Induction Motor stator temperature running high.

[aderemb]

We have two induction motors that are connected to two seperate 4.16kV Medium Voltage Switchgear bus and both MV switchgears are fed from a single step-down main transformer. We experienced some problem with both of the induction motors stator winding temperature on phase "B" running at elevated level in relative to other two phases during the past summer.

We also observe that when any one of the motor in question is off-line, the stator winding temperature reading per phase is uniform across each phase. The difference only appears when either of the motor is running. This tell me that the sensors are calibrated correctly. The voltage and current per phase is not available in our DCS. However, we have few other induction motors connected on both MV switchgears that do not exhibit similar behavior across the stator windings when in service.

Motor Data: WEG Induction electric motor, 600HP, 80.7A rated current, 1188 RPM, 6 Poles, 4kV, 3Ø, 484A Lock-Rotor Current, 40°C design ambient temp.
Stator Winding RTD: Alarm at 130°C and trip at 155°C.

Question- Has any one experience similiar problem and how did you fix this problem. Any help will be highly appreciated.

 

[dpc]

How much higher is the B-phase winding temperature in relation to the other two phases?

How is the voltage balance when the motors are running?

David Castor

http://www.cvoes.com

 

[electricpete]

It’s an intriguing symptom. I have no idea really what could cause it. I can only throw out some questions
1 – Have you checked for current unbalance. At our plant it is fairly easy to check current balance by clamping on each phase current at the input to a 3-phase relay (with suitable site-appropriate electrical safety precautions). Are they balanced with both motors running and are they balanced with only one motor running?
2 – You told us the difference changed, but you didn’t really tell us if the average changes
3 – More difficult checks would be voltage balance and harmonic content.


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(2B)+(2B)' ?

 

[electricpete]

1A - If you have opportunity to compare balance with 1 and 2 running, also check the average current with 1 and 2 running to see if there is a change in loading.

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(2B)+(2B)' ?

 

[aderemb]

The B-phase stator winding temperature is running about 15° F higher in relation to other two phases.

From the switchgear 1 (pump motor currently in service)protective relay, we have the following voltages per phase across each motor: ØA-B: 4.081 kV, ØB-C: 4.104 kV, ØC-B: 4.093 kV and from switchgear 2(pump motor not in service): ØA-B: 4.154 kV, ØB-C: 4.143 kV, ØC-B: 4.3 kV.