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Is there a delay of 1 or 2 seconds in generation of back emf in a dc motor

  1. Sep 20, 2012 #1
    Does the back emf in a dc motor start to appear after a slight delay(say 1 or 2 seconds) after starting up the motor?
    I have had this simple doubt for a long time.According to the "generator principle" both applied emf and back emf will always exist simultaneously.Also it would be in violation of lenz's law for such a delay to take place.Would'nt it?
     
    Last edited: Sep 20, 2012
  2. jcsd
  3. Sep 20, 2012 #2

    K^2

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    Back EMF should depend entirely on phase and angular velocity of the rotor for any given motor. It's not a direct response to the applied EMF. So if it takes a bit of time for the motor to spin up, you might see something that looks like a delay in response. But this has more to do with mechanics than electrodynamics.
     
  4. Sep 20, 2012 #3
    thanks k^2
    So if i were to measure the waveform using an oscilloscope or some other relevant device,i would see that the back emf reading remains close to zero during at least the 1st second of operation of the dc motor after starting it up.
     
  5. Sep 20, 2012 #4

    CWatters

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    Consider a small DC motor connected to a large heavy flywheel. Might take 20 seconds for the system to reach full speed. The back emf takes the same time to reach a maximium. The current during that time can be very high but reduces as the motor speeds up.

    Same motor, no load it will reach max speed very much faster.

    In both cases the back emf starts increasing the instant the motor starts moving.
     
  6. Sep 20, 2012 #5

    K^2

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    Precisely. Oh, and it doesn't matter what causes the motor to spin, so measuring back EMF might actually be easier without applying the voltage, and spinning up the motor by other means.
     
  7. Sep 21, 2012 #6

    rcgldr

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    CWatters is correct. In the case of a sensor based brushless dc motor controller, during startup, after the first 3 or so steps (in case the first step goes the wrong direction), the controller can start using the back emf as feedback. This usually takes less than 1/10th of a second for a typical dc motor, depending on the load.
     
  8. Sep 21, 2012 #7
    Back EMF is directly dependent on the angular velocity of your motor's rotor. Whatever rate your rotor is rotating at in any instant of time will directly relate to the back EMF generated at that instant of time. The time for your motor to reach a given angular velocity depends on the resistance and inductance of your windings that will create a time constant slowing down the time that the applied EMF is attained, and then even when this has been reached, there is another mechanical time constant of the inertia and friction of the motor and its load that will slow down the time that the angular velocity reaches steady state (and so, back EMF too).

    So your thread question is completely dependent on all of these variables to where the direct answer would be no, not 1-2 seconds necessarily, but it is a finite, non-zero, time delay for back EMF to reach any value after an applied EMF in a DC motor.
     
    Last edited: Sep 21, 2012
  9. Sep 21, 2012 #8
    thanks all,
    my doubt has been cleared :)
     
  10. Sep 21, 2012 #9
    Also i was wondering if the back emf from the dc motor could be routed into another circuit using a diode;just like using a flyback diode on an inductor to prevent the sudden build up of counter emf when supply to the inductor is switched off.

    This must be impossible because if it were true, that one could somehow "recapture" back emf or simply prevent it from countering the applied emf then people would have done it and made motors very efficient in the process- possibly by also using the motor as both a motor and generator simultaneously.

    obviously this has nothing to do with regenerative braking or any such mechanism.

    Actually its a very simple experiment because motors and diodes are well understood.yet by doing a google search for routing back emf to an external ckt, all i found was just a large number of "free energy" patents talking about "recapturing" back emf and recharging source batteries instead of dissipating it.i am sure all those claims are false otherwise electricity would cost a lot less & there would be no energy crisis.
     
    Last edited: Sep 22, 2012
  11. Sep 22, 2012 #10

    CWatters

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    Modern brushless motors ARE very efficient. Some have been made that are over 95% efficient. Remaining losses are down to winding resistance, bearing friction, air drag on the rotor etc

    Generators rely on backemf to work.
     
  12. Sep 22, 2012 #11

    CWatters

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    It might be worth understanding how the power of the magnets in a permanant magnet DC motor can effect efficiency...

    Lets say you want your motor to run at a particular speed when a fixed voltage is applied. Recall that these motors basically accelerate until the backemf is close to the applied voltage. The backemf depends on the power of the magnets and the number of turns (think generator).

    So if you have a motor with weak magnets the rotor will need to have more turns to generate the same backemf. More turns = more resistance in the windings = greater losses.

    Swap the magnets for nice strong samarium cobalt magnets. Now you need fewer turns to achieve the same backemf. Fewer turns = lower resistance = lower losses.

    An interesting fact is that if you overheat this type of motor and partly demagnitise the magnets the no-load speed goes up not down. It has to spin faster to generate the same backemf.
     
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