Electrical insulation systems rated by standard NEMA classifications to maximize allowable operating temperatures
Electrical insulation systems are rated by standard NEMA (National Electrical Manufacturers Association) classifications according to maximum allowable operating temperature:
|Temperature Tolerance Class||Maximum Operation |
|Allowable Temperature Rise at full load |
1.0 service factor motor 1)
1.15 service factor motor 1)
- T(oF) = [T(oC)](9/5) + 32
1) Allowable temperature rises are based upon a reference ambient temperature of 40oC. Operation temperature is reference temperature + allowable temperature rise + allowance for "hot spot" winding.
Example Temperature Tolerance Class F:
40 oC + 105 oC + 10 oC
= 155 oC
In general a motor should not operate with temperatures above the maximum. Each 10 oC rise above the rating may reduce the motor lifetime by one half. It is important to be aware that insulation classes are directly related to motor life.
Example - a motor operating at 180oC will have an estimated life of
- only 300 hours with Class A insulation
- 1800 hours with Class B insulation
- 8500 hours with Class F insulation
- tens of thousands of hours with Class H insulation
Temperature Tolerance Class B is the most common insulation class used on most 60 cycle US motors. Temperature Tolerance Class F is the most common for international and 50 cycle motors.