How to use a multimeter to check if the water pump motor coil is burned out? To determine the quality of the motor, three conditions must be measured: the absence of open circuits and short circuits between the coil windings, and the insulation resistance. First, check whether the motor is a three-phase motor or a single-phase motor. The resistance values of the windings in a three-phase motor are the same, while the resistance values in a single-phase motor are different.
Three-Phase Motor
A three-phase motor has three sets of coil windings. When measuring a three-phase motor, set the multimeter to the 200-ohm range. Regardless of whether the three-phase motor is connected in a star or delta configuration, it can be measured using the same method. Simply measure the resistance values between U1, V1, and W1. If the resistance values between U1 and V1, U1 and W1, and V1 and W1 are the same, with a mutual error of no more than 10%, it can be determined that there are no open or short circuit issues in the coil.
Open Circuit: If the measured resistance of one group of coils is infinite or significantly larger than the other two groups, it can be confirmed that the motor coil is damaged due to an open circuit.
Short Circuit: If the measured resistance of one group of windings differs by more than 30% from the other two groups, causing three-phase imbalance, it can be determined that the motor is damaged due to a short circuit.
The resistance value of the coil for motors below 7.5KW generally ranges from 2 to 50 ohms. The larger the motor power, the smaller the coil resistance; conversely, the smaller the motor power, the larger the coil resistance.
Even if the resistance between the windings is normal, it does not mean the motor is good. The insulation resistance of the motor must also be measured, which is the resistance value of the windings to the outer casing. Set the multimeter to the MΩ range and measure the resistance values of U1, V1, and W1 against the casing. If the measured resistance is above 0.58MΩ, the motor is considered good; if below 0.58MΩ, the motor insulation is poor and requires repair.
Single-Phase Motor
A single-phase motor has two sets of coil windings: one for starting and one for running. Generally, the starting coil has a smaller resistance, while the running coil has a larger resistance. The connection points for a single-phase motor also consist of three terminals: one end for the starting coil, one end for the running coil, and a common end for both coils.
Using a multimeter to measure the three terminals of the single-phase motor in pairs will yield three different resistance values. The smallest value corresponds to the running coil, the middle value to the starting coil, and the largest value is the total series resistance = running coil resistance + starting coil resistance. The three criteria for determining the quality of a single-phase motor are open circuit, short circuit, and insulation resistance.
Open Circuit: If the resistance of the starting or running coil is infinite or significantly larger than the other group, it indicates an open circuit.
Short Circuit: If the resistance of the starting or running coil is close to zero, it indicates a short circuit. The measurement of insulation resistance is the same as for three-phase motors.
Single-phase water pump motors generally have a smaller power, usually below one kilowatt, with coil resistance typically ranging from 20 to 200 ohms.
For single-phase motor connections, the starting and running coils are connected in series and then in parallel with a capacitor (the two terminals of the maximum resistance are connected in parallel with the capacitor). When 220V power is applied across the running coil, the motor rotates in the forward direction. When the 220V power is connected across the starting coil, the motor rotates in the reverse direction.
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