Explanation of high starting current in stator for squirell-cage induction motor.

omniscient

omniscient

@omniscient-50iCf9 • Oct 26, 2024
Hi All

Can someone please explain, to me, why there is a high current draw from the power-supply when DOL starting a 3-phase induction motor please?

  • I understand that initially the rotor is stationary, it behaves as a short-circuited secondary wiring in a transformer meaning the initially induced emf/current of the rotor is high.
  • I understand that the 'relative' speed of the stator field to that of the rotor is very high because the rotor is not spinning, thus a large emf is induced in the rotor.
What I don't understand is why this translates to a higher current draw from the power supply to the stator windings.

I've spent a few hours googling - everything explains why the rotor experiences the high emf/current but not the stator. I assume it must be a very basic principle but it escapes me.

Thanks in advance.
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  • aj_onduty

    aj_onduty

    @aj-onduty-BvuwMK • Aug 25, 2010

    You found out that the rotor is stationary and due to that, the stator is short circuited, right? So, what happens in a short circuit? the voltage across the stator tends to be zero. Still we are applying power to the terminals. So, power is equal to P=VIcos(phi). Here, the power applied remains the same. So, when the voltage becomes almost zero, the current shoots up. Any more doubts or if you think I am wrong, let me know in here.
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  • omniscient

    omniscient

    @omniscient-50iCf9 • Aug 26, 2010

    I figured it was down to the stator windings essentially being a short-circuit; thanks for the confirmation!
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  • manoj_2all

    manoj_2all

    @manoj-2all-YWk0FH • Nov 3, 2010

    As there is no Back EMF existing on the motor side due to this the motor consumes high starting current as it connected directly to the power supply.
    To reduce the starting current we use different methods, as the motor is in running it induces an Back EMF by opposing the source EMF which reduces the current value.
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