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April Tech Tip/ Does it matter that two different 1/3 horsepower motors have different Amp ratings?
Does it matter that two different 1/3 horsepower motors have different Amp ratings?
Yes.
Well, that was short and sweet. But there is more to it than that. The type of motor, such as shaded pole, permanent split capacitor (PSC), split phase, capacitor start, capacitor start/capacitor run, three phase, or electronically commutated (ECM), will influence the Amp rating of the motor. This article will focus on PSC motors.
Let us assume that when we select a replacement motor, it has to be the correct voltage, phase, frequency, RPM, number of speeds, mounting, shaft, enclosure, insulation class, ambient temperature rating, etc. as the original. When it comes to selecting the proper output power of the motor, many technicians will base their decision strictly by the horsepower rating on the nameplate of the motor. At first glance that appears to be a perfectly fine method for getting the right strength of motor. However, this could result in selecting a motor that could be too weak or too strong. Having a motor that is either too weak OR too strong could result in shortened motor life. How could that occur? If the replacement is the same horsepower it will be the same output, correct?
Let us look at output power of a motor. Remember, this article is comparing two PSC motors. As mentioned in a previous Tech Tip, all PSC motors, in general, have the same power factor, which is about .9. Watt’s law shows that
Watts = Volts x Amps x Power Factor
With PSC motors having a power factor near 1, Watt’s law is often simplified to
Watts = Volts x Amps
Watts are a measurement of output power, as is horsepower. Watts can be converted to horsepower, and vice versa, using the following relationship
746 Watts = 1 Horsepower
There are a lot of Watts in one horsepower! And the Watts provide a more defined measurement of output power as compared to horsepower. Let us assume that I have two PSC motors of the same speed and same voltage, and they are both rated 1/3 horsepower on the nameplate. However, they have different Amps. Remember, they have the same voltage rating, and let us assume they are both 230 Volts. With that being the case, the motor with the higher Amps rating would be the stronger motor. It would have higher Watts.
Let us assume that Motor #1 is rated 1.2 Amps. It would be equivalent to about 248 Watts, truly a 1/3 horsepower motor. Let us also assume that Motor #2 is rated 1.7 Amps. It would be equivalent to about 352 Watts, nearly 1/2 horsepower in output. If the original motor were properly matched to the load and it had a nameplate Amp rating of 1.2, the replacement motor would be too strong if we used Motor #2. It may not seem like a big deal, but by using the stronger motor with a load appropriate for the weaker motor, the stronger motor, Motor #2, would operate inefficiently when matched with the load. This would result in the motor operating hotter than its design operating temperature. This would shorten the life of the motor.
Most air moving motors in HVAC/R are designed for a very specific load. When replacing one of these motors, we should select the replacement by using nameplate Amps of the original motor and of the replacement motor. The replacement motor should be within an Amp range of equal to or no more than 25% stronger than the original motor’s nameplate Amps.
To assure that the replacement motor is loaded properly, once it is installed (with all guards, filters, doors, and panels in place) the operating or full load Amps (FLA) of the replacement motor should be no more than 10% above or 25% below the nameplate Amps of the replacement motor.
By following these guidelines, you will be able to match power output more precisely between the original motor and the replacement motor. You can rest assured that you have chosen the correct replacement motor!
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