calculating standstill stator emf

Discussion in 'General Electronics Chat' started by daviddickason, Aug 15, 2017.

  1. daviddickason

    Thread Starter New Member

    Jul 11, 2017
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    Hello.How do I calculate the stator emf at standstill when an 3 phase ac induction motor is starting up?
     
  2. profbuxton

    Member

    Feb 21, 2014
    349
    158
    The stator emf will be the applied voltage minus any volts drop in the mains due to starting current
     
  3. daviddickason

    Thread Starter New Member

    Jul 11, 2017
    4
    0
    What are the mains? Is it the rotor?
    So if its a 240 Volt AC its 240 volts minus rotor emf?
    Thanks for the reply you gave me.
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
    16,176
    4,739
    An induction motor at the point of switch-on is a transformer with shorted turn secondaries, causing a short period of saturation.
    Max.
     
    cmartinez likes this.
  5. profbuxton

    Member

    Feb 21, 2014
    349
    158
    Mains?. That is the 3 phase feed to the motor. The voltage used here is a nominal 415v phase to phase volts. So there will be three wires feeding the 3 phase supply to the stator.
    As Max stated, at switch on the motor appears like a transformer with shorted secondary(see construction of induction motor details) so instantaneous current is only limited by the stator impedance and cabling resistance and source(whatever supply you have) impedance.
    If you want to be pedantic you can also add in the impedance of the rotor(reflected back into the stator) to calculate to total impedance the stator presents to the supply on startup. Then you can determine how much supply volts will be at the stator on startup.
    Note that induction motors have a LARGE starting current at standstill.
     
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