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Why is there no induced emf in electric motor?

  1. Jul 20, 2008 #1
    Hello everyone :smile:

    I got a small question. In an electric motor when the coil rotates isn't this a change of magnetic flux. So wouldn't there be an induced emf in the coil?

    Any help would be appreciated. Thanks :smile:
  2. jcsd
  3. Jul 20, 2008 #2


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  4. Jul 20, 2008 #3
    I think it's because in a typical motor (where the rotor is the coils and the stator is the magnets) the field in the coils is being produced by the current passing through it. This field then repels the like field from the magnets. Depending on design, a motor can then be turned to produce an electric current. However the more efficient motor designs can't produce electricity. I think it has to do with the AC currents canceling out.

    Besides the typical and the brushless mentioned above. Look up the e-torque motor. It uses the Lorentz force to its advantage. It's a brushless motor that doesn't need the position indication (it's a DC motor), and is ~95% efficient.

  5. Jul 20, 2008 #4


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    As Jeff mentioned, there is an EMF in the rotating coil of a motor. The effect is to reduce the current through the motor. You can see this for yourself if you connect an ammeter in series with a small DC motor. Power the motor up but hold the shaft to keep it from rotating, and note the current. Then release the shaft to let it spin, and watch the current reading drop.

    p.s. DON'T try this with power tools (eg. saws, drills, etc.). These motors are (usually) too powerful to hold still by hand, and you could really hurt yourself. Instead use a small DC motor similar to what powers a computer fan.
  6. Jul 20, 2008 #5
    There is both an induced and counter emf and both are required.

    When a conductor moves across a magnetic field it will have an emf induced within itself. Since the conductors in the armature of a motor are cutting across a magnetic field as the armature rotates, an emf is produced in the conductors and this emf opposes the current being applied to the armature from the outside source. This induced voltage is called counter emf, and acts to reduce the amount of current flowing in the armature. The net emf is the difference between the applied emf and the counter emf.

    An example of counter emf is a starter-generator. It takes battery voltage to start an engine and acts as a motor. After the engine starts and is running at normal speed, the counter emf produced in the armature becomes greater than the applied battery voltage. The current then flows in the opposite direction and charges the battery.

    Another example is a series wound motor like the starter on an automobile. If you run it without a load on it, the speed of the motor will continue to increase for as long as the counter emf is below the applied emf. The speed may increase far above the normal operating speed of the motor, and may result in the armature flying apart because of the centrifugal force developed by the rapid rotation.

    Source Electricity and Electronics for Aerospace Vehicles
  7. Jul 20, 2008 #6
    Right. It's important to understand that back-emf exists only during "load"

    Load is simply a complete electrical connection with resistance and will most definitely express a back-emf. No load, no back emf.

    Experiment: Take a finely-wound, hollow core solenoid and drop it down a magnetic rod.

    Condition 1: the solenoid is "open-circuited", not electrically connected to anything even itself.
    Condition 2: the solenoid is "closed-circuited", electrically connecting to itself alone(short-circuit) or, say, to a small LED glued to the solenoid.

    The open-circuit solenoid will drop down the magnetic rod virtually unimpeded, but the closed-circuit solenoid will create a back-emf and drop slowly; perhaps even lighting the small LED.
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