Disconnect and lock out the power. Test from T terminal to T terminal with an ohmmeter. If an open circuit is found, it will be in the line or in the motor winding.
Disconnect the motor leads and check for an open winding.
If the open is found in the line, the motor has run on two lines (singlephased). There’s a good chance the motor has winding damage and will look like Figs. 7.9 and 7.10 (see “Burned Windings from Operating on Single Phase” in Chapter 6). Normally, an open winding means the motor must be rewound or replaced.
Open Winding Test (Wye)
Disconnect and lock out the power. An open circuit in a single-circuit wye (high-voltage) connection will test open between the open phase and both normal phases (Fig. 7.9).
If a nine-lead motor is connected low voltage, leads T7, T8, and T9 should be disconnected and tested separately for an open circuit.
Larger motors have more than one internal wye. In this case, the open phase has higher resistance than the other two phases. A microhmmeter or surge tester will identify this problem.
If the motor runs, it has less power, and may not be able to pull its load. The motor’s sound may change (depending on its internal pole-to-pole connection) because of the magnetic unbalance within the winding.
Open Winding Test (Delta) Disconnect and lock out the power. Figure 7.10 shows a complete circuit through two phases. The resistance is higher across Tl and T2 (the open phase) because the circuit includes two phases in series. Both T2 to T3 and T3 to Tl show lower resistance (the resistance of only one phase).
A test light will light between all leads when there’s an open circuit in a delta-connected winding, and won’t indicate a problem.
If the winding has multiple circuits in each phase (e.g., four and eight delta), the test results will be much the same as in the preceding test.
Resistance will be higher across the open phase than across the other two phases. If there’s a small difference in resistance, the rotor should be turned to rule out its effect on the test. A microhmmeter or surge tester will identify this problem
Comparison Test from the Control (Wye or Delta)
Disconnect and lock out the power. Comparison-test with a (correct size) ohmmeter or an inductance-type instrument.
From the motor’s control, compare the test instrument readings from Tl to T2, T2 to T3, and T3 to T1. (They should be identical.) If there is just a small difference, turn the motor’s shaft slowly at least two complete turns while doing each test.
The rotor bar’s position can make a difference in the readings. Note the high and low on all three tests.
A difference in readings between lines indicates a shorted winding or a high-resistance connection between the control and the motor. The motor should be disconnected and tested separately before it’s removed.
There’s no exact resistance value specified in a motor winding (related to its horsepower and speed) because there are many variations of windings and core length designs.
TWO identical test instruments may not give exactly the same readings.
This fact should be considered when using a specific test value.
Comparison-testing between winding circuits is the most dependable way to locate winding problems. This can be done quickly on motors of all sizes, and doesn’t require manufacturer’s specifications.
Rotor Test from the Control
This test should be done if the motor doesn’t have normal power (if it takes longer than previously to start the load, or if the shaft speed [loaded] is lower than the nameplate RPM).
Disconnect and lock out the power. The rotor test can be done during the comparison test. When turning the shaft during the ohmmeter or induction test, look for a departure from the high and low readings. If a larger departure happens at the same shaft position, the rotor has an open rotor bar (or bars). If a departure is noted at several equally spaced shaft positions, it could be the rotor’s design.