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DC motor running in reverse after repair

  1. Apr 15, 2017 #1
    Hi all, so my mercury 25 outboard starter was starting to give it up. I took it apart, and yup, the brushes were badly worn. Well, I have several generic brushes kicking around, so I simply swapped them out. Very simple operation. And yet after assembling the damn thing, it now runs backwards! The flywheel engagement gear doesn't jump up and engage the flywheel because it's run on a spiral grove machined in the motor shaft and the shaft has to rotate in the correct direction to make the gear spin upwards. I pulled the motor again and connecting it directly to the battery, and the gear doesn't jump up unless I put + on the case and negative to the power connection. How is this even possible? I didn't remove the gear assembly when I pulled the motor apart.
     
  2. jcsd
  3. Apr 15, 2017 #2

    jim hardy

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    Got a picture of the brush holders ?

    Sounds like you swapped the brushes, ie got them reversed. Are the leads long enough for that to happen?

    I think you'll get to take it apart again.

    .
     
  4. Apr 15, 2017 #3

    Averagesupernova

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    Is this a permanent magnet starter motor? I had never heard of such a thing until recently but I understand they do exist. If the magnets are moved relative to the brushes this could also happen.
     
  5. Apr 15, 2017 #4

    jim hardy

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    They're becoming common on small engines like lawnmowers. Wouldn't surprise me on an outboard, 25hp is a "small" one nowadays .

    Holy Cow - they're on big motors now, 4 cylinder outboards and outdrives
    http://www.wmjmarine.com/18-5919.html


    probably be in cars soon if not already.
     
  6. Apr 15, 2017 #5
    The leads are right beside the brush holders, so no mixup there. But what I did do wrong is when reinstalling the end plate that holds the brushes, i didn't notice a tiny alignment tab and put it on about 1/8" out of alignment. Apparently that was enough to make it run backwards! I don't understand why, but there it is. Thanks for the help. I tried posting on an outboard forum and nobody had a clue why this could happen, so thought i'd check with experts!
     
  7. Apr 16, 2017 #6
    A google search may give you the answer faster than I could. It is a common thing on DC generators and some starting motors that when it is disassembled there is a loss of field orientation. This will cause a reversed rotation or opposite current flow in the case of generator. On reassembly you need to "flash the coils" this is a direct application of 12vDC to the field coils. It will initiate the correct polarity in the field and then residual magnetism will do it on each additional start.
    With more of the particulars of your motor it should be easy to find the correct procedure. We can possibly work it out if you can't find the correct one online.
     
  8. Nov 6, 2017 #7
    Same thing happened to me with a fractional HP DC servo motor. I just took it apart again and then it spun correctly. Ketch22 seems to have the answer.
     
  9. Nov 6, 2017 #8

    jim hardy

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    necropost ?

    I just learned something so will post it to this old thread .

    That it runs in opposite direction when battery is reversed pretty much nails it as a permanent magnet motor.

    If indeed it is,

    I just had a similar happening with a permanent magnet MinnKota trolling motor.
    The center part of case holding the magnets can go either way, there's no key to assure orientation. I failed to mark it when disassembling so of course put it back half a turn out, and it ran backward.

    Physics of what happened to me and i think happened to OP:

    motor1.gif

    To reverse a DC motor you must change relation between field and armature. You can reverse the direction of armature current or you can reverse the direction of the magnetic field , but not both if you want it to run backward.

    That picture can represent either type of motor, permanent magnet field or wound field.
    The S and N field poles shown can be an electomagnet or a permanent magnet.
    The grossly oversimplified one-turn armature coil is just for demonstration,

    If it's a wound field motor, either series OR shunt,, swapping battery reverses BOTH field AND armature so it runs the same direction because two negative currents in both field and armature, make a positive torque.

    If it's a permanent magnet motor, reversing battery only reverses the armature current not the field so it changes direction. Permanent magnet field has no current to reverse.
    Assembling it with the field magnets upside down though reverses the N-S field orientation but not the armature current so it runs backward .

    It would be convenient for a manufacturer to have a reversible starter for marine applications where one engine rotates opposite the other.

    I suspect @rattlenbang made same mistake i did. I took a manufacturer's field polarity mark to mean "this side up" , not "other side up" . .

    Hmm @ function is not working again...

    old jim
     
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