How to Troubleshoot a 3-Phase 480V Motor

Updated July 20, 2017

Commercial electrical motors typically run on 480-volt, three-phase power. Three-phase power supplies are high-voltage systems that employ multiple phases in a single circuit to supply a flow of alternating current that is virtually constant. Single-phase power, on the hand, uses only one phase, so that the resulting current is discontinuous. Occasionally, a three-phase motor will stop working properly. To troubleshoot the motor, a multimeter is used. In Step 1, you will be working with live electrical current. Follow all appropriate electrical safety precautions. This motor produces enough voltage to kill you. Consult a professional if you are not absolutely sure you can perform the task safely.

Turn the multimeter on, and touch the meter's two probes together. This will cause the meter's gauge to reset to zero, which will give you a more accurate reading.

Set the meter to the voltage reading, then apply one probe to one of the three motor phases and the other probe to the motor ground. Repeat this process for all three phases of the motor. Make sure you read the meter to see if it is being supplied with the proper voltage. Remember that you are testing a live motor to see if the proper voltage is being supplied to the motor. If the voltage is incorrect, shut the power off at the main breaker and inspect the connections and breakers at the power supply.

With the main power off, lock the breaker with a padlock to prevent it from being turned on. Use the voltage reading on the multimeter to ensure that the main power is off. Inspect the circuit breaker and all of the electrical connections to be sure that proper contact is being made between the power leads and their termination points. Check the grounding circuit in the same way. Tighten any loose connections with a flathead screwdriver.

Remove the motor cover with a flathead screwdriver, and inspect the termination points for any loose connections. Tighten any loose connections with a flathead screwdriver.

Set the multimeter to the ohms reading, then touch one probe to the motor winding and the other probe to the motor ground. Any reading below 20 megohms indicates a problem with either the insulation or the connections at the termination points. Any reading above 30 megohms also indicates a problem with either the insulation or the connections at the termination points. Inspect the insulation on the circuit as well as the connections at the termination points. Tighten any loose connections using a flathead screwdriver.

Replace the motor cover, unlock the circuit breaker and restore power. If the motor emits a burning smell, shut the power off, then remove the motor cover, and clean the motor windings and the motor housing with compressed air. If the burning smell persists, the motor must be replaced.

Things You'll Need

  • Multimeter
  • Flathead screwdriver
  • Padlock
  • Compressed air
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About the Author

Eric W. Thompson began his writing career in 1996 and is now a member of the All-USA Academic Team, having been featured in "USA Today" as one of the top 20 community college students in the country. He is currently taking a break from earning an undergraduate degree in contemplative psychology at Naropa University.