Motor Service Factor vs. Temp. Rise

[theshockdoc]

When it comes to Motor Overload Protection and Art. 430.32 - is the Service Factor / Temperature Rise an "and / or" statement. In other words do both the Service Factor need to be 1.15 or greater and the Temp. Rise have to be 40 degrees C or less in order to use 125% ???

Myself, I have used the S.F. only and actually do not recall ever seeing a Temp. Rise on a motor (if I am not mistaken Amb.Temp and Temp.Rise are not the same thing).

From what I have read (and also comments) I believe it is an "or" condition but someone I work with insist it is and "and" condition.

One last note - as I was trying to read up on the subject I noticed that motors with a "better" insulation have a higher temperature rise. It would seem that the higher the temperature rise the more current a motor could in an overload condition.

Thanks for any enlighten on the subject.

 

[Jraef]

Temperature rise is a factor embedded in the motor insulation class, which is required to be on the nameplate. The more simplistic charts only give you the total max temperature, but that can be broken down into three components; ambient, rise, and hot spot. So if the design ambient is 40C (minimum NEMA designs by the way), the rise is 60C, and the hot spot is 10C, the total max internal winding temperature is allowed to be 110C. That by the way is Class A insulation, the lowest of the NEMA classes. But if I use that motor in a place where the ambient is 70C, the hot spot design doesn't change, so my maximum rise over ambient becomes only 30C. But that motor still has a MARKED temperature rise of 60C.

So in response to your second part, technically it is "or", but is situational, that's why they list it that way. For the most part any motor you use in an environment that is 40C or less will have way more than 40C rise capability. The only time you will see a temperature rise MARKED on the motor as less than 40C is when the motor is a custom design for a special machine, such as a cryogenic ally cooled system (LNG submersible pumps come to mind). But if it ALSO has a SF of 1.25, then you can adjust the OL setting if you must use the SF.

Keep in mind however that NEMA design specs allow the motor mfr to state that design life is foreshortened when running into the SF continuously. OEMs do it to keep from buying a larger motor, but the burden of that falls on the end user who has to replace that motor earlier.

In addition, understand that MOST OL relay mfrs ALREADY factor in the 115% to the settings and tell you to use the FLA, so of you plan on running into the SF, read THEIR instructions thoroughly on how to adjust the settings or heater selection, don't just go with a 125% FLA value.

 

[theshockdoc]

thanks for the reply and explanation.