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Limited number of components means fewer problems in three-phase motors

Three-phase motors have fewer components that may malfunction than other motor types. Therefore, three-phase motors usually operate for many years without any problems.

If a three-phase motor is the problem, the motor is serviced or replaced. Servicing usually requires that the motor be sent to a motor repair shop for rewinding. If the motor is less than 1 HP and more than five years old, it is replaced. If the motor is more than 1 HP, but less than 5 HP, it may be serviced or replaced. If the motor is more than 5 HP, it is usually serviced.

Troubleshooting Three-Phase Motors

The extent of troubleshooting a three-phase motor is dependent upon the motor's application. If the motor is used in an application that is critical to the operation or production, testing is usually limited to checking the voltage at the motor. If the voltage is present and correct, the motor is assumed to be the problem.

Unless it is very large, the motor is usually replaced at this time so production can be resumed. If time is not a critical factor, further tests can be made to determine the exact problem.

Figure 1 Troubleshoot three-phase motors with an ohmmeter.

To troubleshoot a three-phase motor, apply the following procedure:

1. Using a voltmeter, measure the voltage at the motor terminals. If the voltage is present and at the correct level on all three phases, the motor must be checked. If the voltage is not present on all three phases, the incoming power supply must be checked.

2. If voltage is present but the motor is not operating, turn the handle of the safety switch or combination starter OFF. Lock out and tag the starting mechanism per company policy.

3. Disconnect the motor from the load.

4. After the load is disconnected, turn power ON to try restarting the motor. If the motor starts, check the load.

5. If the motor does not start, turn it OFF and lock out the power.

6. With an ohmmeter, check the motor windings for any opens or shorts. Take a resistance reading of the T1-T4 coil. This coil must have a resistance reading. If the reading is zero, the coil is shorted. If the reading is infinity, the coil is opened. Since the coil winding is made of wire only, the resistance is low. However, there is resistance on a good coil winding. The larger the motor, the smaller the resistance reading.

After the resistance of one coil has been found, the basic electrical laws of series and parallel circuits are applied. When measuring the resistance of two coils in series, the total resistance is twice the resistance of one coil. When measuring the resistance of two coils in parallel, the total resistance is one half the resistance of one coil. See Troubleshooting guides in additional three-phase motor troubleshooting article below.