Surge test? 5

[buckeye3]

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

 

[electricpete]

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.

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[edison123]

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 *

 

[buckeye3]

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.

 

[itsmoked]

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

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Skogsgurra]

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

http://www.gke.org

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[buckeye3]

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.

 

[Skogsgurra]

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

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[buckeye3]

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.

 

[Skogsgurra]

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

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[buckeye3]

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

 

[buckeye3]

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

 

[Skogsgurra]

Awaiting results with breath held...

Gunnar Englund

http://www.gke.org

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[itsmoked]

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

 

[Skogsgurra]

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

http://www.gke.org

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[Marke]

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

 

[GTstartup]

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

 

[Skogsgurra]

'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

http://www.gke.org

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[itsmoked]

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

 

[Skogsgurra]

Hey buckeye!

Any success?

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

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...