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Running motors as generators

  1. Jul 14, 2011 #1
    The text books say that motors and generators are the same structurally, they just differ in what is input (motion / voltage) and what is output (voltage / motion). I have also read that electric trains use this phenomenon for their 'electric braking', by running their motors as generators by cutting the power to them and connecting them to resistors which draw huge current and help to brake the train.

    To demonstrate this effect to my students, I want to use the pedestal fan in my room (running off the mains).

    Will the following work? -

    I run the fan at its highest speed. Then I take out the power cord from the mains socket and quickly connect it to a small bulb. Will the bulb light while the fan coasts to a stop? Specific query - does the effect work on AC as well as DC motors?

    Can our fans be braked in the same way as trains, by not merely cutting power to them but by drawing power from them by having a third position on the switch where a resistor is connected across the fan to drain its current (and thus its residual motion)?

  2. jcsd
  3. Jul 14, 2011 #2
    Well it is possible... but the output power won't be significant as the pedestal fan being a single phase m/c will introduce high impedance and suppress effective power generation. You can select a fan of higher capacity (eg. industrial fans, etc.) and check the o/p volts with a multitmeter. You can then connect a bulb of suitable rating to produce maximum effect. It could work for demo purposes but effective power generation might not be realised.
  4. Jul 14, 2011 #3


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    Unfortunately that is unlikely to work Ranjit.

    The problems in not just one of AC versus DC but also whether or not the motor is of a type that uses permanent magnets, and also whether or not there is some sort of electronic controller between the supply and the motor. All of these things could be factors in how well the demonstration might work.

    More Details.

    - Generally it's much easier to run a permanent magnet motor (either AC or DC) as a generator/alternator.

    - If the motor is not of the permanent magnet type then usually you can still make it "self excite" if it's a DC motor (particularly a shunt field DC motor) but this is more difficult if it's an AC motor like an induction motor.

    - The presence of an electronic controller could prevent it from working with just about any type of motor. Electronic controllers can certainly be designed to be bi-directional, allowing both motor and generator operation, but there's no guarantee that a controller designed for motor operation would necessarily allow for generator current to flow. In some cases there might be "inverse parallel" diodes that do provide a reverse current path, but there are lots of different possible controller types so I don't think you could really guarantee that.
  5. Jul 14, 2011 #4


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    BTW. I forget to add that in the past I have done this demonstration successfully with just a small 12V DC computer cooling fan and a small red LED. So perhaps you should try that combination.

    Most small computer cooler fans are 12V and around 60 to 150 mA. They are usually permanent magnet types but they also usually have an inbuilt controller (with switching transistor that operate like an electronic commutator). Fortunately these devices usually have inverse parallel diodes, because when I did this demo I tried about 3 different cooler fans and they all worked. In my case I was able to light the LED by simply blowing sharply on the fan to make it spin.
    Last edited: Jul 14, 2011
  6. Jul 14, 2011 #5
    Speakers (headphones included) and microphones have a similar relationship as well. I used a pair of headphones as a microphone once to record narration for a video I made. I also used them to talk with someone over AIM. It was awesome.
  7. Jul 14, 2011 #6


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    I demonstrated this once by driving a permanent magnet brush motor with a half wave rectified (but not filtered) DC supply.

    The power supply keeps the motor turning, but the motor generates a back EMF all the time and this is visible between input pulses when the motor is rotating but not being powered.

    If you observe the waveform across the motor with an oscilloscope, you can see the back EMF of the motor (ie the generated voltage) in between the half wave input pulses.

    Like this:
    [PLAIN]http://dl.dropbox.com/u/4222062/Back%20EMF.PNG [Broken]

    The waveform (shown in red above) is usually more noisy than the diagram indicates due to brush contact with the commutator.
    Last edited by a moderator: May 5, 2017
  8. Jul 15, 2011 #7


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    Ideally, you would use a DC motor (with a permanent magnet) and have a substantial flywheel which will run for some time after the changeover switch has been thrown. This will give you time to show the effect. The only proviso would be that the flywheel should not be so massive that it sstalls / burns out the motor. Or you could run it up slowly with a variable voltage supply.
    Yhis would also be a much safer solution!!
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