Note: No, there are no inverters involved in this question here.
I've perceived that most, if not all induction motors make a "howling" sound in their coils that is different from the constant 60 Hz tone they produce due to the working frequency.
This howl has pitch shifts depending on the rotor speed. In very low speeds it is low-pitched and blends with the 60 Hz hum, but as it speeds up the sound evolves into higher pitches. When accelerating I can hear it standing out when it gets around ~100 Hz and then it starts getting quieter after reaching ~400 Hz. When decelerating it does the opposite (I start hearing at ~400 Hz and then it goes down until it blends with the 60 Hz hum).
I have the impression that in motors with more poles this sound is much more audible, like my 16-pole ceiling fan motor. In it the howling is very audible when its spinning at low speeds, like 60 RPM.
And the more "effort" the motor is doing the louder the howl, reversing the fan during operation makes a much louder howl just before reversing (while it's at a low speed and decelerating, nearly stopping) than just after reversing.
I've also heard that howl in several other induction motors, most of them being 4-pole motors. The motors in vertical washing machines, for example, make a quite audible howl when accelerating (in the centrifugation step).
I know that this is probably the result of the interaction between the applied Electromotive Force and the Back Electromotive Force induced on the stator windings, as they're not in-phase and the speed of the rotor modulates the back-EMF on the stator windings.
The resultant between the EMF and the back-EMF (and the successive iterations of inductions between the stator and the rotor) might produce a higher frequency signal on the windings.
However, I can't figure out exactly how this happens.
Does anyone know how can I model the process making that sound?
Best Answer
The major sources of acoustic noise in induction motors are magnetostrictive effects, torque ripple effects and aerodynamic effects. There may also be some effects due to magnetic forces that are orthogonal to torque. As mentioned by @Lorenzo Donati, Rotor imbalance and bearing noise may also contribute even if the motor is properly balanced and in good condition.
The magnetostrictive effects are driven by the power frequency in the stator and by the frequency of the rotor current which varies with slip.
Torque ripple is caused by reluctance variation with rotor angle in the stator-rotor flux path due to the way in which the stator slots align with the rotor slots. There is also torque variation in single-phase motors due to the inherent variation of power transfer.
Aerodynamic effects are due to the rotor fins and other motor cooling fans attached to the rotor.
There are also a secondary effects due to mechanical resonance of the motor parts.
Noise may also be produced in the motor by load variations and in the load by motor torque ripple. Noise produced in the load may be difficult to distinguish from noise produced in the motor.