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Old fashioned 2 pole ac/dc motor/generator used for classroom demonstrations

  1. Mar 17, 2009 #1
    Hello this is my first posting. I have a device that I acquired from my old high school. It is a demonstration unit of AC and DC electricity generation. It has a small crank on one end. It is not equipped with permanent magnets in either the field or the armature. If I test the voltage off the commutator whilst spinning it without energizing the field-- no electricity is produced.

    By that same token if I energize the field with a 14 volt battery from a cordless drill, and then manually spin the crank, voltage is generated. (this is limited however, by the speed at which i can turn the crank, not very fast)

    I have successfully managed to wire it in series to a 14 volt battery from a cordless drill. This was done in the "DC configuration" It spins quite fast but the field windings get too warm to touch. I try to avoid running it for any extended period of time.

    Figure 1 shows the internal wiring of the device under DC "motoring" conditions. Omitted is the 14 volt power supply, which would be connected to the field windings.
    http://img25.imageshack.us/img25/2695/devicewiredfordc.th.jpg [Broken]

    http://img25.imageshack.us/img25/4782/diviceasac.th.jpg [Broken]
    Figure 2 shows the motor as configured, but without wiring, for presumably AC generation and motoring. I have yet figured out how to safely connect it to AC power. I have managed to energize the field with 14 Volt DC as before, and turn the crank--this generated some AC voltage. At the slow rotational speed the voltage dipped and rose. This proves it was AC.

    At this point I am beginning to wonder whether or not the device truly goes "both ways". I have produced enough proof for myself that it does indeed "motor" under DC, and that it generates DC, and I have proven that it generates AC. I have yet to prove if it can "motor" under AC.

    My ultimate question is this:: Where can I obtain low voltage and low current AC to make it motor under AC?
    (the field will still be energized by 14 volt DC, separately)

    how does AC frequency affect the speed of the motor if i do manage to make it run? 60 hertz with 2 poles? is that 3600 RPM? I am not sure the motor will handle that amount of rotational stress
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Mar 17, 2009 #2
    Hello and welcome to PF. Some electronics distributors (Mouser, Jameco, etc.) carry low-cost, low-power, AC-to-AC wall adapters.
  4. Mar 17, 2009 #3


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    Staff: Mentor

    I agree that you will need a low-speed AC source to experiment with driving that motor. Do you have access to a signal generator? You will still likely need some power amplification to drive the motor with the output of the signal generator.

    You might consider looking through the many electronics kits on the Internet, to see if there is an inexpensive kit for an audio signal generator, and one for an inexpensive audio power amplifier. Or use a combo kits something like these:


    It's great that you are curious and are working through the experiments to better understand how things work.
  5. Mar 17, 2009 #4
    Are any of the three copper bands around the rotor axle a commutator rather than a slip ring? If so, you can get DC out with DC on the field coil. Don't overheat the field coil with too much current. It should never be too hot to touch. Measure the resistance of the field coil, and keep the applied voltage below that number, so A<= 1 amp.
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