Service factor for electric motors


The service factor rating for an electric motor is supplied by the motor manufacturer and is usually available in three ranges:

  • A service factor of 1.00 / 1.10 – most of these are older motors and a majority of them have undesirable aluminum windings.
  • A service factor of 1.15 – this is the most common service factor used in modern motors.
  • A service factor of 2.00/ 2.50 – These motors are seldom in stock and have to be built at a premium price.

Motors are available in a variety of horsepower and kilowatt ratings. Typical horsepower ratings would be 0.5, 1.0, 1.5, 2.0, 3.0, 5.0, 7.0, 10, 15, 20, 30, etc.

As an example: suppose we needed a 17 horse power motor in our application. The fact is that a 15 horse power motor will work in this application because of the service factor (15 x 1.15 = 17.25 horsepower available). Keep in mind that any heat generation computations made by the motor manufacturer were made for the motor when it was running at its rated horsepower and not at the service factor rating. All this means is that the motor will run hotter than anticipated, but still within acceptable limits.

Oil refinery applications use a second factor recommended by the American Petroleum Institute (API). This organization specifies that the factor should be used as an additional safety margin. These factors are:

  • To 25 horsepower (18.7 kw) = 1.25
  • From 30 to 70 horsepower (22.4 to 52.2 kw) = 1.15
  • Above 100 horse power (74.6 kw) or more = 1.10

If we take the same example as noted above and insert the API (American Petroleum Institute) additional requirement we would come up with:

  • If 20 horsepower is needed x 1.25 (API (American Petroleum Institute) specification) = 25 horsepower needed.

There are instances where you can combine the two service factors and come up with a compromise. As an example, suppose that the horse power requirement was 8.7 instead of the 20:

According to the A.P.I. you would need 8.7 x 1.25 = 10.8 horsepower, so you would have to go to a 15 horse power motor because there is nothing in between 10 and 15 horsepower. According to the above information a 10 horse power motor has a service factor rating of 1.15 so, 10 x 1.15 = 11.5 horsepower or more than enough to satisfy the API (American Petroleum Institute) recommendation.

Electric motors are sized taking into consideration the specific gravity of the liquid being pumped. If a low specific gravity pump is tested with water, or any other higher specific gravity fluid, the increase in motor amperage could burn out the motor.