How to use a multimeter to test whether the motor is burned out
1. Measurement method of three-phase motor
Set the multimeter to the 200 ohm barrier. Measure the three-phase resistance respectively. Generally, the resistance of a 750-watt motor is about 20 ohms, and the resistance of a 4000-watt motor is 3 to 5 ohms. The resistance values vary slightly depending on the motor. If the resistance values of the three phases are the same and the difference is not large, it can be initially judged that the three-phase windings are basically normal and there is no inter-turn short circuit.
Then, disassemble the three-phase short-circuit piece of the junction box, set the multimeter to the 20M ohm level, and measure the insulation resistance between the three phases and the three-phase shell. The resistance should be infinite. Because the power supply battery of the multimeter is nine volts, its insulation resistance can only be preliminarily determined. The specific insulation resistance needs to be measured with a 500-volt megger. It is normal for the insulation resistance to be above 5M.
The above is the judgment when the motor is normal. If the three-phase resistance values differ greatly and the resistance value is zero or no resistance value, it can be basically determined that the motor has been burned or has a phase failure, and the motor needs to be disassembled for further inspection. If the insulation resistance is small, it is necessary to determine whether it is a short circuit or moisture.
2. Measurement of single-phase motor
The measurements of single-phase motors and three-phase motors are similar. Single-phase motors only have main windings and auxiliary windings. Measure the resistance of main windings U1 and U2 and auxiliary windings Z1 and Z2. Generally, the resistance of the secondary winding is greater than the resistance of the main winding, but there are also cases where the resistance of the main winding and the secondary winding are the same.
If the resistance is normal, remove the connecting piece. Measure the phase-to-phase insulation resistance between the main winding and the secondary winding of the star and the insulation resistance to the shell. The measurement method is the same as for three-phase motors.
Precautions
1. Before disassembly, use compressed air to blow off the dust on the surface of the motor and wipe the surface dirt clean.
2. Select the working place where the motor will be disassembled and clean up the on-site environment.
3. Be familiar with the structural characteristics and maintenance technical requirements of motors.
4. Prepare the tools (including special tools) and equipment required for disassembly.
5. In order to further understand the defects in the operation of the motor, if possible, an inspection test can be done before disassembly. To this end, put the motor on load for test rotation, check the temperature, sound, vibration, etc. of each part of the motor in detail, and test the voltage, current, speed, etc., then disconnect the load, conduct a separate no-load inspection test, and measure the no-load test. Record the load current and no-load loss.
6. Cut off the power supply, remove the external wiring of the motor, and make records.
