Motor Bearing Currents - GE AF-600 FP Design Manual

4.4.7 Motor Bearing Currents

All motors installed with 110 kW or higher power drives should have NDE (Non-Drive End) insulated bearings installed to eliminate circulating bearing currents.
To minimize DE (Drive End) bearing and shaft currents proper grounding of the drive, motor, driven machine, and motor to the driven machine is required.
Standard Mitigation Strategies:
1. Use an insulated bearing
2. Apply rigorous installation procedures
Ensure the motor and load motor are aligned
Strictly follow the EMC Installation guideline
Reinforce the PE so the high frequency impedance is lower in the PE than the input power leads
Provide a good high frequency connection between the motor and the frequency converter for instance by screened cable which has a 360° connection
in the motor and the frequency converter
Make sure that the impedance from frequency converter to building ground is lower that the grounding impedance of the machine. This can be difficult
for pumps- Make a direct earth connection between the motor and load motor.
3. Apply conductive lubrication
4. Try to ensure the line voltage is balanced to ground. This can be difficult for IT, TT, TN-CS or Grounded leg systems
5. Use an insulated bearing as recommended by the motor manufacturer (note: Motors from reputable manufacturers will typically have these fitted as
standard in motors of this size)
If found to be necessary and after consultation with GE:
6. Lower the IGBT switching frequency
7.
Modify the inverter waveform, 60° AVM vs. SFAVM
8. Install a shaft grounding system or use an isolating coupling between motor and load
9. Use minimum speed settings if possible
10. Use a dU/dt or sinus filter
AF-600 FP Design Guide
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