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Help identifying motor leads.

Discussion in 'Electronic Repair' started by Daniel Rudy, Oct 16, 2008.

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  1. Daniel Rudy

    Daniel Rudy Guest

    Cross Post: sci.electronics.repair, comp.robotics.misc

    *****

    Hello Groups,

    I have recently acquired two heavy duty motors for robotics use. Below
    is a link to the picture of one of them.

    http://www.mpja.com/prodinfo.asp?number=14386+MD

    I have both a left hand and a right hand motor. They are about a foot
    long, half a foot wide, and weigh in at about 40 lbs each (No wonder it
    cost $40 for shipping...UPS ground even.). The motor also has
    serviceable brushes too. There are four wires for these motors, colors
    red, black, white, and yellow. I'm going to assume that red and black
    are power, but what are the white and yellow for? A built in
    tachometer? The gearing is bolted on, but there doesn't appear to be
    any way to dismantle the motor so I can see how it is wired internally.
    Any thoughts or suggestions?
     
  2. Smitty Two

    Smitty Two Guest

    Two of the wires for the armature, two for the field.
     
  3. Daniel Rudy

    Daniel Rudy Guest

    At about the time of 10/16/2008 5:51 AM, Smitty Two stated the following:
    A DC motor that has a field winding? I need to find a way inside these
    motors...
     
  4. Daniel Rudy

    Daniel Rudy Guest

    At about the time of 10/16/2008 4:04 PM, Dave Plowman (News) stated the
    following:

    Interesting.

    I have some additional information. The black and white wires are
    connected to brushes. The DC resistance between the two is about 2.7
    ohms. The red and yellow wires have a DC resistance of 7.3 ohms. So I
    guess they can be wires either series or parallel. What I don't
    understand is if the red and yellow wires are for the field, then why
    bring them out? Why not just connect them internally?
     
  5. Daniel Rudy

    Daniel Rudy Guest

    At about the time of 10/16/2008 4:41 PM, Dave Plowman (News) stated the
    following:
    Sorry, but I'm not up on that theory. How can the field make speed
    control easier? For DC motors, I usually use a PWM scheme for speed
    control.
     
  6. IanM

    IanM Guest

    You cant reverse it if the field connections are internal. Reversing
    the polarity on both windings will maintain the same rotation direction.
    You need to reverse one winding relative to the other to reverse the
    direction.

    REDUCING THE FIELD CURRENT *INCREASES* THE NO LOAD SPEED.
    (dont take it too far, you wont be happy)

    Field loss is a critical failure and if unloaded its likely to
    over-speed till it grenades, otherwise the armature current will
    increase till it melts. Its therfore advisable to have a contactor with
    a low impedance coil in series with the field winding to cut power to
    the armature if the field circuit fails. At the minimum for bench
    testing, switch the armature to reverse, not the field.
     
  7. Daniel Rudy

    Daniel Rudy Guest

    At about the time of 10/16/2008 6:26 PM, IanM stated the following:
    Well, I just finished doing a bench test on this motor. The results
    that I have are listed below. The power source that I used was 4
    standard C alkaline cells wired in series for 6v. I removed the gear
    reduction and tested just the motor.


    field armature
    red yellow white black result
    -------------------------------------------------------
    + - open open no rotation
    open open + - no rotation
    + white yellow - CCW rotation, high torque
    - white yellow - CCW rotation, high torque
    + black - yellow CW rotation, high torque
    - black - yellow CW rotation, high torque
    + - + - no rotation

    Rotation was observed facing the motor. That last one I'm not sure
    about...maybe I don't have enough power. But this gives me a few ideas
    on power control. I'm thinking of placing a DPDT relay in series
    between the armature and field windings for direction reversal, then use
    a the standard PWM control from a microprocessor to control speed.
    Plus, this will also protect the motor too in case any winding fails it
    will open circuit.

    There are absolutely no numbers or any other markings on this motor
    besides a warning about it being hot. But from what I have been able to
    scrounge up on the web. It seems that this motor was manufactured by
    Magnetek and is rated at 24v. Stall current is 20. The lead wires are
    only 16 AWG, so at stall, even the high temp wires would melt...the
    wires are rated at 200c.
     
  8. To observe while facing _away_ from the motor would involve a mirror,
    I suppose. :)
     
  9. PeterD

    PeterD Guest

    No, my mother could have done that easily. A few of my teachers as
    well. All female, so perhaps we men are left out in the cold? <g>
     
  10. BobH

    BobH Guest

    There are absolutely no numbers or any other markings on this motor
    I have a pair of these motors that I bought for a medium sized bot. They
    proved not up to the job and were replaced with wheel chair motors. I
    think that they were made for elevator or subway door operation, several
    ads I saw a while ago indicated this. 5 years ago, they were bringing
    $70 each, now I think they are about $20 each.

    I am in the process of building an antenna rotator with the pair right
    now. I am planning on running the field windings on a constant 12V and
    PWM'ing the armature for control. I don't need a lot of torque, so I
    will try 12V for the initial input power.

    Good Luck,
    Bob
     
  11. BobH

    BobH Guest

    I did some studying on controlling wound field DC motors with the
    armature and the field windings in parallel and then driving the two
    from a PWM source results in sort of an odd torque/speed curve that is
    lacking in low speed torque. Running the field windings on a constant
    voltage and controlling the armature voltage gives a flatter
    torque/speed curve. Also, the adds that I remember seeing suggest that
    the motor can be run on 36-48VDC.

    I needed a place to attach encoders for closing the control loop and
    found that the output shafts are fairly soft steel and drill and tap
    very nicely. The unused side of the output shaft is exposed and flat,
    making it an easy place to attach the encoders.

    Good Luck,
    BobH
     
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