Take measures to reduce the various sounds emitted during the operation of the motor. Theoretically, there should be no sound when the motor is running, so any sound emitted during the operation of the motor can be classified as noise of the motor. These noises include electromagnetic noise, mechanical noise, aerodynamic noise.

The electromagnetic noise is mainly caused by the radial electromagnetic force or unbalanced magnetic pull in the motor that periodically changes the magnetostriction and vibration of the core. Electromagnetic noise is also related to the vibration characteristics of the stator and rotor itself (such as the inherent frequency, damping, mechanical impedance and acoustic characteristics, etc.). For example, when the excitation force and the inherent frequency resonance, even if the electromagnetic force is very small will produce a lot of noise. Electromagnetic noise can be suppressed from a variety of aspects. For asynchronous motors, the first step is to choose the appropriate number of stator and rotor slots. Generally speaking, the rotor slot number and stator slot number difference, that is, the so-called far slot fit, electromagnetic noise is smaller (there are a few exceptions, such as stator 24 slot, rotor 22 slot is also a good fit). For slotted motors, the oblique slot can make the radial force produce phase displacement along the direction of the motor axis, thus reducing the axial average radial force, thus reducing the noise. If double-slotted structure is used, the noise reduction effect is even better. Double-slotted structure is to divide the rotor into two sections along the axial direction. Each segment has the opposite direction of torsional slope. There is also an intermediate ring between the two segments. In order to reduce the flux potential harmonics, a double-layer short moment winding can be used. And avoid the use of fractional slot winding. In single-phase motor, sinusoidal winding should be used. In order to reduce the electromagnetic noise caused by the slot, magnetic slot wedge can be used or reduce the width of the slot of the stator and rotor until the use of closed slot. The three-phase motor should be operated with as much voltage symmetry as possible, and the single-phase motor should be operated with a rotating magnetic field close to a circle. In addition, the motor manufacturing process, should reduce the ellipticity of the inner circle of the stator and the outer circle of the rotor and ensure that the stator and rotor concentric, so that the air gap is uniform. Reduce the air gap flux density and use a larger air gap, can reduce noise. In order to avoid resonance of electromagnetic force with the inherent frequency of the housing, appropriate elastic structure can be used.

Mechanical noise is mainly caused by the rotor and bearings. The bearing is the connecting member of the rotor and stator of the motor, which bears the excitation of various forces in the motor and transmits the excitation force, thus generating vibration and noise. The brush and slip ring or commutator friction of the motor will also produce mechanical noise.

For the motor with higher speed or longer rotor, dynamic balance correction should be carried out. The bearing of this kind of motor should use the motor special low noise bearing, in the motor operation, the inner and outer rings of the bearing should not occur harmful sliding, but also to prevent the bearing and shaft or bearing and end cover bearing room with too tight, in order to avoid bearing radial clearance is too small and bearing inner and outer ring deformation. Rotor shaft bearing block and end cover bearing chamber machining accuracy and surface finish should be high. In order to prevent the rotor from axial runout sound, waveform spring should be used to apply axial pre-stress to the outer ring of the bearing. The bearings must be carefully cleaned before assembly. It is advisable to use hot sleeve or pressure inner ring to mount the bearing to the shaft and choose suitable grease. Low noise motors are preferable to use plain bearings.

The clearance of the brush and brush handle should be properly designed and ensure that the commutator or slip ring has a smooth surface and correct geometry, etc.

Aerodynamic noise includes the fan, rotating rotor and the airflow noise formed when the airflow flows along the wind path. The most important measure to reduce aerodynamic noise is to control the air volume. In the scope of ensuring that the motor temperature rise does not exceed the permitted limit, try to reduce the air volume. Improve the structure of the fan and rational design of the air path system can reduce aerodynamic noise. When the stator and rotor radial ventilation channels are aligned, flute sound may occur, at this time they should be staggered from each other.