[3 phase induction motor] direction of rotation

  1. How is the direction of rotation of rotor of a 3 phase induction motor determined? Assume the rotor to be squirrel cage or any other type of rotor.
    I was able to reason this out, but couldn't actually write it on a paper since my explanation lacked the physics... (it was based on simple understanding of electromagnetic induction)

    I would be thankful if someone could give a good explanation for this.

  2. jcsd
  3. stewartcs

    stewartcs 2,244
    Science Advisor

    It's based on the direction of the rotating magnetic field, which is dependant on the phase rotation. Hence if you swap in two line leads on a three phase motor, the direction of the magnetic field and thus motor will reverse.

  4. Thanks "stewartcs".
    but as I said in my post that I was able to justify the direction. I want to know the physics involved in this. If you could help me with that it would highly appreciable.
  5. stewartcs

    stewartcs 2,244
    Science Advisor

    Here is a simplified version of how they rotate:


    Conceptually, in a multiphase motor such as a three phase motor, the rotor essentially follows the rotating magnetic field generated by the stator. Since the stator windings are 120 electrical degrees apart (3-phase motor), they develop alternating magnetic fields in the stator coils (which are essentially magnets now). The "magnets" pull on the rotor bars and cause it to rotate thus turning the motor.

    Here is a visual of how the field rotates.


    Does that help?

  6. Thanks once again. But I think I'm unable to explain my question. I try once again. "How does the rotor feel the torque?(say for squirrel cage induction motor)".

    Let me tell you my explanation.
    Considering a squirrel cage rotor. Consider one rectangular face of the rotor. There is a slip in an induction motor. As a result there is a relative speed of rotating field with respect to the rotor. Now as the rotating field approaches this rectangle, the flux through this increases, but by "eletromagnetic induction principles" we can say that, the rotor will oppose this increase and will try to move away --> here moving away will mean that it has to move in the direction of the rotating field. But since the speed of the rotating field is more (due to slip) it somehow will move past this rectange. And then the flux through the rectangle will start decreasing. To oppose this the rectangle will move towards increasing the flux --> hence in the direction of magnetic field.
    Hence we can say that the rotor rotates in the direction of rotating field.

    Is this explanation correct? If no, atleast now you would better understand my question.

  7. stewartcs

    stewartcs 2,244
    Science Advisor

    The rotor "feels the torque" due to "flux bunching". In accordance with Lenz's law, the voltage, current, and flux generated by the relative motion between a conductor and a magnetic field will be in a direction to oppose the relative motion. Hence, to satisfy Lenz's Law, the conductors must develop a mechanical force or thrust in the same direction as the rotating flux. For this to happen, "flux bunching" must occur. Thus, the generated flux due to rotor-bar current must be in the opposite direction. The direction of the rotor-bar current that produces the flux can be determined by using the right-hand-rule.

    Hope that helps.

  8. thanks stewartcs. this helps.

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