AC Induction Motors

Motor Insulation Temperature Classes :

Motor insulation classes are set by NEMA , the National Electrical Manufacturers Association. The most common classes are depicted in the table below.

NEMA Motor Insulation Classes
Insulation Class Winding Temperature
A 221 °F  (105 °C)
B 266 °F  (130 °C)
F 311 °F  (155 °C)
H 356 °F  (180 °C)

The numbers in the table are the average temperature of the winding. Some parts of the winding may be up to 18 °F (10 °C) higher than the average temperature. The numbers are based on a rated life of 200,000 hours (22.8 years). That is, a winding with class B insulation operating at 266 °F  (130 ° C) should last for 200,000 hours. The rule of thum is that for every 18 °F (10 °C) over the rated temperature, the life of the winding will be halved.

Class B & Class F insulation systems are common today. Class F insulation systems are special motors for very heavy use at high ambient temperature and/or at high altitudes. Class A insulation systems were used on motors built in the 1960's and earlier.

Shaded Pole Motors

Shaded Pole Motors produce a rotating magnetic field by placing heavy, single turn, shorted copper coils into the steel core of the motor.

Bathroom fans, vent hood exhuast fans, and can openers use shaded pole motors.


Shaded Pole Fan Motor (F04-21).


Disasembled Shaded Pole Fan Motor (F05-20).

New Bathroom Fan Motors Have a Thermal Fuse

New Bathroom Fan Motors Have a Thermal Fuse (F04-21).


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