Nema Insulation Classes

[fifty60]

Are all Nema classes based off of a 40 C ambient? I have a class B motor that is operating at 52.5 C, it has not tripped its thermal overload. I am not sure what the service factor is for the motor. The nameplate does say "Class B" insulation.

How far away from the motor should I make my measurements?

 

[fifty60]

Would the ambient temperature be the temperature the motor would settle to if it were shut off? If so, this would indicate that you have to get out of the "near field" temperature of the motor, so probably 2" or so away.

 

[Jraef]

Are all Nema classes based off of a 40 C ambient? I have a class B motor that is operating at 52.5 C, it has not tripped its thermal overload. I am not sure what the service factor is for the motor. The nameplate does say "Class B" insulation.

How far away from the motor should I make my measurements?

The Class number denotes an allowable temperate RISE above whatever the nameplate stated ambient rating is. USUALLY it is 40C, but it is supposed to be shown on the nameplate (I have seen some cheapies that are rated at only 25C... SURPRISE!!).

So let's say your motor is rated 40C, and has Class B insulation, which means a maximum RISE of 80C at a 1.0 Service Factor, 90C if it is a 1.15SF. The difference is that a a 1.0SF, you have an allowable "hot spot" additional rise of 10C, but at the 1.15SF you have already used that up, so your allowable maximum is not changed. That means that the COMBINED maximum insulation temperature on a 40C ambient rated motor with Class B insulation can be 130C (229F).

If you do not have embedded temperature sensors telling you the INTERNAL temperature at the stator slots however, you cannot tell what the ACTUAL temperature of the winding insulation is unless you do a specific procedure called a "resistance test", outlined in the NEMA standards. If you are measuring 52.5C with an IR gun on the outside of the frame, that is relatively meaningless.

 

[fifty60]

Using this chart, and a class B insulation, would I just add 15 degrees to the class B curve to get my motor insulation lifespan? If I am at 20,000 hours at 130 C, at an ambient of 55 C, I would go to 145 C on the curve which would be approximately 7000 continuous hours?