Motor electromagnetic noise and weakening analysis

The electromagnetic noise of the motor is mainly caused by the electromagnetic force wave generated by the electromagnetic field in the air gap, which causes the vibration of the core yoke and radiates the noise through the motor shell. When the frequency of the excitation force matches the natural frequency of the vibrating component, the vibration and noise will increase. The size of electromagnetic noise depends on the size of the electromagnetic force, the vibration characteristics of the motor structural parts and the acoustic characteristics of the motor itself.

There are many design and fault factors that can lead to an increase in electromagnetic noise, such as magnetic pull imbalance, core saturation effects, magnetic flux oscillation, etc.

Due to the existence of the magnetic field in the air gap, electromagnetic force is generated between the stator and the rotor. Under the action of radial electromagnetic force waves, the electromagnetic force acts directly on the teeth or magnetic poles, and both the stator and rotor cores are deformed. Analysis shows that the main source of noise is not caused by the vibration of the teeth and magnetic poles, but caused by the vibration of the iron core and yoke. Since the rotor is fastened to the rotating shaft and its stiffness is much greater than that of the stator, the noise caused by the vibration of the stator core yoke is mainly studied.

In order to simplify the analysis, the stator core is often analyzed as a cylinder. On the shell, there is a radial electromagnetic force with a wave number n that changes periodically with time, showing a cosine distribution in space. For small force wave orders, when the iron core is deformed by the force wave, the distance between adjacent nodes is large, the stiffness is relatively poor, and the deformation is relatively large, causing large vibration and noise. Under the action of different times of force waves, the stator yoke ring has different natural frequencies. As the yoke height increases, the natural frequency increases; the larger the average radius of the yoke ring, the lower the natural frequency.

The electromagnetic force waves cause the iron core to vibrate and force the surrounding air to vibrate, producing electromagnetic noise. The sound energy is radiated to the space through the casing. If the size of the motor casing is larger than the radiation wavelength, the casing is regarded as a plane radiator. At this time, the electromagnetic noise frequency range is often between 100Hz and 4000Hz.

The following conditions must be met to generate large electromagnetic noise:

(1) The amplitude of electromagnetic force waves that generate vibration and noise is large.

(2) The order n of electromagnetic force waves is relatively low.

(3) The electromagnetic force excitation frequency coincides with the core's natural vibration frequency. Even if the electromagnetic force is relatively small at this time, the electromagnetic noise will be greatly increased due to resonance.

Among the various harmonics, the tooth harmonic, especially the first-order tooth harmonic, has a great influence. On the one hand, because the tooth harmonic winding coefficient is the same as the fundamental wave, it is not easy to use short distance and distribution to weaken it, and the amplitude is relatively large. On the other hand, when the cogging is improperly selected, the frequency of harmonic electromagnetic force waves generated will be very low. Appropriate cogging should be selected to avoid the generation of electromagnetic force with a low number of force waves.

Ways to reduce electromagnetic noise:

(1) Use sinusoidal winding. Sinusoidal windings can be used for both three-phase motors and single-phase motors to reduce certain harmonic components in the magnetic potential and reduce noise.

(2) Reasonably select the cogging of the stator and rotor.

(3) Uniformity of air gap and selection of air gap value. For example, after increasing the air gap from 0.25mm to 0.35mm, the noise level dropped by 4~6dB(A). However, after the air gap is increased, the excitation current or the thickness of the permanent magnet will increase, and the power factor cosφ will decrease. It needs to be comprehensively consider.

(4) Chute.

(5) Reduce magnetic density control resonance noise.

In micro motors, sometimes the natural frequency of a structural part may coincide with the frequency of electromagnetic or mechanical excitation force, so resonance is easy to occur. The noise level is relatively large during resonance. In this case, it is necessary to perform spectrum analysis of the vibration or noise and calculate, Determine the natural frequency of main components.

In addition, tooth harmonics and electromagnetic noise can also be reduced by using strategies such as armature skew slots, uneven air gaps in DC motors, and magnetic slot wedges in AC motors.

In the process of controlling motor noise, we should also pay attention to the problem of mechanical noise sources. Selecting higher-precision bearings, ensuring good coordination between bearings and bearing housing end caps, improving assembly technology, calibrating centers, maintaining good dynamic balance, and taking damping and vibration reduction measures are all effective ways to reduce mechanical noise.