How to determine actual gearmotor horsepower?

Even though electric motors have been around for a long time, they still are the cause of much perplexity, especially surrounding key parameters such as frame size, torque, and horsepower. And gearmotors can add an extra dimension of confusion.

Read motor nameplates carefully and if not entirely sure of what the numbers indicate, ask the gearmotor manufacturer for clarification. (Source: Bodine Electric)

Specifically, when we say gearmotor, we’re talking about the entire unit; the motor and gearing combined. And it’s here that things can get a bit tricky sometimes. When speaking of gearmotor horsepower, oftentimes people think of the motor horsepower alone. This is not accurate because the horsepower for the entire gearmotor is the combined effect of both the motor horsepower plus the action of the gears.

Sometimes manufacturers may list under horsepower only the motor horsepower. This, of course, doesn’t account for the influence of the gearing on total system efficiency and power. The only way to be sure is to ask the company if the horsepower rating is for the motor only or if it includes gear efficiency.

In simple terms, there is a surefire way to calculate the actual gearmotor horsepower. The fairly straightforward approach is to calculate the total gearmotor system power as the motor output power multiplied by the gear efficiency, or:

Power_(total) = Power_(motor) X efficiency_(gear)

In this equation, it’s obvious that a perfectly efficient gearbox (no losses, 1:1 power transfer) would result in the system power being equal to the motor power. Since this is not realistic, the total power will always be less than the motor power, as efficiency is always less than 1.