Service Factor the HP multiplier above which motor exceeds design temp. class or insulation class? 1

[Soloten]

I understand that the Service Factor of the motor is the multiplier above the nameplate HP up to which the motor is rated for a continuous running capability -- my question is, technically how is this continuous capability limit defined and what exactly changes when the motor exceeds nameplate SF?

For e.g. Consider a NEMA class B design 'temperature rise' motor with a class F insulation rating and service factor 1.15. When exceeding the service factor, is the motor exceeding its design temperature rise, or is it exceeding its insulation temperature class..or is SF value based on something else?

 

[waross]

My understanding is that the motor is slightly overbuilt.
eg: A motor designed and built as a 5.75 HP motor will be name-plated as a 5 HP motor with a Service Factor of 1.15

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

Service factor has no good definition. I forget which standard (maybe NEMA MG-1) has service factor, but it more or less just says that it's acceptable for the motor service life to be reduced when running the motor above rated power into the service factor. There is no definition for how much the service life is reduced when the service factor is used.

I would expect you will find that exactly what happens changes for every motor model line each manufacturer has. Heck, it could even change by the HP or frame size within one model line.

 

[jraef]

A long time ago, Service Factor had more definition in what you could do with it in that it was not intended to be continuous, they used the vague term "brief periods" or words to that effect, which you can still see reflected in some articles written on the subject. Then in the 1998 version, under pressure (I assume) from machinery OEMs that did not want that to get in the way of their sizing motors into the SF to avoid going to the next size up, NEMA removed the reference to it being time limited, implying that it could be continuous, but with the concession that one could expect it to be detrimental to the motor life. Here is the actual wording in NEMA MG-1 -1998:

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 and current and breakdown torque will remain unchanged.
A motor operating continuously at any service factor grater than 1 will have a reduced life expectancy
compared to operating at its rated nameplate horsepower. Insulation life and bearing life are reduced by the service factor load.

That's all it says, no further definition.

But it does say that the Temperature Rise for motors with a 1.15SF and higher are 10 degrees C higher than motors with a 1.0SF (except for Insulation Class H, which does not change). So for example Class F is 105C for motors at 1.0SF, 115C for SF 1.15.


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