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Temperature Rise

Excessive heat shortens the life of a motor. Motor losses dissipate as heat and are removed from the motor by the motor cooling system. The motor cooling system includes internal or external fans, fins, and even coolant-based systems for special application motors. While losses increase with the load, the cooling ability of the motor does not; therefore, the temperature of the motor increases with load. When a motor application overloads the motor or restricts air flow over the motor, inhibiting the fans and fins most commonly used to cool the motor, the motor temperature can rise over the designed limit shortening the life of the motor.

Overloading the motor, improper supply voltage (high/low), frequent start/stops, poor input power quality, restricting the cooling system and using variable frequency drives increase temperature rise. This heat destroys motors in two main ways:

Heat reduces the expected life of lubricants and seals. At higher temperatures, the soap component of lubricants that time-releases oil becomes sticky increasing friction losses, reducing the life of the bearing and leaking the released oil onto the motor windings. Lubricating oils deteriorate winding insulation causing premature winding insulation failure.
Winding insulation in motors is classified by its life expectancy at various temperatures. When the rated temperature is exceeded, the insulation breaks down at a faster rate resulting in premature failure.