Shorted Coils Repair Question

So let me get this right... you are working on alternator coils for a 11KV system, and they show as being shorted when megger tested at 10KV DC? Then I guess your repair job may not have been up to what was needed.

Yes... 10KV DC is a greater voltage then 11KV AC... but not by much. 11KV AC is equal to 7,777 volts DC. I do not think their request is far fetched.

What did it call for in your contract?

These are MINIMUM values,not maximum values.The customer may specify the maximum value according to their desired safety margin.

And remember, the boss is not always right, but he is always the boss.

Have you disconnected the neutral(star point) from the ground point?

The OP mentioned the coils were out of the alternator.

The Coils are out of the Alternator and we followed the Alternator Manufacturer's guidelines in repair of the coils. The specs of the insulation material used is as follows:

QUOTE:

Prepreg Mica Tape made of calcined muscovite impregnated with heat resistant epoxy resin and bonded with supporting materials, such as glass cloth, polyester film and polyester fleece. When prepreg mica tape is subjected to heat and pressure, the resin flows and cures to void free insulating materials. Nominal thickness of the tape = 0.18 mm. It needs two controlled heat presses from 165 deg. C to 175 Deg. C at-least for 120 minutes each press.

UN-QUOTE:

We followed the instructions strictly. Also the Star Point is opened.

Best Regards,

Younis Sabir

Yes Sir, The Neutral Point is disconnected also the Potential Transformers Connections are disconnected. Actually the shorted coils are already out of the Alternator.

B.R.

Younis Sabir

Sabir -

You may need to re-rewind the coils if it continues to fail the required test. In fabricating new coils, you need to use a higher grade or better quality of insulated magnet wires! The quality of the materials used, like the layers, types of insulations and the quality of your workmaships are the major contributing factors!

But before resorting to that, have you tried baking the coils overnight to dry? You need to remove all the moisture before conducting your megger insulation tests. you may have to enclose the coil together with a powered 1000 watt bulb while conducting your test to maintain the minimum possible moisture content of the wound coil..

Good luck!