How to test short in alternator rotor winding inside the slots?

[SA07]

Hi
What test can be done to test for short in an alternator rotor winding inside the slots? Alternator 45 MW 11kV 1500 rpm Thanks

 

[panter]

To begin with, you would have to measure resistance of the winding and compare it with the values from generator test sheet. As it is relatively small, you could determine the voltage drop with DC current of for example 100 A if it is less than the nominal values .

 

[LionelHutz]

You can apply AC voltage to the rotor and then measure the voltage on each coil/pole and compare. I don't know how much difference indicates a bad pole, but I know this is a test that sites with a lot of synchronous motors use to determine when a motor has rotor issues.

 

[Gr8blu]

For industrial equipment, the allowable variation is dependent on the type of test.

DC Drop Test (Apply DC to circuit, measure DC voltage across each pole). Allowable variation is 5% from average (most shops will raise concern at 3%).
AC Drop Test (Apply AC to circuit, measure AC voltage across each pole). Allowable variation is 10% from average (most shops will raise concern at 7-8%).

For a "wound in slot" design, the typical tests are:
1) insulation resistance (meggar) which looks for primarily surface ground paths or moisture absorption - performed at relatively low voltage
2) surge (looks for turn-turn fault) - performed at elevated voltage
3) dielectric (hipot) which looks for ground fault via coil insulation breakdown (pinhole) - dielectric test may be AC or DC, and is at significantly elevated voltage

Both 2) and 3) are considered "destructive" tests by some users, because of the el3evated voltage levels introducing added voltage stress to the insulation.

An 11KV alternator probably has a rotor winding rated somewhere between 120 and 500 Vdc. Test 1) would be performed at 500 Vdc. Test 2) at 1500 Vac. Test 3) at 2500 Vac OR 3400 Vdc.