Consider these methods to help protect systems from high-frequency current damage.
by Robert McElveen Jr. & Frederick “Rick” Hoadley
June 20, 2019

The stator winding to frame/shaft current is shown in yellow. This current flows through the stator winding insulation (which capacitively conducts at high frequencies) and through the motor frame, the motor bearing, the motor shaft, the conductive coupling, the load bearing, the load ground and finally to the drive ground. Current flowing in this manner can damage both the motor and load bearings as well as the coupling. The preferred path, to prevent bearing damage, is the stator winding to ground path shown in blue.

In this case, no current flows through the motor or load bearings.

Bearing Current Remediation

There are many possible ways to reduce the magnitude and paths of current flow shown in Image 4. For inverter-fed motors, reducing or eliminating the CMV addresses the problem at its source. However, the CMV is a function of the drive design and cannot be addressed on existing applications.

Pumping applicationImage 5. Pumping application

For this reason, it is necessary to investigate other means to reduce or eliminate this problem. If multiple current paths are present, multiple remediation methods may be required.

Methods that have proven results include:

  • improving the high-frequency ground connection from the motor to the drive and from the motor to the driven equipment
  • insulating the bearing on the opposite drive end (ODE) of the motor
  • using two insulated motor bearings
  • using a shaft grounding brush across the drive end (DE) motor bearing, which could be mounted inside the motor housing or outside

An important ground path is the connection between the motor and inverter. Cables that provide continuous, low resistivity shielding around the three-phase conductors should be used. The termination of the cable shield should be made by landing these connections on a ground surface free of paint at both the drive ground bus and at the motor frame.

paths of common mode currents from stator to ground, back to drivepaths of common mode currents from stator to ground, back to drive through the cable shieldImage 6a (top). Paths of common mode currents from stator to ground, back to drive. Image 6b (bottom). Paths of common mode currents from stator to ground, back to drive through the cable shield.

It is also important to properly ground the frame of the motor. Stator winding to frame currents has the highest potential magnitude of all capacitively coupled currents discussed. Low impedance ground straps should be used to bond the motor frame to the driven equipment frame. A combination of solutions that reduces or eliminates all paths of bearing current flow is shown in Image 6, where most these high-frequency currents will return to the drive through the motor cable shield and little will return to the drive through the building ground.

As shown, an ODE insulated bearing is used with a shaft grounding brush on the drive end. Good high-frequency bonding (such as a flat-braided cable) is used between the motor and load. The insulated ODE bearing prevents potential current flow through it. The shaft brush provides a lower impedance path around the DE bearing. The bonding cable between the motor and load creates a lower impedance path around the bearings in the load.

Conclusion

Bearing currents have existed since the induction motor was invented. CMV induced currents are a phenomenon resulting from high switching frequency drives. Proper grounding is a major factor in preventing bearing damage due to circulating currents. Protecting an installation from bearing current damage requires a thorough understanding of the inverter/motor/load system. Identification of potential high-frequency current paths is crucial to providing an effective solution.

References

  1. C. Pearce, “Bearings Currents - Their Origin and Prevention,” The Electric Journal, August 1927.
  2. R. F. Schiferl &. M. J. Melfi, “Bearing Current Remediation Options,” IEEE Industry Applications Magazine, August 2004.

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