Exploding Surge Packs in Motors

[duki]

I work for a company that has two main air compressors powered by 2 36KW synchronous motors. They are soft started by a medium voltage VFD and placed on an 11KV run bus that has vacuum type contactors. Over the past several years lightning strikes has caused two of the surge packs in the motor T-boxs to explode and damage the motors(analysis from motor factory). We would like to eliminate the surge packs. A friend informed me that there was a test called a "steep front voltage" impluse test that couls be preformed on a sample stator coil. If the coil withstood this impulse then there really was no need to have the surge packs. Yes the surge packs are sized properly with ample margin. Has anyone heard of this test? I would also like some reference soources. Thanks.

 

[electricpete]

The test you are referring to would be an impulse test as described in IEEE522.

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[jraef]

If you have VFDs powering these motors, the VFDs are more likely to damage the surge caps than anything else. I believe the high harmonic current and voltage levels coming from the VFD output can set up resonances in the circuit that will overheat the capacitors. Here is an article about PFC capacitors but the principal is the same. EMC Article link


Why have surge caps on motors fed by VFDs anyway? if there is a line surge it might damage the VFD. But don't forget, the VFD output is akin to being a new power source for the motor circuit, so the only "surges" the motor terminals would see would have to come from the VFD transistors (unlikely) or from a complete catastrophic failure of the VFD, in which case the motor damage would be the least of your worries.

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[duki]

Sorry I didn't mention this, but the VFD only starts the motors. It then runs another motor on variable speed full time.

 

[jraef]

It's still possible to damage the caps in that short amount of time. The same is true of soft starters. No capacitors on the load side.

JRaef.com
Eng-Tips: Help for your job, not for your homework Read faq731-376