AEGIS® Bearing Protection - Why Shaft Grounding?


Bearing Currents


There are two primary sources of bearing currents in VFD driven AC motors:

Capacitive EDM Current: Discharge of shaft voltage capacitively produced by the pulse width switching waveform of the VFD's voltage output. The output voltage is coupled to the shaft by parasitic capacitance, and the shaft voltage can discharge through the motor bearings or those of attached equipment, causing electrical discharge machining (EDM).

  • Any VFD-fed motor, from fractional HP to thousands of HP, may have bearing currents from this source.
  • Voltages can discharge through the motor bearings resulting in EDM pitting and eventual failure.

Best Practice: Ground the motor shaft with the AEGIS® Shaft Grounding Ring to provide a path of least resistance to ground, and divert discharge current away from the motor's bearing.

Ref: NEMA MG1 Part

Total Qualitative Bearing Currents

An Electric Motor works like a Capacitor

  • The pulses to the motor from the VFD create a rapidly changing common mode voltage at the motor terminals. This rapidly changing common mode voltage creates a capacitively coupled voltage on the motor shaft.
  • Shaft voltages are measurable with an AEGIS® Shaft Voltage Tester™ or other digital oscilloscope equipped with an AEGIS® Shaft Voltage Probe Tip.
  • Discharge of these shaft voltages is called Capacitive EDM Current.

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