Frequent Slipring Burning of 400kw Induction Motor

Is this multiple choice? Where are the data from your forensic investigation post ring failure?

Something about slip ring failures you can find here.

Zlatkodo

Need more data for clarity.

1. What does the motor manual say about the permissible temperature limits of slip-ring. Insulation between rings also is subjected temperature rise, what is the insulation class of slip ring.

2. A single clue : based on ur data, motor parameters seem within limits, the secondary currents "b ph" is significantly lower.

3. Is load hunting ( short-time overload) frequent?

4. How & when are brushes/contact assembly being checked, as brush pressure is likely to be different after each check.

Kindly update.

The first place I'd look is the cleaning regime followed by eliminating any ingress of dust from the grinding process.

If the motor has run trouble free in the past then unless the brush grades have been changed look for external influences.

What is the maintenance regime for this motor at your site?

What was 'happening" at the time of failure?

Is this the first time this has happened?

You mentioned vibration concerns. Is that a clue?

Share your theory(ies) with us....

From your description this appears to be a synchronous motor application.

If so;

Slip ring failure can be caused from too much DC excitation voltage being applied to the rings resulting in an arcing condition and/or severe overheating of the rings as well as the motor rotor windings. (Reference motor nameplate rating and adjust accordingly.)

Short brushes, incorrect brush spring tension, incorrect brush grade, incorrect gap spacing from bottom of brush magazine to slip ring, incorrect size of brush, incorrect brush magazine mounting angle, and incorrect brush magazine spacing can cause slip ring failure. (Refer to motor OEM specifications and adjust/repair accordingly.)

Failed or damaged (burned) brush magazine mounting insulators can cause leakage and/or total failure of the rings.

Failure can be the direct result of a mechanical overload during the starting cycle of the motor as synchronous motors cannot handle large loads until after they are up to rated speed and are "synchronized". All ball mill applications wherein synchronous motors are used include a motor protection circuit with associated logic to prevent the motor from running in an "under-excited" state and depending on age, will have either a "pull-out" switch mechanism or if newer will have a programmable motor protection relay. (Reference motor DATA IOM and verify settings/adjustments are per factory design. If you do not have the needed information, it will be necessary for you to contact the OEM motor manufacturer to obtain it.)

"Short-circuited" rotor windings in the pole pieces can also cause this type of damage. (Obtain rotor winding impedance specifications from the OEM and verify readings to be within allowable limits. If readings are not within limits, it will be necessary to either pull and send the motor to a qualified re-winding facility or have the repairs performed in place.)

Loss of stator horsepower/torque caused from shorted (damaged) windings also cause this type of failure. (Obtain stator winding impedance specifications from the OEM and verify readings to be within allowable limits. If readings are not within limits, it will be necessary to either pull and send the motor to a qualified re-winding facility or have the repairs performed in place. Be sure to include a Polarization Index test on all of the motor winding component insulation to verify condition and insulation capacity.)

Check and verify the motor rotor grid resistor impedance (adjustable) to be within OEM specifications. Look for loose wiring, loose terminations, resistor(s) damaged or burned open, etc. (Obtain rotor resistor impedance specifications from the OEM and verify readings to be within allowable limits. If readings are not within limits, it will be necessary to repair or replace the resistor(s).)

Good luck and be sure to get competent technical help if needed.