AC motor failure rate

  • Thread starter João Lourenço
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Thread Starter

João Lourenço

Could anybody tell me what is the typical failure rate or MTBF of an AC three-phase asyncronous motor?

Thank You

João Lourenço

MIIT - Manutenção Industrial Informatizada e Tecnologia, Lda. Inspecção e Controlo
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1 - 20 x 10-6 failures per hour, is listed in one of the appendices in Reliability, Maintainability & Risk, by D.J. Smith. Obviously, it depends upon MANY factors!

Paul Gruhn, P.E., C.F.S.E.
Siemens, Houston, TX

Bouchard, James [CPCCA]

That depends very much on the application, design and choice of motor. We had 300HP TEFC motors that ran for 15 years without a failure because they were properly chosen and maintained. ( after 15 years they were relocated to another location so they may still be running ) Another case I had a 200 HP ODP motor that was located in a damp area that also had caustic fumes. It failed every 2 to 3 years until I replaced it with a TEFC motor which lasted another 12 years before it was transferred. On smaller motors ( 10 to 100 HP ) we have much the same results. If properly applied and maintained they will last decades. If you abuse them, don't do the maintenance etc they may only last a year.

James Bouchard
Johnson & Johnson Products
Do you mean synchronous or induction motor? IEEE, in the Gold Book, I believe keeps track of some large motor characteristics. There is not an easy answer to your question though. If you consider large synchronous motors(over 2500 HP). I beleive the MTBF is typically in the range of 5-7 years. Small induction motors, say 1/2 HP though are lucky to live 365 days in rough service. Smaller bearings mainly affect for small motors. I worked as a Reliability Engineer and tracked this statistic for several years for a motor population of around 1400 motors ranging in size from 25 HP-3500 HP(mainly induction motors but [email protected] Hp synchronous). I recall the MTBF being 1100-1200 days.

Hope this Helps.

Bob Peterson

I'd like to suggest something else here as well. Motors are made to different levels of quality, even motors made in the same frame size, HP, and enclosure, by the same manufacturer can have varying levels of quality
between different models.

And many maint men have an almost instinctive dislike of a certain economy brand of motor, preferring the longer life of several other brands, because over time they have learned that the less expensive motor just does not last as long.

Also keep in mind that using a motor sized closely to the actual load used may cost less, and be more energy efficient, but going to the next size up gives you a motor with a lot more iron in it to dissipate the heat, but is
somewhat less efficient. While the correctly sized motor may give you several years of good service, the oversized motor may last indefinately.

In intermittant service, it may not matter a whole lot as the motor may only be run a few hours a day.

One other thing, in larger motors that sit for any substantial fraction of the time, are in damp areas, or outside, its advisable to add winding heaters to them to keep them dry. The difference in cost is small, but keeping the
windings dry will extend the life of the motor a LOT.

Bob Peterson