Insulation Classes and Limits

Insulations used in Electrical machines have been classified into 4 classes. They are A, B, F, H. The classification has been made by NEMA( the National Electrical Manufacturers Association).

All insulation are standardized to be operated at 40 ºC. However, the operation temperatures of a motor or a generator will be higher than this. The maximum operating temperature for the different classes of insulation are

Class

Max. operating temp(ºC )

Max. Permissible temp rise ºC

Allowance for Hot spot

ºC

A

105

60

5

B

130

80

10

F

155

105

10

H

180

125

15



The Maximum operating temperature is the maximum temperature the insulation can reach during operation. It is calculated as
Max. operating temperature = standardized operating temperature(i.e. 40 ºC) + maximum permissible temperature rise + Allowance for hot spot in winding.

The Maximum permissible temperature rise is the temperature by which the operating temperature temperature can exceed the standardized temperature i.e. 40 ºC.

The allowance for hot spot refers to the allowance in temperature allowed for hot spots which may form in the centre of the winding.

Therefore, a class F insulation will have the maximum value calculated as 40+105+10 = 155 ºC

Exceeding the maximum operating temperature will affect the life of the insulation. As a rule of thumb, the lifetime of the winding insulation will be reduced by half for every 10 ºC rise in temperature




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