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Every 3 phase motor has six (6) terminals with the supply voltage connected to three (3) of those terminals. The most common configuration of a three-phase motor is the Delta (∆) – Star (Wye) configuration with the Delta side connected to supply voltage. The terminal configuration of a 3 phase motor is shown below:
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| Terminals Configuration of a 3 Phase Motor |
The W2U2V2 terminal set is the star side of the 3 phase motor while the U1VIW1 is the Delta side of the motor connected to the supply voltage.
The 3 phase motor is a rugged piece of equipment but as with everything man made, there comes a time when this beautiful piece of machinery fails either due to old age, misapplication, mal-operation or any other adverse cause.
The most common failure mode of a 3 phase AC motor is burnt winding or shorted winding leading to the damage of the motor. Often it is required to test the winding of the 3 phase windings with the aid of a multimeter or ohmmeter to determine whether the motor is still good or burnt or shorted.
How to Test the Winding of a 3 phase Motor
To determine whether a 3 phase motor is still good or has gone bad, a simple ohmmeter test across the windings of the motor will reveal its true state of health. As shown below, the indicated terminal matrix (blue lines) shows the way the windings of a 3 phase motor should be tested with an Ohmmeter:
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| How to test the windings of a 3 Phase Motor with an Ohmmeter |
The first thing to do before testing the windings of the motor is to remove the links linking terminals W2U2V2 and the disconnect the motor from supply (L1, L2, L3). A multimeter terminals placed across this matrix of terminals will indicate the following readings for a good 3 phase motor:
(a) Terminals W1W2, U1U2, V1V2 will indicate continuity for a good motor
(b) Every other terminal combinations should indicate Open for a good motor
(c) Readings between any of the six (6) terminals and the motor frame signifying earth
(E) should indicate open for a good motor.
Ohmmeter Readings for a Bad 3 phase Motor
In the case of a burnt or bad 3 phase motor, this matrix of terminals should indicate the opposite readings for a bad motor:
(a) If any of the terminal combinations W1W2, U1U2, V1V2 should indicate open then
the motor is bad.
(b) If any other terminal combinations should indicate continuity instead of open, then
the motor is bad.
(c) If the reading between any of the six (6) terminals and motor frame (E) should
indicate continuity, then the motor is dead.
