induction motors question (1 Viewer)

[Steven12]

ok let me get this right.

with induction motors. no commutator or split rings are used because the current is induced onto the coils.(by a magnetic field)

the stator is a circular piece of metal with 2 curved pole pieces(electromagnet) to which the current is supplied to. the current on the pole pieces produced a radial magnetic field throught the rotor.

to start an induction motor, a force must be applied(starting device) to it first so that a current can be induced and then the magnetic can exert force on this current, so the rotation goes on forever.

(im right so far?)

now here is what i dont understand. if the current supplied is single phase alternating current. the changing current would produced a changing magnetic field. NOW if that happens, wouldnt the force exerted on the current reverse(up and down), so that the rotor rotates back and forth?

this is wat i think, the changing Magnetic field would INDUCED AC(changing) current on the coils of the rotor so that continous movement is allowed?
but i am not cos the text book explains brief.

 

[Steven12]

and also

can somebody tell me the difference between

Magnetic flux and magnetic field intensity?

 

[Xayma]

Generally the magnets will "rotate" (multiple magnets that turn on and off in sequence) and that induces the current, in order to minimise this change the squirrel cage will begin to rotate in the same direction as the magnets are being turned on and off (to reduce the different velocity) if a load is placed on it will be slowed down.

Induction motors are self regulating and unless you change the speed in which the magnets around the exterior are turned on and off you can't control the speed, it is only controlled by the load placed on.

Magnetic flux is a measure of the strength of a magnetic field.

Magnetic field intensity is the amount of magnetizing force a magnetic field has.

Magnetic flux density is a function of magnetic field intensity.

 

[Steven12]

xayma
thanks for explaining how the induction motor works. But i dont think you have answered my question.

but for the speed of rotation part. You said that speed of the rotation of the motor cannot be controlled. but what if you control the rate of turning on and off around the rotor. applying more currents to each electromagnet to increase the torque. or is that not possible?

 

[Xayma]

If you have multiple magnets around the outside by controlling the speed in which they turn on and off (effectively changing the relative speed). So you can turn them on and off at different speeds to control the speed.

As for changing the torque I dont know.

I know that when the AC flips it doesnt slow down as they rely on an AC current, when I get arround to it I'll double check my Engineering studies notes as it goes into it in a bit more detail.

 

[Steven12]

well ok, but for the induction motor. once you give the rotor a push. it will start to rotate without anymore energy input. is this correct ?

 

[Xayma]

No, there will always be energy lost indeed a motor wouldn't be any use if it kept rotating always as no energy would be transferred to what you need it to be ie to wheels on a road.

 

[xiao1985]

the speed of the rotor, if no load has being applied, is solely dependant on the "rotational" motion of the stator... it has nothing to do with the torque being applied to the squirrel cage, nor the magnetic field strength induced by the stator...

once a load is applied, then the rotational energy is used to do work... to recover the slip speed, is where the torque is important...

 

[Steven12]

By"rotational" motion. do you mean the speed at which the Electromagnets around the rotor switch on and off?