shorting contacts sparking (slip ring induction motor)

[sipho68]

there is a situation of a 15MW slip ring motor (uses liquid rotor starter) that has its slip ring shorting contacts sparking/ flashing - mostly when the motor trips from full load. On few occations they also spark during startup but this is not frequent. As a result the contacts need to be replaced now and again as they get badly damaged on the contact surface. This is a recent problem, the motors slip rings were replaced about 5 months ago as part of major overhaul. Can anyone suggests why this could be happening and what can be done to resolve this problem?

 

[waross]

Do mean the brushes are sparking or the contactor that shorts the rotor circuit when the motor is up to speed is sparking?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[sipho68]

the rotor shorting contacts are sparking when the motor trips

 

[electricpete]

So, the contacts are closed when the machine is running normally (after starting sequence complete).
And the contacts are opened while the machine is secured.

During transition from running to secured (trip), the contacts immediately open?

In that case I would think a time delay is needed between tripping the motor and opening those contacts. Just thinking out loud (I don't work with these type machines)

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[edison123]

Any tripping on load is going to flash. If you do not want the rotor contactors sparking, then trip only the stator.

Muthu

http://www.edison.co.in

 

[sipho68]

electricpete, thanks for that suggestion. I think delaying the openning of the contacts (after the motor has tripped)might resolve this.

edison123, keeping the contacts closed all the time will adversely affect the starting current (will be high) and starting torque (will be low). For a slip ring motor we want to start the motor with some resistance added to the rotor circuit for high starting torque, this is then shorted out (by the rotor shorting contacts) once the motor has attained running speed. The trouble now is that as soon as the mtor trips for some condition, these shorting contacts open (as they should)but a flash also occurs accross the contact faces - destroying them in the process.

 

[edison123]

Not all the time. I meant at the time of tripping the motor. Open the rotor contactor after a few seconds of tripping the motor.

Or go for a higher rated rotor shorting contacts to handle the rotor current, which is close to DC.

Muthu

http://www.edison.co.in

 

[panter]

Sipho68,
I have similar HV motor, *only* 3550kW, and it have special device mounted on the shaft with small motor for shorting rings and lifting brushes when it came to the nominal speed. After tripping of the motor,device simply letting down brushes and motor is ready for new start. Your motor is bigger and it must have something similar or, like Muthu said, you must make scheme for opening rotor contactor after few second of tripping of the motor and also block start of the motor when the rotor contactor is close. Good luck!