Methods for Judging the Condition of an Electric Motor with a Multimeter
A three - phase motor has six taps for its three - phase windings. First, we need to remove the connecting plates on the motor's terminal blocks. Use the continuity mode of the multimeter to measure the continuity between the start and end of each phase winding. Then, use the multimeter to measure the resistance between each pair of phase windings. The resistance should be at least greater than 0.5 megohms, and the larger the better. Finally, measure the insulation resistance between each phase winding and the motor housing, which should ideally be infinite. Based on these measurements, we can basically judge the condition of the three - phase motor.
A single - phase motor has only two windings: a starting winding and a running winding. For a single - phase motor with three wires, one of them is the common wire, which means one end of the two windings is connected together. The other two wires are the taps of the other ends of the two windings.
The resistance between the two taps should be the largest and equal to the sum of the resistances of the two windings.
Of course, we also need to measure the resistance between the coil and the motor housing to check if it has been broken down. Naturally, the larger the resistance, the better. Similarly, it should not be less than 0.5 megohms.
For the six - wire single - phase motor, in fact, two of the wires (Z1, Z2) are for the centrifugal switch. This type of motor also has six terminal blocks, so we also need to remove its connecting plates. Remove the capacitor as well, and then measure the start and end of the two windings and the resistance between the two windings respectively. The method is the same as that for the three - phase motor.
Using a multimeter to judge the condition of a motor can only provide a preliminary assessment. The insulation resistance between windings and the insulation resistance of each phase winding to the ground should be measured more accurately with a megohmmeter.
Moreover, a short - circuit between turns in the winding cannot be detected. We can only make a guess by comparison to see if there is a turn - to - turn short - circuit. For example, if the resistance of one phase winding is much smaller than that of the other two phases, we can suspect a turn - to - turn short - circuit.
