duty cycle for motor longevity?

[jarvis]

If a motor was ran for ten seconds on and ten seconds off would it last longer than having it run continuously. Would it be different for ac and dc motors? How much would starting load factor into it? Thanks for any input.

Ben Englund

 

[SteveKW]

First thing I can think of...

In your 10 second repeating on/off scenario, the inrush current (starting) would overheat the motor windings.

 

[aolalde]

jarvis

It is important to know the starter, motor type, speed, load inertia and load torque curve to make an analysis.

For a traditional SCIM handling an average load, a 10 sec run, 10 sec stop repetitive cycle is more severe than a continuous running service due to the high inrush current involved. The mechanical stresses and the heat generated are proportional to the second power of the current.

 

[jarvis]

How about one minute on and one minute off? I don't have any particular application in mind, but it is just something I have been pondering. thanks for your replys

Ben Englund

 

[kassad]

Jarvis,
always remember that it is always less severe for a motor to be running continously rather than on an on/off cycle, no matter what the on/off cycle might be.

 

[jbartos]

Suggestion: There is a great flexibility in the motor design. A motor for such duty might be designed and actually are designed for process industry. However, an off the shelf squirrel-cage induction motor or dc motor will experience an overheat and a lower life cycle.

 

[electricpete]

Here is some info from NEMA on allowable starting duty (starting not allowed as frequently as horsepower increases, speed increases, inertia increases).

http://www.joliet-equipment.com/allowable_starts.htm

I think that "always" and "never" are amost always never right ;-)

If the motor were very highly loaded above the service factor, then I would think it would certainly be preferable to start it once a year for a few hours than to run it continuously.

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[electricpete]

(also let's assume my hypothetical overloaded motor is in-doors and outfitted with space heaters that come on when the motor is off, and has a very low inertia load which does not challenge it during start)

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[jbartos]

Comment: Custom made or very special motors do not necessarily have to comply with NEMA MG-1. E.g. NEMA does not cover 15 or 18phase motors. They are beginning to be applied to elevators.