Methods of Motor Protection

Since we understand how bearing currents are generated and what they can do to our motor bearings, as well as a couple of ways to determine whether or not there may be a problem, one may probably wonder how to do to protect the motors.

Protecting motor bearings from an unpredictable occurrence of bearing currents is not an exact science, but rather a process of risk assessment and cost analysis. There are certain measures you can take in order to reduce the risks — but not without additional cost. Since the occurrences are unpredictable, one has to weigh the cost of protection against the costs in the event that the problem was to arise.

For example, installing a shaft grounding assembly in each of 15 direct-coupled fans with 100-HP motors would be much cheaper than replacing those 15 motors three months after start-up. On the other hand, replacing 15 smaller 3-HP motors would be much less expensive, so waiving the extra cost of shaft grounding assemblies might be an acceptable risk.

If protection is warranted, the following recommendations are a few of the most frequent that have been used successfully in various industries and applications.

SHIELDED CABLE

SHAFT GROUNDING

INSULATED BEARINGS

FARADAY SHIELD

The best way to protect the equipment is to understand the causes and effects of EDM damage in order to make informed decisions during the specification, requisition and installation stages. While there is no magic recipe to follow, in this article we offer some guidelines that at the very least will bring these issues to light for discussion and consideration.