Message Board Thread - "Skin vs Insulation Temp"

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Skin vs Insulation Temp danny_b 2/6/2004
Does anyone have any knowledge or references on figuring out the temp of the insulation of a motor based on skin temp of the motor.

What is expectable for the skin temp of a motor with F insulation class?
Re:Skin vs Insulation Temp walterdaniel 2/6/2004
From experience, I check about 40ºC on wound in the chassis, around the conection bus of the motor. Then to the max. temperature reading subtract 40ºC plus your segurity percentage.
Re:Skin vs Insulation Temp jkstyron 2/9/2004
Most motors are designed to run in a 40 C ambient maximum. They will also be rated for a "Class Rise", B Rise or F Rise. New motors are typically rated B Rise on 60Hz power and F Rise on VFD power. Never the less they all typically have Class F insulation. NEMA will designate what B Rise and F Rise are. I think that B Rise is 20C and F Rise is 40C???? This temperature rise is a rise above ambient for any operating point on the motor. Naturally, the internal windings of the motor are typically the hottest. So, for a motor rated at 40C ambient and F Rise, the hottest spot internal to the motor should never be more that 80C. If I remembered the Class Rise correctly.

Skin temperature is going to vary widely based on; ambient, load, mounting (belted or direct coupled), VFD power and how it is being cooled; TENV, TEFC or TEBC. Of which, TEFC, cooling is dependent on motor speed, due to the fan being shaft mounted. As you can see, there are a lot of variables.

Remember, on motors you can look at the stator body to see if it is over loaded. You can also look at the motor leads for over load or phase imbalance. Direct coupled bearings shouldn't ever get too hot unless something is wrong. Belted connections should be warmer, but not dramatically.
Re:Skin vs Insulation Temp Gary Orlove 2/10/2004
See also

Gary Orlove
Infrared Training Center
Re:Skin vs Insulation Temp Andrew 3/4/2004
Motor shin temperature depends on the type of motor and surroundings, such as ambient temperature, wind speed around the motor, etc. Class ‘F’ AC motors are normally rated: ambient 40°C, rated rise 60°C and maximum allowed 145°C. Personally I would be concerned if on typical ‘small’ industrial motors the skin temperature was in excess of 120°C. At this temperature the motor is likely to be operating at or above the maximum allowed winding temperature. In dirty industrial environment there is a possibility that the motor cooling fan gets blocked. So, if you see a hot motor always check the motor fan before you take any more drastic action.

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