Motor Selection


Electric motors operate at their best power factor and efficiency when fully loaded so you do not want to purchase a motor that is too big, and common sense dictates that one that is too small is even worse.

In the following paragraphs we are going to learn how to select the correct motor for your centrifugal pump application.

Let’s assume we will be selecting the motor for the pump described by the pump curve shown below.

The first thing we must do is decide what diameter impeller we will be using. The above curve shows impeller diameters from “A ” to “E”.

I have selected letters rather than numbers so that we can work the examples in either metric or inch units.

For our example we will use impeller size “A”. You will want to look to the right hand side of the curve to select the last efficiency line. In this case it is the 50% line. This will give you the maximum capacity for that size impeller.

Note the capacity at this point (400) and then transfer this capacity and impeller size to a second graph (many times this information is part of the pump curve or located very close to the pump curve) that is supplied by the pump manufacturer.

The second graph will look something like the one illustrated below.

The numbers on the left side of the graph show either the brake horsepower or the kilowatts being consumed. You can select the appropriate units for your application. According to this graph we will be using about 20 (brake horsepower or kilowatts) at the last efficiency line (400).

One assumption we made during this selection process was that the specific gravity of the fluid we were pumping was one (like fresh water). If the fluid has a higher or lower specific gravity we must multiply the number on the left-hand side of the graph by the specific gravity of the fluid to get the correct horsepower or kilowatts for your application.

If the pump were sized correctly for the application it would run within ten percent of its best efficiency point. For impeller size “A” that would be approximately 325 (as shown on the first graph) so we are going to take advantage of the pump service factor (I’ll explain that in a few minutes) to give us the needed horse power if we should occasionally run at this higher capacity (400) or get into any other temporary overload condition such as starting a pump that is rotating backwards.

The service factor rating 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.

Our graph showed that we needed a 17 horse power motor, but 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
  • A 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 (A.P.I. 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. (American Petroleum Institute) 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 considering the specific gravity of the liquid being pumped. If a low specific gravity pump is tested with water, or any higher specific gravity fluid, the increase in motor amperage could burn out the motor.