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How to Test a Variable Speed 3-Phase Motor

Variable speed three-phase motors are usually tested the same as a single speed motor. That is because most three-phase motors can become variable speed motors simply by adding a frequency controller circuit between the motor and the power supply. The important things to test are the resistance of each field winding and the resistance between each field winding. This will tell you if any of the coils have problems, such as shorts or opens.

Things You'll Need

Digital multimeter

Instructions

- 1
  Shut off power to the motor at the main power panel. Visually inspect the motor. Look for any signs of motor damage, such as broken or melted insulation. Also check for broken pieces and proper ventilation.
- 2
  Disconnect the wires feeding the motor and be sure to make note of how the motor was wired on a piece of paper. Set the digital multimeter to read resistance.
- 3
  Start with the first field coil and measure the resistance of that field coil by placing one digital multimeter probe at each end of that specific coil. Write down the resistance value. Repeat this step for each of the three field coils.
- 4
  Measure the resistance between each field coil by placing one probe on one end of field coil "A" and the other probe on one end of field coil "B." Repeat this step for each of the three possible coil combinations. Write down the results for each measurement.
- 5
  Compare the three values of each column. Each value for the field coils should have a deviation of no more than 5 percent. The same holds true for the values measured between the field coils. Large deviations mean that coil is bad and will have to be repaired.

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