Oscillating current in 6300KW wound rotor motor for ball mill

[fiaz1404]

We have 6300KW, 13800V, 1194 rpm Slip ring SIEMENS MOTOR used for cement grinding ball mill. Recently while mill was in operation we noticed that current started to oscillate from 280 Amp to 320 Amp. We also observed the voltage oscillation ranging from 13.6 KV & 14 KV. Current is absolutely balance on all phases.

To rule out that current fluctuation is caused due to the fluctuation in voltage and therefore a power supply issue we started another grinding unit with same rating motor on same power source. Motor current was quite stable, variation range from 278 amps to 283 amps and voltage source was 13.8kv stable.

Following checks were performed on motor

1. Checked slip ring, liquid resistance starter and short circuit breaker, found ok.
2. Checked the rotor resistance with micro meter it was balance on all phases
3. Measured the stator resistance it was balance.
4. Disconnected the power factor compensation capacitors and started the motor without Capacitor and reactor but oscillation persist
5. Started the motor without coupling mill, motor and voltage found normal

Some current recording are attached for reference.

Request to experts on this forum for their opinion.

 

[Compositepro]

Your current oscillation has a period slightly less than one second. It is quite possible that the load on the motor is fluctuating at this frequency. The balls in the mill may be cascading in waves due to a mechanical resonance caused by an interaction between mill rotational speed and ball/cement loading. You should see the same fluctuation if you plot mill rpm versus time.

 

[waross]

If the higher current coincides with the higher voltage it may be a supply issue.
If the higher current coincides with the lower voltage it may be load related.
I agree with Compositepro.

Bill
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