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From my experience just any electric motor when under huge load would make recognizable growling sound. This would happen regardless of the motor type - brushed/brushless, AC/DC, small size/huge size - it would just happen.

Where does this growling sound come from exactly and how does it form?

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  • \$\begingroup\$ ""just any electric motor"" this is not my experience, but nevertheless the working principles of different kinds of electric motors are so different, that a simple answer is not possible. This should be discussed at se.electric and along with the type of motor. This discussion here is just guessing . \$\endgroup\$ – Georg Jun 21 '11 at 9:34
  • \$\begingroup\$ @Georg: I believe all electric motors follow the same principle and how the windings and core are structured is of course important but doesn't have that much influence on that sound. \$\endgroup\$ – sharptooth Jun 21 '11 at 9:36
  • \$\begingroup\$ I deleted a couple of comments which were not constructive. \$\endgroup\$ – David Z Jun 21 '11 at 17:10
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Under high load significant force is 'trying' to move magnets & individual wires inside coils - so if there is even tiny loose piece - it start to move back & forth with the frequency = frequency of magnetic field change effectively working as a speaker.

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    \$\begingroup\$ I don't mean to contradict it just from experience. If a motor has lose coils it will burn up pretty fast the friction on the insulation shorts the coil and you get smoke this and metal fatigue in the copper are the primary reasons for winding failure. \$\endgroup\$ – Fortunato Jun 21 '11 at 8:00
  • \$\begingroup\$ 'Lose' might be just few microns of movement... This might last long without failure... \$\endgroup\$ – BarsMonster Jun 21 '11 at 8:09
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    \$\begingroup\$ In fact copper coils in motors were compressed into the grooves by slim wedges, up to deformation of the cross section. Today fixation is achieved by filling with some resin. \$\endgroup\$ – Georg Jun 21 '11 at 9:30
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Ball Bearings could be an answer.

No motor is perfectly symmetric around its axis of rotation. As the load increases the precession about the axis increases and this in turn places greater stress on the bearings, producing sound energy.

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Transformers hum because of magnetostriction. Electrical current induces a magnetic field which cause the iron core in most transformers and motors to physically change shape (magnetostriction); when the current returns to zero (typically at mains frequency) it returns to the original shape. Heavier loads cause higher peak current which cause a larger peak change in shape which sounds louder.

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If you use a doctors stethoscope you can locate the source of a baring failure vibration or bushing wobble. But most often than not if it’s a “Growling” as you describe it its harmonics. I have dealt whit this problem in brad new motor assemblies. The motor balance is set for a max RPM so it’s good. The pulley/gear/sprocket is balanced in the same way. Now put them together and there’s a point in watch this 2 minutely of balances create the harmonic. Manufactures tray to make this happen outside the Min/Max rpm and Max load rage but it’s not 100%

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  • \$\begingroup\$ When ball bearings make some "growling" sound, You have seconds to minutes left before they overheat or block for other reason. \$\endgroup\$ – Georg Jun 21 '11 at 9:26
  • \$\begingroup\$ @Georg Plz read what a wrote bearings can have a vibration "Growling" is harmonics. I know I'm not the best writer but heel its in the 1st line \$\endgroup\$ – Fortunato Jun 21 '11 at 9:52
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You can get the growl out a hard working transformers too.

Somewhat like @BarsMostner thinks I think it the whole assembly moving as speaker.

The spider of the motor and the shaft are somewhat flexible.

We had a weapon system project with a 10kW brushless motor driver that I suggested we cold use a an audio output device. So it could growl (actually quite loudly) when you get close to it.

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In a heavily loaded motor the case and rotor are oppositely REACTING from either the cogging forces of a brushed motor and/or the AC cycle forces of any AC motor, and even the CURRENT waveform of a PWM supply. When a motor is lightly loaded these currents are less as are the forces. The rotating load also feels the same forces. Various resonances may also come into play if the motor speed changes. The same thing happens with gasoline engines and other devices.

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