Electrical Testing
Using a Simple Horsepower Calculation

Using a Simple Horsepower Calculation

Performing a horsepower calculation will help you determine which steps you need to carry out when replacing a motor.

Suppose a process line experiences motor failure every few weeks. The motor drives a grinding system that converts the raw materials from chunks to powder. After some analysis, the line’s process engineer tells you three things:

  • It’s not the motor’s temperature rating, because the motor isn’t exposed to process heat.
  • The feed rate has been increased to satisfy demand for higher throughput. Consequently, the system occasionally jams.
  • To solve the problems, the process engineer wants you to replace the motor with the next size up.

It’s a 40HP motor operating at 480V. Can you just install a 50HP motor in its place? Or do you need to increase the size of the branch circuit?


A quick way to answer this is to perform a horsepower calculation. This is a simple division problem; you’re going to divide A by B.

To determine A, multiply the horsepower by 746.

To determine B, multiply these four numbers together: 480V, 1.73, % efficiency, power factor.

Then, divide A over B.

But what are those quantities in B all about? It’s a 480V motor, so no mystery there. The 1.73 is the multiplier for three-phase power (if this were a 2-phase, 4-wire system the multiplier would be 2; there is no multiplier for single phase or DC).

The % efficiency can be assumed or you can divide output over input to calculate it. If you don’t have the output or input figures, you’ll have to assume. You could pick an arbitrary number such as 85%, or you could take measurements on a similar motor if you have one in a similar application.

You should use a power analyzer to measure the actual power factor as close to the motor as possible. Power factor correction capacitors at the service don’t correct for low power factor at the load.

Using 85% efficiency and 90% power factor, you calculate that a 50HP motor would draw about 71A. You look in the circuit panel and see the existing breaker is rated at 60A.

To increase the motor size, you have a bigger project than simply swapping out the motor. And that bigger branch circuit breaker raises another question. Is the feeder large enough to handle the additional load? If not, the project grows in scope again.

You determined all of this without having to contact a motor manufacturer for nameplate data. You did a quick calculation based on the horsepower rating. And it proved that the solution to the motor failure problem is not as quick and painless as the process engineer seems to believe.

It is probably better to install a feed rate control system that keeps the load within the capabilities of the 40HP motor the system was designed with. And make the next replacement a premium-efficiency motor, because such a motor will last longer in this application.

Throttling back on the throughput rate will actually increase overall throughput because you won’t have those motor failures. And if the company went through the expense of increasing the motor size, what would break downstream of that?

TAGS: Design Motors
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