Induction Motors: Rotating Magnetic Field & AC Frequency

[Glenn G]

Hi,
Can I please ask how the rotating magnetic field (due to the 3 phase ac) is changed? Is it due to the frequency of the ac supply that it can speed up and therefore drag the rotor along due to induction. If it is due to the changing frequency input that mimics a faster rotating magnetic field then how is this produced? In fact I don’t even know a 50hz supply is changed by a signal generator?
Regards
G. X

 

[russ_watters]

Hi,
Can I please ask how the rotating magnetic field (due to the 3 phase ac) is changed? Is it due to the frequency of the ac supply that it can speed up and therefore drag the rotor along due to induction. If it is due to the changing frequency input that mimics a faster rotating magnetic field then how is this produced? In fact I don’t even know a 50hz supply is changed by a signal generator?
Regards
G. X

An ac motor and generator are essentially the same device, but one is the doing the pushing and the other being pushed. In short, moving a coils over magnets causes the field to vary and the varying field causes coils to move over magnets.

https://en.m.wikipedia.org/wiki/AC_motor

 

[Merlin3189]

I'm not clear on what you're asking. But for most induction motors, the field frequency does not change. The supply is 50 Hz (or whatever in different countries) and this produces a constant speed rotating field. This changing (direction) field induces currents in the rotor, which in line with Lenz's principle oppose the changing field. Therefore there is a force on these conductors pulling the rotor in the direction of the rotating field.
The rotor accelerates in the same direction as the rotating field, but just as Lenz intended, this reduces the rate of change (direction) of flux and therefore reduces the induced currents and thence the force. Eventually, when the rotor is moving fast enough, the force from the induced currents is only just enough to overcome the load and the rotor no longer accelerates.
It is then moving at a rotational speed less than the field: the difference is called the slip speed, which depends on the load. With a bigger load the slip speed increases, so that there is a greater rate of change (rotational) of flux: with a smaller load the slip speed decreases to give a lower rate of change of flux.
Even under no load the rotor can never reach exactly the same rotational speed as the rotating field, because then there would be no induced currents and no force to overcome any friction.

 

[CWatters]

Hi,
Can I please ask how the rotating magnetic field (due to the 3 phase ac) is changed? Is it due to the frequency of the ac supply that it can speed up and therefore drag the rotor along due to induction. If it is due to the changing frequency input that mimics a faster rotating magnetic field then how is this produced? In fact I don’t even know a 50hz supply is changed by a signal generator?
Regards
G. X

Are you asking how to vary the speed of a 3 phase induction motor?

These are normally designed to run at a constant speed but it is possible to vary the speed. Efficiency can suffer...

https://www.electricaleasy.com/2014/02/speed-control-methods-of-induction-motor.html

 

[Babadag]

The stator produced rotating magnetic field velocity depends on supply system frequency, indeed, but system frequency may vary in permissible limits or sometime even beyond these limits. The electricity generation is produced by synchronous generators activated by prime movers as turbine or diesel generators which could lose speed at overload. The limits are stated by national or international standards as MG-1 [for U.S.A.] or IEC 60034-1.
On the other hand, if the motor supply system is provided with a Variable Frequency Device you can change the frequency as need it.

 

[Fisherman199]

Hi,
Can I please ask how the rotating magnetic field (due to the 3 phase ac) is changed? Is it due to the frequency of the ac supply that it can speed up and therefore drag the rotor along due to induction. If it is due to the changing frequency input that mimics a faster rotating magnetic field then how is this produced? In fact I don’t even know a 50hz supply is changed by a signal generator?
Regards
G. X

These questions you're asking are too broad and should be answered through your own study. It's best to come to PF with specific questions after having studied the material. That being said, magnetic field frequency is a result of the current frequency and cannot be decreased nor increased independently from it. Should one require a different frequency they would need a variable speed drive.