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Question on Back emf generated by motor

  1. Jun 4, 2009 #1
    1. Is there an electric motor of any type that does not produce back emf.
    2. Is it possible to design a motor that does not generate back emf.
    3.What are the demerits of designing a motor that do not produce back emf.
    4. What are the merits of having a motor that generate back emf.
    5. Suppose a 9v dc source is connected to a dc motor, now due to back emf that is generated in the motor winding, the effective volts is less than 9v. Hypothetically, if the motor does not produce back emf, does this mean the effective voltage in motor would be 9v and is the motor going to rotate much faster.
  2. jcsd
  3. Jun 4, 2009 #2


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    Sounds like homework...what are your thoughts?

  4. Jun 17, 2009 #3
    Back emf or induced emf is the nature of rotating machine (e = - dø/dt) which named dynamic induced voltage against static induced voltage that absorbed in transformers, therefore everywhere that there is magnetic moving part we front the back emf. In starting condition of motor, back emf is zero which cause maximum flux density/mmf and required maximum electromagnetic torque for overcome the rotor static inertia.

    V – Eb = Ra.Ia
    Eb = 0 (in starting condition)
    V , Ra = constant
    Ia = maximum
    Ts æ ø æ Ia = maximum

    Creative thinking is enjoyable, Then think about your surrounding things and other thought products. http://electrical-riddles.com
  5. Jun 17, 2009 #4
    1. Salient pole synchronous motors can be phased to be either inductive or capacitive at no load, so my answer is synchronous motor. The only one I have seen was 400 HP. Read about synchronous capacitors.
    2. Unless you are designing a salient pole synchronous motor, it will be inductive at no load, so it has back emf.
    3. Will draw too much current at no load.
    4. reduced current and power dissipation at no load.
    5. And it will get very hot.
    Check into merits of series-wound vs. shunt-wound dc brush motors. Compare to PM dc motor.
    Last edited: Jun 17, 2009
  6. Jul 21, 2009 #5
    hi friends,.
    good morning,
    i have just started my engg. and my question is how an inductor generates "BACK EMF" when it is connected to an voltage source??
  7. Jul 21, 2009 #6


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    I rigged up an interesting demo of this.

    You get a small, permanent magnet DC motor and run it from a half wave rectified AC waveform, without any filtering. So there is a diode directly connected to the motor.

    Now, observe the waveform across the motor with an oscilloscope. You will see the peaks of the half wave rectified AC, and this is what drives the motor.

    BUT, in between the peaks, where there is no supply voltage, you will see the back EMF. It is caused by the rotating windings in a magnetic field just like a generator (which it is).
  8. Jul 22, 2009 #7
    1) Since when has a induction motor produced back emf when no AC voltage is driving the motor, experts? The core has no initial flux. What should induce flux in the core? I don't see a source.

    I don't see a back emf in an induction motor with open leads.
    Last edited: Jul 22, 2009
  9. Jul 22, 2009 #8


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    Back EMF is just the generator action of a conductor moving in a magnetic field.

    A squirrel cage motor will not produce output as a generator because the field coils are not being excited and there is no magnetic field. Any output would be inaccessible anyway because there is no connection to the armature coils.

    However, when it is operating, back EMF plays an important part in its operation.

    The currents in the armature occur because of transformer action from the field coils. The coils in the armature are short circuited and will draw a lot of current if the motor is not turning.
    This causes large currents in the field coils, just like it would if the secondary of a transformer was short circuited.

    However, once it is turning, then back EMF gets generated in the armature coils and reduces the otherwise very large currents in the armature and in the field coils.
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