When should I derate (oversize)?
A. All inverters generate more heat in the motor than a pure sine wave does. This was particularly true with older 6-step inverters where derating the motor by one horsepower was standard practice.
With Pulse Width Modulation (PWM) inverters, the heating is less, allowing closer horsepower matching, most of the time. However, some motors should not be run at full load powered by an inverter even at 60 hertz.
Extreme caution should be used if the motor has class F temperature rise or a 1.0 service factor. Derate these motors or go to the next higher rating. Motors to be used on inverters should be wound in F or H insulation but operate with only a class B rise at full load. They should also have a 1.15 service factor. If the ambient temperature is high or the altitude is more than 3,300 feet, still derate.
For help in selecting the right motor and inverter for your application, contact your local Applied® service center.
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