Use 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 as shown in the figure. In the figure, μA is the 50μA block of the multimeter (the 1mA block can also be used). Connect it to any of the three-phase stator windings, and then slowly turn the motor rotor by hand to make the rotor rotate at a uniform speed. Since there is always a certain amount of residual magnetism in a used motor, when the rotor rotates, it cuts the magnetic lines of force, and an electromotive force is induced on the stator winding. Current passes through the ammeter, causing the needle to swing left and right. The number of swings is exactly the number of pole pairs P of the motor. . For example: the rotor rotates once and the multimeter pointer swings twice, indicating that the current direction has changed four times, and it 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 smaller than the synchronous speed, generally 2% to 3% lower than the synchronous speed, so that the rotor speed can be easily calculated.