Motor Temperature Rating 1

[kingjoey]

Hello,
I have a motor with a temperature rating on the nameplate that says:

AMB 50 Degrees C

Is this the maximum allowed temperature rise above ambient or is this the maximum allowed temperature of the motor?
No other termperature reference is on the motor.

This is an older motor from the late 1960's.

Thanks for any help.

Kingjoey

 

[dpc]

It means the motor was designed to operate at full nameplate output at an ambient temperature of 50 deg C. Standard NEMA motors are rated at an ambient of 40 deg C. So the motor is basically oversized to operate at a higher ambient temperature while maintaining the rated winding temperature.

 

[kingjoey]

Thank you dpc.

Another question - The motor has class F insulation. Does this mean that the motor windings can be operated at a 105 deg C rise above a 50 deg C ambient since class F insulation is capable of 155 deg C?

Thanks again.
Kingjoey

 

[Skogsgurra]

I would say no. The hot spot is 155 C for a class F insulation. That remains, regardsless of ambient. So, as dpc says, the motor is oversized so it will not heat above 155 C if run at 50 C ambient. Normal motors would get hotter (165 C), which is too much for class C.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[kingjoey]

Thank you Skogsgurra.

I quess that my basic question is:

How hot can I operate this motor?

I am using an infrared device to measure the outside temperature of the motor. I hope that the readings are representative of the winding temperatures.

Thanks again,
KingJoey

 

[LionelHutz]

I'd guess and say the allowed case temperature would be somewhere around 80C to 90C but only after the motor has run at steady state operating conditions long enough for the temperature to be stable. The case temperature will not be as hot as the windings and will only be representative of winding temperatures during steady state operation. The case temperature will not follow the winding temperature for short duration loading or starting.

The big problem is that it is highly unlikely that anyone can tell you exactly what the case temperature will be when the windings are close to 155C. I doubt any motor manufacturer would even tell you.

 

[dpc]

I totally agree with Gunnar. The maximum allowable insulation temperature plus hot-spot allowance doesn't change because of the ambient temperature rating. Class F is Class F, and it doesn't know how hot the ambient air is.

The max allowable hot spot temp is 155 deg C. That is the key figure. Everything else is worked out backwards from there. With a 10 deg hot spot allowance, that gives an average temperature by resistance of 145 deg C. With 50 deg C ambient that gives an average temp rise of 95 deg C.

The 50 deg C ambient rating means that a 100 hp motor can operate at 100 hp in a 50 deg C ambient. For a normal 40 deg C motor, the 100 hp rating would have be reduced at the 50 deg C ambient.

 

[jraef]

I agree that you cannot accurately represent winding temperature from an IR scan of the case. Too many variables involved such as surface area, thermal transfer rate, air flow (both internal and external as applicable), radiation fin design, obstructions etc. etc. The only valid measurement of winding temperature is with embedded sensors, preferably RTDs.

 

[aolalde]

The temperature shall be measured in the windings, not the external motor surface. Taking the resistance of one phase with the motor at room temperature and then comparing it to the same phase resistance after working the motor under load is a way to measure the winding temperature. See IEEE Std 112 for details of the procedure.