Westinghouse Motor Service Factor

[motorgirl]

I have been trying to establish a standard on how we operate our AC induction motor and have run into a snag.

The main snag is that I need clarification on how Westinghouse states there Service Factor. It appears that on several of our motor it is stated as:
TIME: CONT 115deg C rise at 1.15 SF.
I am not sure I understand this statement because my understanding of SF is that it is a overload factor that should only be used for short periods of time not as a load limit.

Will someone please explain the Westinghouse statement and if is really a SF of 1.15 continous? If possible point me to documentation please.

 

[dpc]

It depends on what you mean by "continuous". See NEMA MG-1 for definitions and limitations. Under certain conditions, the motor may carry the service factor load for an extended period of time. But this assumes motor is operating at rated voltage and ambient temperature and also running on a sine wave. The motor will not necessarily maintain specified efficiency and power factor when operating at the service factor load. Operating at service factor load will generally reduce the motor life.

 

[electricpete]

It is simply a 1.15 SF.

If you have class F insulation, the rating is 105C rise above 40C ambient (continuous).

At service factor, it is allowed to be 115C rise.

This is standard approach for service factor rating per the standards (not just Westinghouse). Perhaps what is unique is that Westinghouse is reminding you of the temperautre rise at the service factor load... others may not include this info.

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[electricpete]

At service factor, it is allowed to be 115C rise.

To clarify, let me add a word:

At service factor load, it is allowed to be 115C rise.


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[electricpete]

Maybe I should twist around the sentence a little more for clarity:

To achieve a 1.15 SF rating with class F insulation, the temperature rise cannot be more than 115C

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[jraef]

There you go. You had me worried for second there Pete.

Temp rise chart


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[motorgirl]

Thanks for all the post. I understand what you all are pointing out/ saying, but the main issue is that when operations reads the Westinghouse statement they take it to mean they can operate at a 1.15 SF for ever and ever.

Does any one have any documentation that explains the westinghouse statement?

 

[edison123]

If you mean by "for ever and ever", a continuous operation, yes, you can operate continuously, provided the temp rise < 115 deg c.

This would definitely reduce the winding life at a much faster rate than the operation at SF of 1.0.

Lokk at jeff's chart.

 

[edison123]

Here is another link to the sf definition

http://www.engineeringtoolbox.com/service-factor-d_735.html

 

[rovineye]

I simplify it for my guys. I tell them they may operate the motor at 115% IF the ambient temperature is not above 40 deg C. I know there will be some shorter life expectancy.

Conversely, when we have a space requiring 50 deg C ambient motors, and I am provided a 45 deg C motor, I will accept it if it has 1.15 sf and we run it at 1.0 but I add a nameplate at the motor.

 

[electricpete]

The presence of the temperature rise again is just extra information, assuming the motor has class F insulation.

Look here:

http://www.motorsanddrives.com/cowern/motorterms4.html

Look at table 1. Class F insulation materials have a continuous temperautre rating of 155C.

Most of the entries in the table have a "hot-spot allowance". For example class F TEFC at 40C ambient and 105C rise still has 10C hot spot allowance before reaching the 155C rating. But class F 1.15 SF at 40C ambient temperaure and 115C rise is already at the insulation rated temperautre.

Why do we need hot spot alowance? Because you are reading an average temperature by resistance (or in some cases an actual temperature by RTD but still will not be the hottest spot since the hottest spot is directly adjacent to the conductor, not in between coils where the RTD's are... also there are only maybe 6 RTD's per motor but many more slots than 6).

So there you have it. With hot spot allowance, you will probably exceed the rated temperature at SFA. By how much? If we apply the other hot spot allowances we guess about 10C. Does that mean instant failure? No... that corresponds to roughly halving of insulation life (at least by the Arhennius equaition which applied to organic insulation resins, not so much for epoxy and polyester resins and mica insulation). And none of this is exact since aging characeristics are not so accurately known.

And there may be other effecs on mechanical components besides the insulation. What does NEMA say about it?

“A general-purpose alternating current motor or any alternating current motor having a service factor is suitable for continuous operation at rated load under the usual service conditions given in part 14. When the voltage and frequency are maintained at the value specified on the nameplate, the motor may be overloaded up to the horsepower obtained by multiplying the rated horsepower by the service factor shown on the nameplate.
When the motor is operated at any service factor greater than 1, it may have efficiency, power factor and speed different from those at rated load, but the locked rotor torque, current and breakdown torque will remain unchanged.
A motor operating continuously at any service factor greater than 1 will have a reduced life expectancy compared to operating at its rated nameplate horsepower. The service factor load reduces insulation and bearing life. “

For additional info, try pushing the search button while you are in the motors forum. Select the button:
"Electric motors & motor controls engineering Only"
"keywords in subject line only"
and search for "service factor"
many entries. should be a FAQ
and

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[electricpete]

Sorry, I didn't see your message. I think you have it right. As long as you recognize that in both cases you are partially taking advantage of service factor loading beyond rated, which involves some limited loss of life and is generally not where you want to operate 24-7.

If you have temperautre indication on the motor windings, it brings a whole 'nother dimension to the discussion. It might be argued that this is a more direct indicator of insulation aging rate than is the curent loaidng. Particularly since you may be at a different voltage than namneplate, and motor conditions may have changed including dirt buildup which tends to make things run hotter. Also loosening coils make coils run hotter (less heat transfer accross a loose gap than a tight gap). And core degradation during winding burnout for rewind can make the motor run hotter.

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[tommy75]

Wow!!

 

[starkopete]

The old rule of thumb was that for every 10 degree C temperature rise the life of the insulation would be halved.