How to use a multimeter to measure the resistance between the coils of a three-phase motor
The phase-to-phase resistance of the motor cannot be measured with a megger, because the voltage of the megger is too high, which may break down the insulation of the coil. Usually, we use a multimeter to measure the resistance between the coils of the three-phase motor. Whether it is a star connection or a delta connection, the resistance between the three-phase windings should be small, and the windings are normal.
But it should be noted that the three-phase windings are actually integrated into one. Except for the motors with low power, which are connected internally in a star or delta, high-power motors need connecting pieces to connect, or use star-delta step-down For control wiring, if the connecting piece 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 in order to measure accurate data.
The three-phase winding is insulated from the ground, which can be measured with a megohmmeter. It is usually infinite, at least greater than 0.5 megohm. The larger the motor, the higher the insulation resistance requirement. For a 45 kW motor, the insulation resistance should be above 1 megohm.
The ground insulation mentioned here refers to the connection of the winding to the motor body, not the insulation to the ground wire or the earth. So when we measure, we only need to measure the resistance of the winding and the body.
