Bearing current is the discharge of the shaft voltage created by the inverter through the bearing to the frame. The sum of the three phase voltages at the inverter output is not zero at every instant; this difference, called common-mode voltage, is transferred to the shaft through the motor's internal capacitances. When the shaft voltage reaches a certain level, it breaks through the lubricant film in the bearing and each discharge leaves a microscopic mark on the ball and raceway.

Over time, regular lines resembling a washboard form on the raceway surface. The signs are rising bearing noise, vibration and darkened grease. Because the failure looks like a mechanical problem, its real cause can be missed.

Measures taken:

  • Insulated bearing: Usually fitted on one side to break the path of current circulating through the bearing.
  • Shaft grounding: The shaft is connected to the frame by a brush or ring; the current is routed outside the bearing.
  • Correct cabling: Symmetrical, shielded motor cable with the shield bonded over a wide area at both ends.
  • Filter: Sine or common-mode filters soften the voltage pulses.

The HEM catalogue states that large frames may need an insulated bearing or shaft grounding. From which frame size it is needed is assessed according to the application, so inverter use and cable data should be stated in the order. For bearing maintenance and noise monitoring, see the motor maintenance and overheating questions guide.