1 Power-off method

　　Utilizing the characteristics of electromagnetic noise that varies with the strength of the magnetic field, the size of the load current, and the level of conversion, the power supply is suddenly cut off after a period of quiet listening to the motor running at no load. As the power supply is cut off, the noise will disappear immediately.After a power failure, the noise generated by the motor continuing to run by inertia is mechanical noise.Repeat several times in order to be confirmed.

　　2 Change the voltage method

　　When the power supply voltage drops rapidly to a certain limit (no major changes in the speed), if electromagnetic noise is the main part of the motor noise, it will vary greatly with the voltage, while other noises are basically unchanged.

　　3 current test method

　　If the stator winding is asymmetrical or internal phase failure or short-circuit between turns, the three-phase current is unbalanced; if the rotor cage is broken or the three-phase rotor of the wound motor is asymmetrical, the stator current fluctuates to identify electromagnetic noise.

　　4 drag method

　　Use a low-noise motor to drive the motor under test to rotate, and lift and lower the carbon brush several times to identify the effect of carbon brush noise.

　　5 disassembly method

　　For the stable characteristics of aerodynamic noise, it can be distinguished by the change of noise before and after removing the fan (small motor) or the external blower (large and medium motor).In addition, by replacing fans with different outer diameters and types, distinguishing the difference in noise at different speeds, the fan noise can also be identified.