How to check the amperage draw on an AC motor

Written by stephen benham
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How to check the amperage draw on an AC motor
Check the amperage output from your AC motor using a multimeter. (multimeter image by dinostock from

An AC motor uses alternating current to power it, and AC current changes directional flow 50 times a second. Three electrical windings on the outer part of the motor enable the central rotor to turn, otherwise they would simply vibrate moving backwards and forwards as the current alters direction. The amount of energy drawn to drive the motor is measured in amperes and is known as current. The greater the amperes the motor requires, the more powerful the motor. Don't confuse amperes and voltage, or assume that low voltage means low amperes. A car starter motor operates on 12 volts, but the amperes required to rotate the starter motor and the car's engine often exceed 50 amperes.

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Things you need

  • Protective rubber gloves
  • Multimeter

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  1. 1

    Read the amperage draw your AC motor requires, if it's operating according to the manufacturer's specification. The amperes are on the label on the AC motor.

  2. 2

    Set your multimeter to measure amperes. Set it to the correct ampere range for the AC motor you're checking. For example, if the motor draws 20 amperes, set your multimeter to read between 10 and 30 amperes.

  3. 3

    Put on rubber gloves to protect you from an accidental electric shock. Run your AC motor, otherwise you can't check the amperage draw.

  4. 4

    Locate the terminals on the AC motor. The positive terminal is labelled "+" and the negative "-." The wires connected to the AC motor are coloured red for positive and black for negative.

  5. 5

    Place the metal sensor on the end of the black wire from the multimeter onto the negative terminal of the AC motor, keeping your hands clear of all moving parts. Place the metal sensor on the end of the red wire from the multimeter onto the positive terminal of the multimeter.

  6. 6

    Read the multimeter display and then immediately remove the sensors from the AC motor. Turn off the motor. If the ampere reading is within the range you set on your multimeter, the AC motor is drawing the correct amperage. If it's below the range, get your motor checked as it may require maintenance such as new brushes. The amperes won't exceed the top figure in the range as the motor is unable to draw more amperage than set by the manufacturer.

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