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Is Counter-EMF a good thing?

  1. Aug 1, 2012 #1
    Hallo everyone,

    I know as a motor speeds up! It acts like a generator and due to that fact. It produces counter-EMF. However, I'd like to know a few things about this effect!

    Would counter-EMF cause the motor to draw more current?
    Would it effect the motor negatively?

    Finally,(I doubt this is possibile) If counter-EMF was greater! Then input what would happen?(THEORETICALLY).

    Thanks all,
     
  2. jcsd
  3. Aug 1, 2012 #2
    why would you suspect a higher current draw from a counter EMF?
    Counter EMF is what causes the motor current to drop from the inrush levels once the motor starts spinning.

    Counter EMF is inherent to a motor, so I think you'd be hard-pressed to argue that it has some adverse effect on the motor.

    What do you think would happen if counter EMF was higher?

    http://en.wikipedia.org/wiki/Counter-electromotive_force
     
  4. Aug 1, 2012 #3

    uart

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    If by "input" you meant the return of energy to the source (also known as generation or regeneration) then yes it is very possible, and something that is very commonly done. In this case however the mechanical "load" must provide the driving torque while it's regenerating.
     
  5. Aug 1, 2012 #4
    I knew what COUNTER-EMF is... But I knew it wouldn't be higher then input.

    But Theoretically what would happen? Just imagine. Will it turn off the motor or what do you all think?
     
  6. Aug 2, 2012 #5
    My theory is since the counter EMF (THEORETICALLY) is greater then input, the motor would not operates anymore because that force is countering the input?
     
  7. Aug 2, 2012 #6

    NascentOxygen

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    If you power a motor from a battery, and conditions cause the motor to produce a higher counter emf than the battery voltage, then nett energy is being delivered to the battery.

    On steep ascents, e.g., slopes of foothills or escarpments, it is sometimes necessary to timetable electric locomotives so that at the time one heavily-laden train is ready to climb the steep incline, another on a parallel track is descending and thus generating extra voltage for the overhead electricity cable.
     
  8. Aug 3, 2012 #7
    I noticed from this thread and reading about this matter. That its actually a good thing and not something to worry about! The greater the C-EMF the less input current to be drawn!

    Overall I thought C-EMF was a bad effect... However, it really it isn't.

    (Please correct me If I'm wrong... At the end we all are here to learn.)

    Thanks everyone! :approve:
     
    Last edited: Aug 3, 2012
  9. Aug 3, 2012 #8
    Let me use an example to illustrate my understanding:


    I have an electric motor and a heavy wheel acting as the load.

    Now when the electric motor is turned on without the wheel attached as a load, it will instantly run easily and goes fast because there is no mechanical resistance of the load. So the motor would eventually speed up and not need more current because of C-EMF. We can agree that the hardest part was only the beginning for the motor to start.

    Now when I attach a physical load load on the motors shaft/rotor. In our case that's the wheel... The motor starts to draw more and more current for torque! So there is a force for it to start and rotate, and the RPM significantly drops and the C-EMF would be less generated.However, if the "LOAD" was not a major "resistance" to the motor... In a sense the motor can "handle" the wheel as a "load". It will need more current at the start up with the load because it needs a stronger rotational force to "move" the object from rest. Eventually it will speed up and speed up and becoming less of a load, then the C-EMF would generally reduce its motor input current and then the only thing needed between both RPM & Torque is really the higher RPM and low torque since the wheel is already in rotational motion. Or should we say: Rotational inertia.



    Is this a good example clarifying my understanding? Hope it is.

    Thank you!
    Mo,
     
    Last edited: Aug 4, 2012
  10. Aug 11, 2012 #9
    That means the motor would either stop or counter rotate. Because I'm trying to imagine what you're saying its difficult o see this happen.
    The rotor itself would become a generator. So in a sense when CEMF is higher it can provide power back to the battery.However, the motor would stop its motion I think or counter rotate because of this change... Any opinions?
     
