How to defend against bearing currents?


Destructive bearing currents can happen to any motor application controlled by VFD, ultimately leading to catastrophic motor bearing failure. When the accumulated shaft voltage is discharged through the bearing, the motor bearing is damaged, resulting in an EDM (Electric Discharge Machining) effect. Although high carrier frequency, constant speed operation, inadequate grounding and other factors increase risk and accelerate damage, it is still unpredictable when and where the bearing current becomes a problem.

EDM damage can be checked upon by special vibration analysis techniques. Vibration analysis can be used to confirm bearing fluting damage caused by EDM. In the early stages of the bearing fluting damage, the high frequency, high resolution spectrum of the bearing will present an "energy mound" in the frequency range of 2-4 kHz. Also, measuring the actual voltage and current present on the shaft can be helpful in determining the likelihood of damage from EDM. 

If there is a bearing current problem in the installed equipment, or if the calculated risk is high enough, you can protect your equipment with one method or combinations of multiple methods. Shaft grounding devices, insulated bearings and Faraday shields are the most popular and successful methods.

  • Shaft grounding devices. A low impedance path is provided from the motor shaft to the motor case to ground the shaft by installing a grounding device. This effectively directs current away from the bearing. By not allowing voltage build-up on the rotor, the shaft voltage is significantly reduced, thereby reducing bearing current.
  • Insulated bearings. Insulated bearings get rid of the path that grounds current through the bearings. However, the shaft voltage does not go with an insulated bearing installed, and it will still find the ground path with the lowest impedance. This can still cause problems if the path passes through the driven load or some other component.
  • Faraday shields. Faraday Shield is created by putting copper foil tape or copper paint or other grounded conductive materials between the rotor and the stator, which leads to an electrostatic shielded induction motor (ESIM). By this, the capacitively coupled currents are reduced and the shaft voltage minimized.  

Determining whether or not protection is warranted and determining the appropriate methods of protection is a shared responsibility based on cost analysis and risk assessment. Educating ourselves and each other about the cause, effect and protection of bearing currents is the best line of defense.

Deciding whether or not the protection is included in the warranty and it is a common responsibility of parties involved to perform cost analysis and risk assessment to determine the proper methods. The best defense is to learn about how bearing currents happen and put into place the protection methods.