How to Troubleshoot Three-Phase Motors

Three-phase motors are found in many industrial applications and are one of the workhorses of medium and light industry. As such, they are the first element to be blamed when a "problem" occurs, whether or not they are at fault. Part of the troubleshooting process involves the elimination of that possibility, part of the process involves safety and part of the process simply requires some basic electrical skill.

Difficulty: Challenging

Instructions

Things You'll Need:

• Volt meter
• Ohm meter
• Screwdriver
• Combination wrenches

1. Measure voltage at motor terminals with a voltmeter. If the voltage is within 10 percent of the rated value, the voltage is correct.

2. Turn off and lock out the power to the motor. Use the accepted lock-out-tag-out procedures for the facility.

3. Disconnect the load from the motor.

4. Turn on power and try starting the motor. If the motor runs, the problem is with the load, not the motor. If the motor does not run, turn off and lock out power to the motor.

5. Open the case of the motor by removing the screws from the back of the case. Check the windings with the ohm meter to determine whether there is an "open" or a "short" at the windings. If the resistance is "0," there is a short. If the resistance is infinite, there is an "open." If the resistance reads between 0 and infinity, the bearings are frozen and the motor should be repaired or replaced.

Tips & Warnings

• Take a spare motor to the location of the problem. If the motor doesn't run without being under a load, replace it with the new motor and take the old motor back to the shop to have the windings checked. If the windings are good, set the motor aside for a bearing rebuild.
• This project involves electrical work and appropriate caution is advised.

References

• Fluke Electrical Testers Application Guide

Resources

• Refrigeration and Air Conditioning Technology