Generation of electromagnetic noise in three-phase asynchronous motor

1. Electromagnetic noise of three-phase asynchronous motor

Electromagnetic noise is mainly caused by the periodic radial electromagnetic force or unbalanced magnetic pull in the motor, which causes magnetostriction and vibration of the iron core. Electromagnetic noise is also related to the vibration characteristics of the stator and rotor themselves. When the exciting force resonates with the natural frequency, even a small electromagnetic force will generate a lot of noise.

2. Electromagnetic noise excited by magnetic field force waves

If there are two harmonic magnetic fields with the order of νa and μa, they will cause two rotating force waves with the order of νa+μa and νa-μa. Low-order force waves are the main cause of electromagnetic noise in motors, because the lower the force wave order, the longer the distance between the two adjacent support points of the iron core bending deformation caused by it, the worse the corresponding stiffness of the iron core, and the greater the deformation and vibration.

3. Magnetic field radial force and noise radiation

Any magnetic field in the air gap can be expressed as a sinusoidally distributed rotating magnetic field. The radial force generated by the interaction of these magnetic fields can be expressed as

fγ=FγCOS（γθ-ωt-φf）

Where Fγ——radial force wave amplitude

γ——Force wave number (i.e. force wave pole pair number)

θ——The angle between the direction of sound wave propagation and the positive normal of the imaginary closed surface area element around the sound source

ω——angular frequency of force wave φf——spatial phase of force wave

The sinusoidally distributed rotational force wave represented by equation (1) rotates along the circumference of the air gap at an angular velocity ω/γ, and the frequency of the radial force at any point on the circumference of the air gap changing with time is f=ω/2π.

4. Radial force generated by fundamental magnetic field

The fundamental wave (p-pole pair) magnetic flux amplitude is the air gap magnetic flux Bδ, and the radial force wave order γ=2p, that is, the force wave number is 4p, the force wave frequency f=2f1, and the force wave amplitude Fγ=Bδ2/4μ0. On the unit longitudinal length of the core, the average radial force acting in one "force pole" of the air gap circumference is

Fγa=2/π·Fγ·πDi1/4p

In formula (2), Di1 is the inner diameter of the stator core.

5. Vibration caused by radial force

The alternating radial force wave acts on the stator and rotor cores, causing the cores to deform periodically over time, i.e., vibrate. The vibration frequency is the frequency of the radial force wave. The rotor core has good rigidity and its deformation can be ignored. The stator core has a thin-walled ring shape and poor rigidity, and its vibration is the main cause of electromagnetic noise.

6. Noise Radiation

The vibration generated by the electromagnetic force is transmitted from the motor to the surrounding space, causing the vibration of particles in the air to generate sound waves. When the frequency of the sound wave is between 20Hz and 20kHz, it can be felt by the human ear, which is electromagnetic noise.