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Service factor: What is it and what does it do?

  • August 2015
  • Number of views: 3338
  • Article rating: 1.6

Jim Bryan
EASA Technical Support Specialist (retired)

There are many misconceptions about Service Factor (SF) in the industry. Some feel it is meant for temporary excursions into overload conditions; others consider it to be an allowance for permanent overload. The truth is that it is neither. As defined in the EASA Technical Manual and NEMA MG1, the definition of service factor is:

"A multiplier which, when applied to rated power, indicates a permissible power loading that may be carried under the conditions specified for the service factor."

The NEMA MG1-2011 theory of SF says that a motor is thermally capable of overload to that point within the insulation class at normal service conditions.

Since any increase in load increases the current, this overload will increase the operating temperature of the motor. For every increase of 10ºC, the motor winding expected life is reduced by one-half. It does not matter what the source of that increase in temperature is; overload, poor ventilation, low voltage or high ambient temperature are just a few.

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