Shaft Voltage Measurement - Case Study

Motor; 460kW Leroy Somer Permanent Magnet Compressor Motor. 400V, 3600rpm.

• Initially overhauled in May 2017, when 2 x Insulated Bearings were replaced, Excessive Bearing Failure was once again detected in November 2017.
• It was agreed to complete a Shaft Voltage Test, as the motor is powered by VSD, Shaft Voltage of 54V Peak to Peak was detected.
• Upon inspection of the power cable between the Motor and VSD, the earth cable was found was found to be connected to the VSD end, but not on the Motor end.
• Once this cable was properly earthed to the motor, the shaft voltage decreased immediately to 3.6V Peak to Peak.
• Further reduction to less than 1V was demonstrated by applying an Earthing Brush to the Motor Shaft.

Initial Shaft Voltage: 54V Peak to Peak

Improved Shaft Voltage: 3.6V Peak to Peak

Conclusion: Shaft voltage measurement is the definitive way of detecting harmful electric current discharge through bearings.