How do I classify the temperature limits of an Electric Motor?

BS EN 60034-1 classifies the temperature limits (and rise limits) of insulation materials. It specifies the maximum permissible temperatures that the various classes of insulation materials can withstand.

The temperature rise of an induction motor is the permissible increase in temperature above its maximum ambient which is normally 40°C. Insulation classes B, F and H have attainable temperature limits of 130°C, 155°C and 180°C respectively. These are the maximum temperatures to which the insulation materials within the motor can be exposed for short periods of time without the risk of permanent damage. The insulation materials used in classes B, F and H are generally the same as each other, these are materials such as mica, glass fibres and asbestos, with the main difference lying in the high-temperature binders which bond the materials together.

Insulation Class Max. Ambient Temp. Max. Temp. Rise (1.0 service Factor) Max. Temp. Rise (1.15 Service Factor) Max. Attainable Temp.
B 40°C 80°C 90°C 130°C
F 40°C 105°C 115°C 155°C
H 40°C 125°C Not defined 180°C

Submersible AC squirrel cage motors tend to come with class “F” insulation. However they also need to have an ingress protection rating of IP67 or greater, depending on the length of time they are submerged. These motors are designed to operate at an ambient temperature between -20°C and +40°C. Higher ambient temperatures up to +60°C would require a reduction of the power output as listed below.

Ambient Temp (°C) Power Output %
40 100
45 95
50 90
55 85
60 80

The maximum temperature that a motor may run at is also linked to its service factor, this is a percentage multiplier of the motors power rating. For example; motors with a service factor of 1.15 may operate at 115% FLC for short intervals only.

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