A critical factor in the reduced life of electrical equipment is heat. The type of
insulation used in a motor/generator depends on the operating temperature that the motor will experience. Average
insulation life decreases rapidly with increases in motor internal operating temperatures.
NEMA has established safe maximum operating temperatures for motors/generators
based on an average 20,000 hour lifetime. These maximum temperatures are the sum of the ambient and maximum
temperature rise ratings of the motor.
There are four NEMA insulation classes based thermal endurance of the system for
maximum temperature rating purposes. These are listed on the motor's nameplate and are either A, B, F, or H. These
codes indicate the maximum temperature the motor insulation can withstand without failure.
Class A insulation was the standard insulation used on older U Frame motors
between 1952 and 1964. Since 1964, T Frame motors use class B insulation as the standard insulation. Most common
fractional horsepower motors use either insulation class A or B.
Class B is used on most integral horsepower motors. Classes F and H are generally
used for NEMA motor designs which are special applications. Do not confuse the NEMA insulation classes with the
NEMA motor designs which are also given by letters.
NEMA insulation ratings assume the motor is operating within its rated ambient
temperature. Ambient temperature is the air temperature surrounding the motor and is also indicated on the
Motors should be replaced by motors with the same or higher insulation class to
avoid reductions in motor life and nuisance tripping of the motor overload device. Each 10 Degree C rise above the
motor's rating can reduce motor life by one-half.
Class A insulation has a recommended temperature limit of 105 Degrees C or 221
Degrees F. Class B goes to 130 Degrees C or 226 Degrees F, Class F to 155 Degrees C or 311 Degrees F, and Class H
to 180 Degrees C or 356 Degrees F. Insulation classes are directly related to motor life.
For example: A motor operating at 180 Degrees C will have an estimated life of
only 300 hours with a Class A insulation system. If Class B insulation is used, estimated life is increased to
1,800 hours. If Class F insulation is used, 8,500 hours of life can
be expected from the motor and with Class H insulation
motor life will increase to tens of thousands of hours.
All Hercules Generators have CLASS H