Service Factor 2

Service Factor (SF) is a concept uniquie to NEMA design motors and is defined as a percentage by which the motor can withstand an increased load while still maintaining rated torque and speed within an accepted tolerance, even though current is expected to rise. Some believe this to be a short time rating, however you will never find an AMOUNT of time discussed, mostly because it is nebulous. What is said is that when using a motor's SF, an increased temperature rise is to be expected and therefore the motor life will not be the same as if the SF was not used. Many OEMs choose to use SF as if it represented an increased HP at the shaft, mostly in order to avoid stepping up to the next size motor if their application calls for just a little more than a standard motor design allows. Those OEMs are generally not concerned that the motor will last 5 years instead of 20 years, because either way it is out of their warranty by the time it fails.

the reason why you don't see SF listed for 50Hz motors is because IEC, the governing rules for the rest of the world, make no such claim of a motor being rated for anything other than what it says it is. So in effect, all IEC motors are 1.0SF.

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friend81; Adding to jraefs fine dissertation I would say if you have a load like say... an air compressor that generally runs at 5Hp but right at it's maximum pressure requires 5.5hp a 5HP motor with a service factor greater than 1.0 might be a better choice as periodic overloading still leaves the motor operating within it's design region.

Keith Cress
Flamin Systems, Inc.-

Thanks for ur replies!

i read somewhere that a service factor motor is that the winding design will be with F class insulation but while running at full load the temperature rise will be equal to B class.The temperature rise will be equal to f class in SF load.is it true!

That may be a design theory used by one or more manufacturers, but it cannot be said universally. Many/most motors over 5HP are now Class F insulation anyway; automated manufacturing processes make it more economical for the manufacturers to standardize on one insulation material for their magnet wire. So what you may have seen is someones way of saying that they have a 1.15SF on their motor by essentially derating the insulation value to Class B even though they know it is Class F. That of course would NOT work if the motor nameplate says it is Class F AND 1.15SF. In that case, only a slightly larger frame (more mass) is going to suffice to increase the power capability.

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No, not necessarily - not unless you specify it that way.

Also, all motor efficiencies and other performance parameters are based on operation at 1.0 not 1.15. All NEMA says is that the motor will operate satisfactorily at 1.15 - as jraef said.

As consultants, we typically specify a motor with a 1.15 SF, but require the bhp of the driven equipment should never exceed the nameplate motor hp under any operating conditions. We allow a 1.00 SF motor if the bhp never exceeds 87% of the motor nameplate horsepower - but we don't see that option used too often

Friend81;

The following was taken from NEMA MG1 standard:

“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. “

Thanks for your classifications.

As my job is related with Submersible pumps & motors(deepwell) can anyone of u please tell me what will be the class of Insulation of Water filled rewindable submersible motors (Winding is made of water proof insulated copper wire). Normally manufacturers claim it as Y class?

But i have never seen anywhere the temperature ratings for Y class insulation.