In section 1.42 of NEMA MG 1, service factor (SF) is defined as “a multiplier, which, when applied to the rated horsepower, indicates a permissible horsepower loading, which may be carried under the conditions specified for the service factor.”
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.
The National Electrical Manufacturers Association (NEMA) states that “A motor operating continuously at any service factor greater than 1.0 will have a reduced life expectancy as compared to operating at its rated nameplate horsepower. Insulation life and bearing life can be reduced by the service factor load”.
There are several factors that can lead to overload – usually, this is the result of the increase in motor winding temperature. Mechanical overload (moving heaving materials), poor ventilation, low voltage, or high ambient temperature are just a few of the root causes leading to overload. For every 10ºC, the motor winding expected is reduced by one-half.
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.
Service factor: What is it and what does it do?
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...
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The National Electrical Manufacturers Association (NEMA) states that “A motor operating continuously at any service factor greater than 1.0 will have a reduced life expectancy as compared to operating at its rated nameplate horsepower. Insulation life and bearing life can be reduced by the service factor load”.
There are several factors that can lead to overload – usually, this is the result of the increase in motor winding temperature. Mechanical overload (moving heaving materials), poor ventilation, low voltage, or high ambient temperature are just a few of the root causes leading to overload. For every 10ºC, the motor winding expected is reduced by one-half.