Asynchronous Motor: Short Circuiting Rotor & Magnetic Fields

[hisham.i]

If we have a magnetic field and a ferromagnetic material subjected to this field, the body will rotate until its magnetic axis is aligned with that of the field..
My question is: In asynchronous 3-phase motor, we have a rotating field.. so why we need to make a short circuited rotor and we don't apply the same principle of aligning the magnetic fields in such a motor? why we need a current to flow in the rotor ?

 

[vk6kro]

The rotor is soft iron which cannot be permanently magnetized.

The flow of current in the shorted turns turns the rotor into a number of electromagnets which will then try to follow the rotation of the magnetic fields from the stator.

You could have permanent magnets in the rotor, but they cannot vary their magnetic direction, so you would have to bring the rotor up to synchronous speed for it to work properly.

 

[hisham.i]

But ferromagnetic materials have a property that they can be aligned to the magnetic field very quickly, when the axis of stator magnetic field is aligned with that of the rotor, then the rotor will follow the rotating magnetic field at each instant...

 

[uart]

But ferromagnetic materials have a property that they can be aligned to the magnetic field very quickly, when the axis of stator magnetic field is aligned with that of the rotor, then the rotor will follow the rotating magnetic field at each instant...

Yes but there won't be any aligning torque if the rotor has cylindrical symmetry. If you include "saliency", either by non-cylindrical shape or by use of axial laminations, then you can get torque this way. This type of motor is called a "reluctance motor".