Always disconnect and lock out the power when testing a motor that is connected to its control. Test the two-speed motor using the same test methods as with a nine-lead mot0L The first test should be from a motor (T) terminal to the conduit or control box with an ohmmeter or megohmmeter.
If a ground is indicated, the motor should be disconnected and checked (separately from the conduit). A solid ground means the motor must be replaced or rewound. Fifty megohms or less means the motor should be cleaned and dried.
FIGURE 7.2 The winding analyzer is used to perform the step voltage test. Electrom Instruments.
If no ground is indicated, the windings can be tested using the line-to-line comparison test. A comparison test can be done from Ll to L2, 1.2 to L3, and 1.3 to Ll. (It may be necessary to turn the shaft while testing.) The controller contacts can be blocked or held closed to check the two speeds separately. A difference in test results requires further testing.
The numbers in the two-speed winding schematic can be used as a guide for comparison testing. Both constant-horsepower and constant-torque windings have an equal number of turns or coils between T4 and T5, T5 and T6, and T6 and T4. An equal number of turns or coils is also found between Tl and T2, T2 and T3, and T3 and Tl.
Any fault that is found should be verified by disconnecting the motor from the controller and testing the motor by itself.
If a surge test is used, run the test on both speeds. This will test all the turns that are connected to the lines. (These turns receive line spikes, and might have turn-to-turn insulation damage.)
The variable-torque motor should be tested in the same way as the constant-torque and constant-horsepower motors. (It may be necessary to turn the shaft during the test.)
Tests should be made from Tl to T2, T2 to T3, and T3 to T1. Another test series should be made with T 1, T2, and T3 tied together. The test is now made between T4 and T5, T5 and T6, and T6 and Tl.
Troubleshooting Multispeed
Multiwinding Motors
Always disconnect and lock out the power when testing a motor that is connected to its control.
The first test is a ground test from each winding to the conduit (Fig. 7.3). The next test should be from one winding to the other. (Be sure they are not connected to a common terminal in the control or internally to a common lead.) Failure of either test requires disconnecting the motor for further testing. Both problems (windings to ground and winding to winding) require rewinding or replacing the motor.
Caution must be used when a test is done that involves voltage. Voltage is transformed into the winding that isn’t being tested and could be hazardous.
If the leads of the idle winding touch each other, the test results (on the winding being tested) will be distorted.
FIGURE 7.3 Schematic for a two-speed two-winding motor.
Each winding of a multispeed multiwinding motor should be tested as a separate motor. The windings normally aren’t connected to each other internally. They may, however, use a common terminal in the control. (In this case, the windings should be isolated from each other before testing. The ground [lead-to-conduit] test can be made without the windings being isolated.)
All windings should be tested (with the correct schematic) as described in the preceding section, “Thoubleshooting the Thro-Speed One-Winding Motor.