Measuring the speed of three-phase motor with a multimeter

    This circuit describes the principle and application of using a multimeter to measure the speed of a three-phase motor. 

The circuit for measuring the speed of a three-phase motor with a multimeter is shown in the figure. In the figure, μA is the 50μA range of the multimeter (1mA range can also be used). Connect it to any winding in the three-phase stator winding, and then slowly turn the motor rotor by hand to make the rotor rotate uniformly for one circle. Since there is always a certain amount of residual magnetism in the used motor, the rotor cuts the magnetic lines of force when it rotates, and an electromotive force is induced on the stator winding. Current flows through the ammeter, causing the needle to swing left and right. The number of swings is exactly the number of magnetic pole pairs P of the motor. For example: the rotor rotates one circle, and the multimeter pointer swings twice, indicating that the current direction has changed four times, which is a 4-pole motor. The number of magnetic pole pairs is 2, and the synchronous speed of the motor is n=60f/p=3000/2=1500 (r/min). The actual speed of the motor rotor is less than the synchronous speed, generally 2% to 3% lower than the synchronous speed, so the rotor speed can be easily calculated.