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Current and voltage relationship in an electric motor

  1. Jun 24, 2017 #1
    I can not understand that a motor (pumping) requires a current of 63 A under a voltage of 390 vac, and requires a current of 72 A under a voltage of 365 vac. Normally when the voltage increases the current increases with (ohm law). Is that the power delivered by the engine which is 37 kw is maintained.
  2. jcsd
  3. Jun 24, 2017 #2
    Power (3 phase) = 1.732 * V * I * PF
    Power in watts, I=Amps, V=Volts, PF= power factor

    Creating 37 kW of shaft power requires less current when the motor is supplied at a higher line voltage, and needs more current at lower line voltages.

    An added consideration is shaft speed varies proportionally with line voltage, but pump power varies as the cube (see: pump affinity laws).

    Induction Motor Characteristic - Voltage Effects.jpg
    Is the motor voltage rating 380V? If so, then 390V is 2.6% above nominal, and 365V is 3.9% below nominal nameplate rating.
    Inspect the generic curves above to gain a sense how voltage variation affects motor performance.
  4. Jun 24, 2017 #3
    Hi Asymptotic,
    thank you for your good explanations,

    the motor voltage rating is 400v.

  5. Jun 24, 2017 #4


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    Staff: Mentor

    Three phase motors operate at fixed RPM and draw whatever current they need to generate the torque they need to keep spinning or fail trying.
  6. Jun 24, 2017 #5
    thank you,
    Therefore it is the velocity that imposes the torque therefore the current, it is a natural regulation of the phenomenon. Nothing is created, nothing is lost all changes.
  7. Jun 24, 2017 #6

    jim hardy

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    Induction motors have something called a "Speed-Torque" curve. It looks like this.
    (the obvious annotations are mine)

    Note how small is the operating range , circled in purple.
    When you lower voltage, the motor runs now on a lower torque curve.
    So it has to 'walk back up' that curve by slowing down a little bit , in order to make desired torque.
    Walking up the torque curve also walks up the current curve. It will require more than normal running current to make full torque at reduced voltage.

    That's why real long extension cords can cause an induction machine like a fridge or pump to burn up its motor.

    hope this helps

    old jim
    Last edited: Jun 24, 2017
  8. Jun 25, 2017 #7
    thank you jim hardy
    your explanation is very good, and it's ok for me. I begin to see things clearer
  9. Jun 25, 2017 #8

    jim hardy

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    That's what counts. Keep up your good work !

    old jim
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