    Last edited: Aug 11, 2012
  11. Aug 11, 2012 #10

    NascentOxygen

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    If an electric locomotive is going down a steep slope, gravity speeds it up faster than the motor would drive it at that speed setting. So the back emf is greater than the battery voltage — a condition where current is now forced into the battery terminal instead of coming out of it. There is no reason for the motor to stop or reverse direction. It's a case of the vehicle wheel's turning the motor, and in these circumstances it's called a generator.
     
  12. Aug 11, 2012 #11

    NascentOxygen

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    Right so far. (see minor correction)
    I don't understand that.
     
  13. Aug 12, 2012 #12


    Ok, is there a way to control/decrease/re-direct C-EMF? Under the same speeds its in. I was thinking if the C-EMF was 20 V and my input was 10, I could possibly used a step up transformer to make it 21 V. So the C-EMF would not be the HIGHER potential. The motor starts to rotate very very fast reaches it potential! It would not draw any current a all. So we could apply a higher EMF then the produced C-EMF and still the motor would not consume any current. :approve:
     
  14. Aug 12, 2012 #13
    Sorry didn't describe it properly. I ment that the load, would only require torque only in the beginning of motion. When it moves faster and faster and faster. It somewhat become's easier to rotate. Not much torque is then needed.
     
  15. Aug 12, 2012 #14

    NascentOxygen

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    That's right, with no load the motor has to develop only sufficient torque to spin its own rotor, overcoming bearing friction and air resistance around the rotor windings. You maybe could encase the motor in a vacuum if you wanted to minimize air resistance losses. :smile:
     
  16. Aug 12, 2012 #15

    NascentOxygen

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    Back emf is usually a desirable thing! By putting power back into the battery, it allows the driver of an electric car to recoup some energy when coasting downhill. Charging the battery exerts a braking effect, so this slows the car without needing to wear out brake linings. In electric trains, they something switch a big resistor across the motor-generator to waste that generated power when slowing or going downhill, by doing this it still affords the braking effect. If you don't want braking, then just disconnect the motor and it has nothing to absorb the current it is trying to generate. It freewheels and the locomotive rockets faster and faster downhill without braking.
     
  17. Aug 12, 2012 #16
    In phyiscs you cant say this thing is desirable, and this thing isn't desirable. Physics is all related together. If you accept the Faraday's Law of elctromagnetic induction or you think it is a good thing, then back-emf is just its consequence. If there was no back emf, (well it can't be), then nothing would work. Because, not having back emf means, physics theories not working, which means, we know nothing.

    On more technical terms, The Work Output of a DC motor = Back emf * Current
    If there was no back emf (which occurs when the motor is stalled), all the input power is lost in the heating.
     
  18. Aug 12, 2012 #17
    Im studying about all the effects of C-EMF. I know its desirable. However, in some causes you must know how to control it.

    I thought of an idea few hours ago... How about using a "DIODE"? To block/redirect the C-EMF? Is that possibile?

    I understand its a good thing but not in all causes. So using a "DIODE" could possibile force the C-EMF to go with the flow of the original EMF?
     
  19. Aug 12, 2012 #18
    Not necessarily.
    Maybe in some severe-circumstances something could differ.
     
  20. Aug 12, 2012 #19

    jim hardy

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    What's emf? It's a force on a charge, be it magnetic chemical or electrostatic in origin..
    What's current? Charge in motion.

    back emf is kinda unavoidable. it comes from the same thing that makes the motor turn, namely the force on a charge that's moving in a magnetic field.

    Charge travelling forward inside the conductor feels a force perpendicular to both the magnetic field and the direction of its motion. That mutual perpendicularity is called a "Vector Cross Product", though the name isn't really important. It pushes the charge sideways which makes the motor try to turn.

    When the motor starts to turn, the wire itself now has motion relative to the magnetic field, and THAT motion creates another "vector cross product" which pushes the charge backward against its forward motion along the conductor. That opposes the applied voltage, trying to reverse current flow. So it's called 'back emf' or counter emf.

    And it's really quite a nice thing , it makes motors and transformers practical.
     
    Last edited: Aug 12, 2012
  21. Aug 12, 2012 #20
    It truly is a wonderful effect. For some reason I though it was... A burden. However, a very useful effect.

    Now my previous question is: Is it possibile to use a "Diode" to force only the input EMF? And re-direct C-EMF to another position?
     
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