hi-pot testing of 4500 HP electric motor

[dharry]

While hi-pot testing a 4500 HP motor, it was noted that there was considerable "popping and cracking" noted and I could visibly see arcs jumping throught the front end of the rotor to the stator. The operator stated this was "normal and, as long as the hi-pot machine didn't trip, was acceptable. Any thought???

 

[edison123]

Was this a new/rewound winding or an old stator winding ?

For new/rewound windings, audible corona (like a steady rain) is ok but visible corona is not ok.

For old stator windings, visible corona indicates failing stress-grading system in the end winding and nothing can be done about it till the next rewind.

 

[dharry]

This is a new motor, 13.2 kV TEAAC. Corona was visible on the drive end on the stator and rotor during hi-pot testing on each phase. This was the only end I could see as the bearing shild plate was not installed on this end at the time of testing.

 

[edison123]

If this is an acceptance test by the supplier, I would not accept the motor. Why the rotor was inside the motor and why only one side of the end plate was removed ?

 

[electricpete]

You didn'nt say but I assume with cracking and popping it was an ac hi-pot? (vs dc)

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[dharry]

Yes it was an AC hi-pot (27400 VAC). This test was the final package test after assembly. Only one side of the end plate was removed because we had performed a bearing and seal inspection after all the other run tests had been performed and the small end plate around the bearing housing hadn't been reinstalled at that time. The motor has a large air cooler that was removed for the bearing inspection and had not been reinstalled so the top of the stator was open. This may be one of the reasons the arcing sounded so loud. On previous testing of similar motors I ahve witnessed, all the end plates and cooler were installed and there was no visible access to the rotor/stator interface. I have heard the "rain" before but never likened it to "Frankenstien's Laboratory". I had to hold onto one of the client EEs to keep him from running off. He was spooked.