Asynchronous Motor: Short Circuiting Rotor & Magnetic Fields

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Discussion Overview

The discussion revolves around the principles of operation of asynchronous 3-phase motors, specifically focusing on the role of short-circuited rotors and magnetic fields. Participants explore the interaction between magnetic fields and ferromagnetic materials, as well as the implications for rotor design and performance.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions why a short-circuited rotor is necessary in asynchronous motors, suggesting that the principle of aligning magnetic fields could suffice.
  • Another participant explains that the rotor, being made of soft iron, cannot be permanently magnetized and that the flow of current creates electromagnets that follow the stator's rotating magnetic field.
  • A different viewpoint emphasizes that ferromagnetic materials can quickly align with magnetic fields, proposing that the rotor could follow the rotating field if aligned correctly.
  • Further, a participant notes that without non-cylindrical shapes or saliency, there would be no aligning torque, indicating that specific rotor designs are necessary for effective operation.

Areas of Agreement / Disagreement

Participants express differing views on the necessity of short-circuited rotors and the conditions under which ferromagnetic materials can effectively interact with rotating magnetic fields. The discussion remains unresolved with multiple competing perspectives.

Contextual Notes

Some assumptions regarding the properties of ferromagnetic materials and the design of rotors are not fully explored, and the discussion does not resolve the implications of rotor shape on performance.

hisham.i
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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 ?
 
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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.
 
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...
 
hisham.i said:
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".
 

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