Electric Motor Service Factor

Motor service factor is probably the most misunderstood value on a motor nameplate.

Electric motor service factor.

To most it seems quite simple. Likewise for each power and speed the mg 1 standard defines a frame number which is linked to the service factor. The reference point is a service factor of 1 which would mean a motor cannot operate over its rated horsepower or hp without overheating. The service factor sf is commonly misunderstood as an allowable overload for electric motors for example you may find a 100 hp motor with a sf of 1 15 driving a 110 hp load under the incorrect assumption that it tolerates up to 115 hp.

Typical service factors sf for ac motors. The service factor sf is a measure of periodically overload capacity at which a motor can operate without damage. In the u s the defacto standard for ac motors is the nema standards. 1 15 x 1 1 15hp anything above this load will certainly result in motor overheating and consequent damage of the ac motor.

The canadian electrical code defines service factor as a multiplier that when applied to. A service factor or sf is a factor representing the capacity at which a motor can operate without overloading or damaging its core or components. For example the maximum motor load for a 1hp electric motor with a service factor of 1 15 is. Traditionally totally enclosed fan cooled tefc motors had an sf of 1 0 but most manufacturers now offer tefc motors with service factors of 1 15 the same as on odp motors.

Most hazardous location motors are made with an sf of 1 0 but some specialized units are available for class i applications. The nema national electrical manufacturers association standard service factor for totally enclosed motors is 1 0. Motors of 10 hp and up usually have a service factor of 1 15. When the motor is operated at any service factor greater than 1 it may have efficiency power factor and speed different from those at rated load but the locked rotor torque and current and breakdown torque will remain unchanged.

A motor operating continuously at any service factor greater than 1 will have a reduced life expectancy compared. A motor operating continuously at a service factor greater than 1 will have a reduced life expectancy compared to operating at at its rated nameplate horsepower. Most motors have a duty factor of 1 15 for open motors and 1 0 for totally closed motors. As per the standards a motor s nameplate should indicate the service factor as sf.

The rated horsepower of an ac motor to the rated armature current of a dc motor or.