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PM generator direction vs. polarity

  1. Dec 10, 2009 #1

    I have a 24v 250w PM DC motor that is being used generate electricity.

    Problem is that the way it is installed, it can be turned in either direction which of course gives me a polarity problem.

    I'm sure there is a simple solution, but could someone tell how I maintain a constant polarity regardless of the motor's direction.

    Furthermore, the generator will be charging a battery so will need a diode of about 25amps.

  2. jcsd
  3. Dec 10, 2009 #2
    You will need a 4-diode bridge, with about a 1.2 volt drop, to make the voltage output completely independent of direction. Also, neither side of the motor (e.g., motor case) output can be a good ground. The ground will be one side of the bridge. (I presume you are talking about a brushed dc motor with a PM stator).
    Bob S
  4. Dec 10, 2009 #3
    Thanks Bob,

    I just had a look at a http://upload.wikimedia.org/wikipedia/commons/1/13/4_diodes_bridge_rectifier.jpg" [Broken] for a bridge rectifier and it does make perfect sense!


    ps the motors are brushless but I suppose that makes little difference?
    Last edited by a moderator: May 4, 2017
  5. Dec 10, 2009 #4
    It make a lot of difference. A BLDC (brushless dc) motor your size probably has a Hall Effect sensor (to sense rotor position) in it, along with an H-bridge polarity switch, to control the stator windings. The motor control circuitry requires a voltage within narrow(?) limits to operate. Your motor may also have a speed control based on PWM (pulse width modulation). Is your motor bi directional? Have you used this motor already for generating electricity?
    Bob S
  6. Dec 10, 2009 #5
    Ah I see, the motor we are using is this http://www.monsterscooterparts.com/24v30mowisp.html" [Broken].

    To answer your question regarding generating electricity! Yes, we belive it or not we used them to do http://www.bbc.co.uk/programmes/b00p8469" [Broken]!

    Look forward to your reply.

    Last edited by a moderator: May 4, 2017
  7. Dec 10, 2009 #6
    Nice motor. I have one. it has brushes. It will generate dc power in both directions. It may have 4 (four) brushes (I forget). Bob S
  8. Dec 11, 2009 #7
    Yeah they are an excellent motor, well built and made from decent metal!

    Bob I was wondering, do you think http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=2278621" would suit?

    Really appreciate your help. You'd think after powering an entire house with these we'd know what we are talking about, its just that we've never had to use them in a b-direction manner.

    Last edited by a moderator: Apr 24, 2017
  9. Dec 12, 2009 #8
    Last edited by a moderator: Apr 24, 2017
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