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Synchronous motor - replace dc field winidng with low impedance short-circuited coil

  1. Nov 8, 2008 #1
    Hi, I have a question about synchronous motors. If the DC field winding of a synchronous motor were to be replaced by a low-impedance short-circuited coil and the stator were to be supplied with 3-phase AC, would the rotor spin?

    If the answer is yes, would the rotor follow the direction of that of the RMF and would the rotor speed be the same as that of the RMF, like in a synchronous machine?

    Any explanations and insight would be much appreciated.

    Thanks!
     
  2. jcsd
  3. Nov 8, 2008 #2
    Re: Synchronous motor - replace dc field winidng with low impedance short-circuited c

    I do have the feeling Lenz Law is working against you, but I am not sure, especially at higher frequencies. So the current induced in the coil will try to prevent the field from changing. If the field drops on a stators shoe, the coil will produce a field in the same direction and thus be attracted to the stator. When the field rises the coil will produce an opposing field, which would turn the coil away, but this causes a drop in the coils field again which might pull it back. Maybe you could you make the coil a LC-Oscillator. But all of this is speculative
     
  4. Nov 8, 2008 #3
    Re: Synchronous motor - replace dc field winidng with low impedance short-circuited c

    What you are describing is called a repulsion-induction motor. A repulsion-inductor motor starts as a universal motor with the field and armature wired in series (I think). After it is up to speed the brushes are short circuited and it run as a synchronous motor locked to the line frequency. I don't think this type of motor has been manufactured for quite a long time. It boasted large starting torque with regulated speed. It is technically a repulsion-start/induction-run motor.
     
  5. Nov 9, 2008 #4
    Re: Synchronous motor - replace dc field winidng with low impedance short-circuited c

    Yes, indeed this way is one of ways for running of synchronizing motor which called induction starting. Of course low impedance induction path is not suitable for good starting and in practice one resistor adds to induction path for improving of starting torque.

    For more information you can refer to Machine Riddle No.12 from http://electrical-riddles.com
    :smile:
     
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