During a chance test, a customer who was inspecting a motor on-site raised a question. The reason was that while the noise test results for a certain type of motor met the product's technical specifications, the customer found the jarring electromagnetic noise unacceptable. When the on-site testing personnel argued that the test values were within acceptable limits, the customer retorted: "I only trust my ears."
Ms. Can encountered similar electromagnetic noise problems many years ago. To experience what it felt like, Ms. Can tried to stay near a motor that emitted a sharp electromagnetic noise for a while, but she couldn't even last 15 minutes before feeling seasick and nauseous.
At the time, electromagnetic noise from motors was largely ignored, naturally assumed to be an inherent characteristic of motors that couldn't be eliminated, and besides, it wouldn't affect usage. Nowadays, everyone is very concerned about environmental protection, and noise is undoubtedly a key focus for customers, naturally unacceptable or intolerable. In light of the above , Ms. [Name] will discuss how electromagnetic noise is generated, providing a theoretical basis for effectively curbing it.
Electromagnetic noise of squirrel-cage induction motors
Electromagnetic noise is mainly caused by the magnetostriction and vibration of the iron core due to the periodically changing radial electromagnetic force or unbalanced magnetic pull in the motor. Electromagnetic noise is also related to the vibration characteristics of the stator and rotor themselves.
When the excitation force and the natural frequency resonate, even a small electromagnetic force can generate a lot of noise.
All types of motors have electromagnetic noise problems, but the electromagnetic noise of induction motors is the most important. Its frequency is usually distributed in the range of 700 to 4000 Hz. The ear has a high sensitivity in this range, so it will cause a strong noise sensation.
Electromagnetic noise generated by magnetic force waves
If there are two harmonic magnetic fields with orders νa and μa, they will induce two rotating force waves with orders νa+μa and νa-μa, respectively. Low-order force waves are the main cause of electromagnetic noise in motors because the lower the order of the force wave, the longer the distance between two adjacent support points in the bending deformation of the iron core, the worse the stiffness of the iron core, and the greater the deformation and vibration.
Radial force of magnetic field and noise radiation
Any magnetic field in the air gap can be expressed as the formation of a sinusoidally distributed rotating magnetic field, and the radial force generated by the interaction of these magnetic fields can be expressed as...
fγ=FγCOS(γθ-ωt-φf) . . . . . . . . . . . . . . . . . . . . (1)
In the formula, Fγ represents the amplitude of the radial force wave.
γ — Force wave number (i.e., the number of force wave pole pairs)
θ — The angle between the direction of sound wave propagation and the normal to the area element of an imaginary closed surface surrounding the sound source.
ω — Angular frequency of force wave
φf — Spatial phase of the force wave
Equation (1) represents a sinusoidal rotating force wave that rotates along the air gap circumference at an angular velocity ω/γ. The frequency of the radial force at any point on the air gap circumference as a function of time is f=ω/2π.
Radial force generated by the fundamental magnetic field
The fundamental (p-pole pair) magnetic flux density amplitude, i.e., the air gap magnetic flux density Bδ , generates a radial force wave with an order γ = 2p , i.e., a force wave number of 4p, a force wave frequency f = 2f1, and a force wave amplitude Fγ = Bδ²/4μ₀. The average radial force acting within one "force pole" of the air gap circumference along a unit length of the iron core is...
Fγa=2/π·Fγ·πDi1/4p . . . . . . . . . . . . . . . . . . . (2)
In formula (2), Di1 represents the inner diameter of the stator core.
Radial force is generated by the interaction of stator and rotor harmonic magnetic fields.
Let the amplitude of the combined force waves be Fγs. Then, the average radial force acting under one force pole along the unit length of the iron core circumference in the air gap is:
Fγa=2/π·Fγs·πDi1/2γ=Fγs·Di1/γ . . . . . . . . . . . . . . . . . . . (3)
Vibration caused by radial force
Alternating radial force waves act on the stator and rotor cores, causing periodic deformation of the cores over time, i.e., vibration. The vibration frequency is the frequency of the radial force waves. The rotor core has good rigidity, and its deformation can be ignored; the stator core, being a thin-walled annular structure, has poor rigidity, and its vibration is the main cause of electromagnetic noise.
noise radiation
Vibrations generated by electromagnetic forces propagate from the electric motor to its surrounding space, causing airborne particles to vibrate and producing sound waves. When the frequency of these sound waves is between 20 Hz and 20 kHz, they can be perceived by the human ear; this is electromagnetic noise.
For large electric motors with a radiating surface size much larger than the wavelength of the radiated sound waves, the sound waves can be approximated as radiating from an infinitely large plane along its normal direction, i.e., plane radiation; for short electric motors with a length-to-diameter ratio of 1, the radiating surface can be approximated as a sphere, and the sound waves radiate spherically; for medium and small electric motors with a length-to-diameter ratio greater than 1, the radiating surface can be approximated as a cylinder, and the sound waves radiate cylindrically.
