Every motor user would probably answer that question the same way: “Until we don’t need it any more.” That could be one year — or half a century. Unfortunately, there has never been general agreement either on what the life span of a typical motor is (or should be), or on what constitutes the end of that motor’s life.

To some users, life ends the first time the motor goes to the shop — for whatever reason. If repairs can resurrect the “dead” motor, the clock starts running again. Most people assume that motor life ends when the winding “burns out” and needs replacing. That’s only one type of major breakdown, however, and may not be the most expensive. Furthermore, we often see surveys revealing that some motors are “rewound several times during their lifetime” — meaning that useful life is considered to extend well beyond that first failure.

One “authority” contends that polyphase a-c motors “are designed to last 40,000 hours.” There’s no truth to that, or to any other claim of a specific “design life” in hours or years. Insulation thermal tests do predict average life, but only for the insulation system itself. Comparing such tests to those for similar insulations, used in actual motors for which historical data exists, leads to insulation life estimates far beyond 40,000 hours. But such figures can’t be termed “design” life. And again they involve the life of only one component, not the entire motor.

A common practice is to assume a typical “time to failure” (such as a winding burnout, shaft breakage, or major bearing damage) of 10 to 15 years for the average general purpose medium sized polyphase induction motor. Yet many such motors have remained in service two to three times that long. Many others, usually through neglect, fail far sooner.

Various IEEE reliability surveys have shown two general life relationships for industrial motors. One is that annual repair/replacement rates for the overall motor population tend to be 5-6%. That implies a 15-20 year average life span. However, these surveys did not make clear what was accepted as “end of life,” nor did they indicate what percentage of the “failed” motors went back into service after repair. The other IEEE survey result was that motor starters tended to fail more often than the motors themselves.

Based on information of that sort, a reasonable approach seems to be to consider motor life in the historical sense as typically at least 15 years. But there can be no implication that such historical experience provides any basis for “designing” specifically longer (or shorter) life spans into any motor. Even if there were, the fact remains that we have no accepted definition of what constitutes motor “death.”

Richard L. Nailen, P. E