How to measure the resistance between three-phase motor coils with a multimeter
The phase-to-phase resistance of the motor cannot be measured with a megometer, because the voltage of the megometer is too high and may breakdown the insulation of the coil. Usually, we use a multimeter to measure the resistance between the coils of a three-phase motor. Regardless of whether it is a star connection or a delta connection, the resistance between the three phase windings should be small and normal between the windings.
But it should be noted that the three-phase windings are actually integrated into one. Except for smaller-power motors that are internally connected in a star or delta shape, high-power motors require connecting pieces for connection, or use star-delta voltage reduction. Control wiring, if the connecting lug is removed, the resistance between the three phase windings should be greater than one megohm.
Therefore, when we measure the resistance of the three-phase winding, we must remove the connecting piece to obtain accurate data.
The insulation of the three-phase windings from the ground can be measured with a megohmmeter. It is usually infinite, at least greater than 0.5 megohms. The larger the motor, the higher the insulation resistance requirement. For a 45 kilowatt motor, the insulation resistance should be above one megohm.
The insulation to ground mentioned here is the bond between the winding and the motor body, not the insulation against the ground wire or the earth. So when we measure, we only need to measure the resistance of the winding and the body.
