Surge Comparison Test

[petronila]

Dear All

I would like to know your inputs about the following:

Some workshop received a two speed, two preformed windings 690-690 V generator. During the initial tests both windings were surge tested at 1500 V diagnostic was low speed winding burned.

Due to economic matters the costumer prefer to rewound only the low speed winding,then the work shop removes the low voltage winding and clean all surfaces deep. Again the good winding was surtested at 1500 V no problems were detected. High speed windig is at least 7 years old but was not overheated and with out any discolaration o posible failures in the winding.

The low voltage winding was installed and before varnishing both windings were surge tested at 5000 V with failure in the old high speed winding (Turn to Turn short circuit).

Seems is clear but please tell me your inputs about it.Also your experience surge testing old windings and voltage levels used for it.

Thanks in advance

Carlos

 

[biggeorgetechnical]

Hello,

A few questions for workshop supervisor should be:

A) Why did they select 1500V for the test voltage on the incoming tests and analysis?

B) What other tests were performed? i.e. visual inspection, DLRO, meg-ohm, etc, etc.

c) Why did they select 5000V for the outgoing quality control tests?

D) Did they advise you as follows:? due to heating/contamination effects on the overall winding from the burned low speed winding, don't expect a repair to be "good as new".

The workshop should be able to answer these questions with some reference to accepted test standards, or those referenced in the user manuals of their surge comparison testers.

If they can't do that for you, I can provide suggestions if you want.

Thanks,

George

"When you go looking for problems, you will find them... But they may not be the problems you went looking for!"

 

[petronila]

Hi George

Thanks for your inputs, during incomming testing both windings were tested following the same sequence that you are indicating; Winding Resistance, Insulation resistance(500 V), P.I.(500 V)and surge test (1500 V). With the results that I was explained before.

Now like you said, the question is why after the workshop rewound the L.Speed winding they used 5000 V for last surge testing for both windings (new and old)?

Most of tester manufacturers recommend use a test voltage of 2 Un + 1000 in this case the generator is 690 V so the test voltage should be 2400 Volts, another sources like IEEE indicate the test values in the table attached.

In my own experience in field testing of used motors I have use 0.6 (2 Un +1000) so if I have doone the test I would do it at 1500 V.

a test level of 5000 Volts is very high for 690 Volts winding?

Your suggestions are well apreciated.

Thanks

Carlos