How to calculate three phase motor current using motor winding resistance

Written by will charpentier
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If you need to find out how much current is being used by a three-phase motor, know the motor winding resistance of the three-phase motor and the voltage the three phase motor is operating at. Calculating the current -- which is measured in amps, or amperes -- is little more than a process of confirming the exact voltage and resistance with a volt meter or a multimeter, then applying the formula electricians call Ohm's Law.

Skill level:

Things you need

  • Volt meter
  • Multimeter
  • Calculator

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  1. 1

    Use a volt meter to measure the incoming three-phase voltage. The voltage can be measured at the motor's brushes by turning the voltmeter dial to the correct voltage range and touching the probes to the motor's power connection.

  2. 2

    Confirm the motor winding resistance using a multimeter. The resistance can be measured by holding the probes of the volt-ohm meter on the brushes while the power is off.

  3. 3

    Use Ohm's Law to determine the current. Current flows on the surface of a conductor (like a motor winding) from negative to positive and is expressed in amperes, or amps. Divide the voltage by the motor winding resistance to determine the current used.

    If the voltage is 240 VAC and the motor winding resistance is 15 ohms, the current used is 240 / 15 = 16 amps.

Tips and warnings

  • Taking direct measurements from operating electrical equipment can be hazardous. Use appropriate caution.

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