect against the imbalance cause by missing gate firing.

RE: Surge test?

(OP)
I am going comit a sinful act and install "inside the delta". All die!

RE: Surge test?

(OP)
Oh Yea? Then why is the OEM raising the current rating of the thyrisors? I am hoping to get a discount for doing the research.

RE: Surge test?

It is about Iavg and I for 60 degrees. Each thyristor conducts only one 1/6th of the period - or less. So thermally a 9000 A load on the SS may look like a 1500 - 2500 A load for each thyristor.

The uncertainty is because thyristor losses are not purely resistive and not purely constant voltage either, but a mix.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Surge test?

Sorry 1/3rd. A 4500 A SS load may look like a 1500 - 2500 A thyristor load.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Surge test?

I don't really agree with you Gunner. For a rectifier or DC drive, yes your 1/3 number applies, but not for a soft-starter. The phase current in a soft-starter is divided between 2 devices so each one only sees 1/2 of the phase current. Above that, you can still over current the devices for short periods of time without damage. The capsule or puck type thyristors are amazingly rugged devices and can take a lot of abuse.

If the manufacturer is changing device size and giving you the runaround then it's time to look for a new soft-start manufacturer. Honestly, the unit you are working on is still set-up using pots and is likely pretty poor technology and now you are indicating the manufacturer doesn't seem to know what to supply.

A good starter for that motor size would provide a nice text display and provide a full array of motor protection features as well as a number of features to protect against the starter itself failing.

By chance is this a RAM soft-starter? I'm just trying to think of who uses 9 position switches to pick the ramp time. If it is, well, I was taught if I can't say anything good then don't say anything at all.

RE: Surge test?

Yes, you are right. One conducts during positive half-wave and the other during negative. So, you shouldn't worry at all about overstressing the thyristors.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Surge test?

The real issue when it comes to SCR ratings on soft starters, is the junction temperature at the end of the start. This is a function of the SCR characteristics and the heatsink characteristics. The average current rating is only an indicator of the SCRs current carrying ability on a given heatsink under continuous load conditions.
It is not easy to determine the useful rating of a soft starter from the SCR average current rating only.

Best regards,

Mark Empson
http://www.lmphotonics.com

RE: Surge test?

(OP)
Mr. Skogsgurra, in reference to your earlier suggestion that the surge test may not be needed because the high current is only during start and not run was correct. I had the motor "surge test" tested by professionals and it passed. The OEM stated that was something they would do under similar circumstances.

One thing the professionals did that I did not want done was couple the motor to the load and start it. They recorded 5200A during start but I did not get a report about run but the motor started and ran. I locked out the motor again after finding out what was done. They did this with my good soft start and it may have sustained some damage or at the least was horribly stressed. It must be a loose connection somewhere in the switchgear or peckerhead. The taped up peckerhead connections show no sign of heating and that is what I would expect with a loose connection. Does anyone have any other sugestions for the cause of high starting current? The load turns freely by hand and I do not suspect it because I think 2800A start current is too high for a uncoupled 460V 700HP induction motor. I think start current should be in the area of 1200-1300A for the uncoupled motor.

RE: Surge test?

FLA of a 700 HP, 460 V motor would be around 850 Amps.  2800 A inrush current during start for such a motor, even uncoupled, is quite normal.


* I would go green if only I were not yellow *

RE: Surge test?

OK, the starting current depends on the motor, the load, and the operation of the soft-starter.

I believe you are dealing with a low end soft-starter and in that case it is very possible that the current can reach the levels you have recorded. A number of the low-end starters only provide limited current control capability so you get what you get.

It's also possible that you have set-up the soft-starter incorrectly which can also lead to high current levels. You posted you did not know what the settings of the ramp dial means and that it was set to the minimum. Are there other pots on this starter whose settings you have not verified? Typically, in the best case the "default" settings are not the optimal settings and in the worse case they are completely wrong.

SCR's typically don't degrade. They are usually OK unless you stress them to the point of failure. Then when they fail, the puck type SCR's typically fail shorted.

RE: Surge test?

My learning is that inrush is not related to load.  If this motor were started at 460 volts, had a KVA code of F, it would draw 4000 amps or so on start.  If the voltage were limited to half, the current would be half.  If the voltage were 500 vac,then the current would be 5% higher.  I can see 5000 Amps if the instantaneous voltage goes up.

RE: Surge test?

(OP)
Time to end this thread:

I've got the OEM engineer answering all of my questions with, "NO!" and angered some in here but I got the nerve to start and run it under load and was successful. I was wary because I replaced the the shorted SCR in the original soft start and its unshorted twin but the same exact same SCR shorted again. I then discovered the cause of the shorts is a compromised thin sheet of plastic that acts as the insulator between between the SCRs and heat sink. I was concerned that an external component (motor) caused the insulator to fail.

As a fast remedy I installed a known good exact same soft start and found the high currents during start with the motor uncoupled and (in my opinion) extremely high currents when coupled. I was making comparrisons with another manufacturer with the same ramp times but the currents are wildly different during start.

Thanks again to everyone for their valuable inputs and giving confidence to someone who is still learning about big motors.  

RE: Surge test?

Oftenlost has it right. The current seen during starting is basically only controlled by the voltage applied.

The currents you have measured during starting the motor with the load indicate that the starter is basically going to full-voltage during the acceleration. So, have you verified all the settings of the starter to get the best start? To me, having the ramp time set to the minimum is not the optimal setting for a large motor that has a fair bit of inertia.

Is this a RAM starter? If so, verify the bypass contactor is not energizing before the motor has reached full-speed. That can be the cause of your high current.

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