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How to Size an Electric Motor for a Combustion Engine

The horsepower rating of an electric motor is not directly comparable to that of an internal combustion engine driving the same load. Internal combustion engines are rated for maximum attainable horsepower, while an electric motor is rated according to the horsepower it can supply on a continuous basis. Electric motor ratings also include a power factor, which is basically a measure of the efficiency of the motor in comparison to other electric motors. In addition, physical sizing should be taken into account to ensure that the electric motor will fit and operate properly without requiring major modifications.

Things You'll Need

Combination wrench set
Calculator
Allen wrench set
Tape measure
Caliper

Instructions

- 1
  Use the combination wrench set to unbolt and remove the original internal combustion engine. Locate the nameplate to determine the horsepower and rpm ratings. Match the rpm rating, plus or minus five percent. Multiply 0.6 by the original engine horsepower, then multiply the result by the power factor of available electric motors to determine the minimum electric motor horsepower rating for the equipment.
- 2
  Measure the distance between mounting holes in the motor mount plate. Narrow your search for a new motor to those which have a compatible footprint.
- 3
  Use the Allen wrench set to remove the set screw from the pulley on the original motor. Slide the pulley off the engine shaft and use the tape measure to determine its outside diameter. Use the caliper to measure the diameter of the motor shaft. If you can match the shaft diameter with the new motor, you can reuse the original pulley. Otherwise you will need a new pulley with the same outer diameter and a bore to match the shaft of the new motor.

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