How to measure the resistance values between the coils of a three-phase motor using the resistance range of a multimeter?
Resistance between Motor Coils
The inter - phase resistance of a motor cannot be measured with a megohmmeter because the voltage of a megohmmeter is too high and may break down the insulation of the coils. Usually, we use the resistance range of a multimeter to measure the resistance between the coils of a three - phase motor. Whether it is a star - connected or delta - connected motor, the resistance between the three - phase windings should be very small, indicating that the windings are in normal condition. The above figure shows a delta connection.
It should be noted that the three - phase windings are actually independent. Except for small - power motors that are internally connected in a star or delta configuration, large - power motors require connection pieces for connection or use star - delta step - down control wiring. If the connection pieces are removed, the resistance between the three - phase windings should be greater than one megohm.
Therefore, when measuring the resistance of the three - phase windings, we must remove the connection pieces to obtain accurate data. The above figure shows a star connection.
The insulation resistance of the three - phase windings to the ground can be measured with a megohmmeter. Usually, it should be infinite, and at least greater than 0.5 megohm. The larger the motor, the higher the requirement for the insulation resistance. For a 45 - kilowatt motor, the insulation resistance should be above one megohm.
The "insulation to the ground" mentioned here refers to the insulation of the windings to the motor body, not the insulation to the ground wire or the earth. So when measuring, we only need to measure the resistance between the windings and the motor body.
