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Short circuit induction motor

  1. Dec 2, 2015 #1
    Just a curiosity about what happens to the flux, when you short circuit an induction machine.
    Say there is an initial flux and you're rotating the shaft of an IM and then you short circuit the stator, so no doubt you'll get large currents in the stator, but those large currents, will they cause a large stator flux? And will that stator flux oppose the rotor flux?
    So say you energised a squirrel cage motor, then made it act as a generator, then shorted the stator? what would happen? I imagine there would still be all sorts of induced voltages and currents between rotor and stator?
    Thanks
     
  2. jcsd
  3. Dec 2, 2015 #2
     
  4. Dec 3, 2015 #3

    NascentOxygen

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    Flux from induced current always opposes that which causes it. If this wasn't the case, then you'd have a motor that could power itself, and this defies the laws of physics.
     
  5. Dec 3, 2015 #4
    Heh, I'm not interested in breaking the laws of physics.
    Yeah ok, so thinking about the small period of time before the power dies out from losses. Wouldn't the situation be something like: there is initial flux and constant mechanical rotation being put into the generator, then there is an opposing voltage generated in the stator, then there is an opposing voltage in the stator, then the flux from the rotor induces another opposing voltage in the stator, then another flux is imposed on the rotor to create another opposing voltage on the rotor to oppose the stator flux.
    So there are two opposing fluxes, so I take it there is very little dΦ/dt total in both the stator and rotor? and the induced voltages and currents pretty much die out?
    Is this a fair assessment of a short circuit squirrel induction generator?
     
  6. Dec 5, 2015 #5

    NascentOxygen

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    You will need to distinguish between switching the power off (and leaving the stator open-circuit), and actually shorting the stator windings. I expect the rotor would come to a standstill faster in the latter scenario, and along the lines you describe, the rotor inducing current in the stator windings.

    Where there is a need to halt rotation of a 3Φ motor as quickly as possible, either routinely or in dire emergency, this can be achieved by momentarily swapping any pair of the mains cables, then disconnecting power before rotation reverses. This braking manouvre stresses the rotor and bearings. (see "plugging")
     
  7. Dec 5, 2015 #6
    Sorry, I'll start again:
    Rather than switching off the power, say it was runing as IM squirrel cage generator, that experienced sudden short circuit on the stator. Since the rotor is by design always short, what happens to the flux in the machine, and does the current in the stator build up, or die out?


    Ah, so a short swap from positve sequence voltage to negative sequence excitation, then open circuit, interesting.
     
    Last edited: Dec 6, 2015
  8. Dec 6, 2015 #7

    jim hardy

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    Think about that for a minute.

    You postulate short circuiting a motor while it's powered? That shorts the supply.

    That drives terminal voltage to zero
    so the motor behaves as a generator , stator current reverses and and adds to the fault current while its rotor field collapses.


    Once its rotor field is gone it's just a spinning mass.
    Presumably something has disconnected the supply by then.
    In my old plant our switchgear would do that within 5 line cycles. Presumably there's faster stuff out there now.
     
  9. Dec 6, 2015 #8
    Hey Jim, hope your health is coming along!
    Well say it was acting as a generator from the beginning, a squirrel cage IM can be a generator can't it? (if you energised it at first)

    [so what you're saying is: if it is acting as a motor, but there is also torque being put in the same direction of rotation, that when the stator excitation shorts, the stator field wil reverse direction, then the rotor field will collapse?....not reverse too?]

    Cheers
     
  10. Dec 6, 2015 #9

    jim hardy

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    reducto ad absurdum and oversimplify, then apply stepwise thinking and the magnetic basics you've already learned...

    crowbar the supply; at zero stator volts can there be a stator flux?

    it makes no difference whether the machine was motoring or generating
    you have a rotor spinning and it has current in its bars that make a mmf
    that's rotating
    and any resulting flux would cut the stator windings making stator volts
    so current flows in stator to oppose rotor mmf just like in a shorted transformer
    until the rotor's mmf dies off meaning rotor current went to zero

    observe energy can come from rotating inertia in addition to what's stored in the magnetic fields
    so the decay time constant is not simply Rrotor X Lrotor

    that mental model should help you single step through this brief introduction:

    http://apps.geindustrial.com/publibrary/checkout/SC-Motor?TNR=White Papers|SC-Motor|generic

    they say fault current into a short persists for only 4-5 cycles
    voltage decay on a motor that's open circuited is much slower because stator current doesn't push against rotor mmf


    old jim
     
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