What Is the Insulation Class of the Motor?

Temperature insulation classes give an indication of the maximum temperature that an
insulation material may be exposed to without deterioration. They are as follows:

Class A: 105 degC. Hot spot margin 5 degC

Class B: 130 degC. Hot spot margin 10 degC

Class F: 155 degC. Hot spot margin 10 degC

Class H: 180 degC. Hot spot margin 15degC

In a motor, this will refer to the insulation on the winding wire (enamel), supplementary
insulation and the material used to wedge the windings into their slots. The hot spot margin
reduces the motor maximum allowable operating temperature by the indicated amount, as a
safety factor.

Typical squirrel cage induction motors are constructed with Class F insulation materials and are
designed for a Class B temperature rise at full load (assuming 40 degC ambient temperature).
This gives a 25 degC margin for most of the windings and a 15 degC margin for hot spots.
These margins ensure a long insulation lifetime and allows for short term overloads of the
motor without damage.