What is AC Motor Service Factor (S.F) Rating?

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The service factor (SF) of an AC motor  is a measure of continuous overload capacity at which a motor can operate without overload or damage, provided the other design parameters such as rated voltage, frequency and ambient temperature are within the manufacturers’ specifications.

Motor service factor is crucial in determining the overload capacity of any given motor. The maximum load an AC motor can carry without overheating or otherwise damaging the motor if rated voltage and frequency are supplied at the motor’s leads is given by:

Maximum  Load  Capability of Motor = S.F x Rated or Nameplate Horsepower Rating
For example, the maximum motor load for a 1HP electric motor with a service factor of 1.15 is :
1.15 x 1 = 1.15HP
Anything above this load will certainly result in motor overheating and consequent damage of the AC motor.

Typical Service Factors (SF) for AC motors.
In the U.S, the defacto standard for AC motors is the NEMA standards. For totally enclosed motors (TEFC), the standard NEMA service factor is usually 1.0. Many manufacturers of TEFC motors often manufacturer motors with service factor of 1.15.  Below are typical service factors for Drip Proof Motors (DPM):
Horsepower Rating (HP) Service Factor(SF)/Synchronous Speed (RPM)
3600 1800 1200 900
1/6 , 1/4, 1/3 1.35 1.35 1.35 1.35
0.5 (1/2) 1.25 1.25 1.25 1.25
0.75 (3/4) 1.25 1.25 1.15 1.15
1.0 1.25 1.15 1.15 1.15
1.5 (1,1/2) or greater 1.15 1.15 1.15 1.15

Service Factor (S.F) Versus Motor Replacement
As seen above, the maximum load capability of an AC motor is = S.F x Nameplate Hp rating. Given this, never make the mistake of replacing an electric motor with the same horse power rating but with a lower service factor! It is recipe for disaster!
Rather when replacing an AC motor, the rule of thumb as far as service factor is concerned is:
NEW Motor MUST HAVE a maximum HP rating (= S.F x Rated or Nameplate HP) equal to or greater than that of the OLD Motor (bad motor) being replaced.

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