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Pumping Systems on VFD Need Bearing Protection

9 Jun 2020  |
In most pump applications, it is not always necessary to run the pump at full speed. Flow rate has traditionally been decreased with flow control valves, but this has disadvantages.

Nowadays the flow is often controlled by running the pump’s motor on a variable frequency drive (VFD). Besides well-controlled flow rates, VFDs offer:

  • Energy savings (from reduced motor speeds)
  • Improved seal and impeller life and reduced vibration and noise (from reduced pressure)
  • Greatly reduced water hammer (from smooth acceleration and deceleration)

But VFDs have their own downsides. One big problem is that they create stray voltage on the motor shaft. This shaft voltage can cause electrical damage to motor and pump bearings by discharging (arcing) through them. This electrical damage accrues over time, causing premature bearing failure.

The solution is to electrically bond the shaft to the motor frame with a shaft grounding device. This gives charge on the shaft a low-resistance path to ground, so current flows through the grounding device instead of arcing through the bearings.  (Large motors, over 100 hp/75 kW also require one insulated bearing to prevent circulating VFD-induced currents.)

AEGIS® Shaft Grounding Rings are the most reliable form of shaft grounding available, and the only one that offers a 2-year extended warranty.  AEGIS rings can be installed on existing motors, and several manufacturers also offer motors with AEGIS rings factory-installed, including vertical hollowshaft, severe duty, and even explosion proofmodels.

To learn more about the problem of VFD-induced bearing currents and the solution using AEGIS rings, see these case studies and best practices, or see the AEGIS Youtube channel

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