Every electrical appliance converts energy -- stored as electricity -- into another form of energy; these can include movement, light or heat. An electric motor converts electrical power into movement, although some energy will be lost as heat and light. Knowing how much power an electric motor uses is helpful when calculating the size of the wires and circuit breakers to use with the motor. It may help you estimate the motor's running costs.
Amp usage will vary when a load if placed on a motor. If the motor does more work, it uses more amps.
If your motor is able to run at different voltages, the current will halve when the voltage doubles.
Always set a digital multimeter to the highest range setting before using it. For example if it can measure zero to10 amps through zero to100 amps, start with the 100-amp setting. Reduce it if your reading falls into a lower range.
Even low voltages can inflict a painful electrical shock. Switch off the power supply before connecting or disconnecting wires and meters.
Check the casing of the motor for a small plate. Most motors have a technical-data plate attached. This gives details of the motor's power, voltage and wattage. There may be two values for the wattage. One is running wattage, and the other is the watts used when the motor is starting. This will be the higher of the two values.
Calculate the amps used by dividing the wattage by the voltage. For example a 500-watt motor -- running on 50 volts -- will draw 10 amps. A motor with the same wattage -- running on 20 volts -- will use 25 amps. This is the theoretical number of amps used by the motor.
Switch off the motor, and then disconnect one of the wires powering the motor. Attach your ammeter or digital multimeter set to measure DC current; attach it between the motor and the detached wire. Always wire ammeters in series with a circuit, placing them between the power source and the appliance. Switch on the motor and observe the reading on the meter display. The reading will rise as the motor powers up, and then drop as it settles into normal running mode.
Use the amps calculated from the wattage plate if you need a quick "average" value for the amps used by the motor. Use a meter reading if you need an accurate value for the current drawn under specific conditions.
About the Author
David Robinson has written professionally since 2000. He is a Fellow of the Royal Geographical Society and the Royal Meteorological Society. He has written for the "Telegraph" and "Guardian" newspapers in the U.K., government publications, websites, magazines and school textbooks. He holds an honors Bachelor of Arts in geography and education and a teaching certificate from Durham University, England.