I was working with a Motor that drives the shaft of a centrifuge. The motor shaft was jammed for some reason. Its a common occurrence that whenever a motor's shaft is jammed and power is switched on, a humming sound comes from the Motor. I wish to know why does that humming sound come?

thanks in advance

 

Grishma... what you hear is the movement of the magnetic structure at twice the supply frequency!

Regards, Phil Corso

 

That's the motor getting ready for the magic smoke!

 

Perhaps your centrifuge controller has the Stuxnet virus? That was intended as a joke by the way, I wasn't serious.

As the other posters have mentioned, this is due to flexing of the structure of the motor by strong magnetic fields. You will also hear a hum from most power transformers or AC resistance welders for similar reasons.

As has been stated by the others, in the case of an AC motor the currents may be excessive and overheat the motor. Whatever is driving the motors should limit the current to safe levels. For motors driven directly from the line, the motor overload protectors should be set to trip if the motor stalls. If the motor is driven by an variable speed drive, the drive controller may have some protective settings which must be set to suitable values.

In any case, if the motor is routinely being subjected to stall conditions which result in high overload currents, the life of the motor may be drastically reduced, or indeed simply catch fire.

If this is a "common occurrence" as you state, then there is quite likely a problem that needs to be addressed in a more permanent fashion.

 

It comes from the fact that it is an AC motor and all the magnetics are changing intensity at the line frequency. When rotating these forces are aligned so as to rotate the rotor. When the motor is not rotating they tend to make things vibrate back and forth and it makes the humming sound. If you have an AC motor that starts slowly, the humming starts louder and decreases as the rotor and field come into sync.

Regards
cww

 

Curt... there are two sources of "sound!"

The first, called "white-noise," is that sound produced by mechanical motion, when for example, the cooling fan is rotating.

The second, and most offensive to human hearing, is caused by magnetic flux through the core laminations. Known as magnetostriction, its effect changes lamination dimensions... twice per cycle of magnetization. This dimensional change, although quite small, produces "sound" characterized as "hum."

Many harmonics are generated at "even" multiples of excitation frequency. Therefore, the fundamental "sound" heard is 100 cps for 50 Hz systems and 120 cps for 60 Hz systems.

Phil Corso

 

Yes, exactly, but I was explaining to someone who wonders why motors hum :^). And there are even more noise sources excited by the vibration.

Regards
cww

 

Thank you all for helping me understand a basic phenomena. While reading more on Magnetostriction, I also had a chance to read about Electrostriction.

Thanks again.
Regards

 

Sorry, to dwell on this thread again but would request to kindly also tell if the buzz in the High Tension cables is also due to magnetostriction or it is due to some other reason?

Regards and thanks in advance