Dc Resistance Of A Conductor

NEMA motor insulation classes describes the ability of motor insulation in the windings to handle heat. There are four insulation classes in use namely: A, B, F, and H. All four classes identify the allowable temperature rise from an ambient temperature of 40° C (104° F). Classes B and F are the most common in many applications.

Temperature rises in the motor windings as soon as the AC motor is started. As shown in the table below, the combination of ambient temperature and allowed temperature rise equals the maximum rated winding temperature. Allowable temperature rise is made up of the maximum temperature rise for each insulation class plus a hot-spot over-temperature allowance. If the motor is operated at a higher winding temperature, service life will be reduced. As a rule,
a 10° C increase in the operating temperature above the allowed maximum can cut the motor’s insulation life expectancy in half.

The table below shows the different insulation classes as defined by NEMA:

Class	Maximum Ambient Temperature (°C)	Maximum Temperature Rise (°C)	Hot-spot Over Temperature (°C)	Maximum Winding Temperature (Tmax)(°C)
A	40	60	5	105
B	40	80	10	130
F	40	105	10	155
H	40	125	15	180

Hot-spot Over Temperature Allowance

Each insulation class has a margin allowed to compensate for the motor’s hot spot. The hot-spot is a point at the center of the motor’s windings where the temperature is higher. As can be seen from the above table, hot-spot over temperature allowance for A, B, F and H are respectively 5°C, 10°C, 10°C and 15°C

When replacing a motor, extreme care must be taken not to choose a motor with the wrong insulation class. It is advisable therefore to replace a motor with one having an equal or higher insulation class. Replacement with one of lower temperature rating could result in premature failure of the motor.