DC motor running in reverse after repair

In summary, the conversation discusses an issue with a Mercury 25 outboard starter that runs backwards after the brushes were swapped out and the motor was reassembled. It is discovered that the alignment of the end plate holding the brushes was off, causing the motor to run in the opposite direction. This is due to the motor being a permanent magnet motor, where reversing the battery only changes the armature current and not the field, causing it to run in reverse. The solution is to correctly align the end plate or "flash the coils" by directly applying 12vDC to the field coils during reassembly.
  • #1
rattlenbang
2
0
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.
 
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  • #2
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.

.
 
  • #3
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.
 
  • #4
Averagesupernova said:
I had never heard of such a thing until recently but I understand they do exist.
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.htmlprobably be in cars soon if not already.
 
  • #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!
 
  • #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.
 
  • #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.
 
  • #8
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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1. Why is my DC motor running in reverse after repair?

There could be several reasons for this issue. It could be due to incorrect wiring or connections, a faulty switch, or a damaged commutator. It is best to double check all connections and consult a professional if the issue persists.

2. Can a DC motor be easily reversed?

Yes, DC motors can be easily reversed by simply swapping the positive and negative wires. However, it is important to note that this may cause damage to the motor if not done correctly.

3. How do I know if my DC motor is reversible?

Most DC motors are reversible, but it is always best to check the manufacturer's specifications to confirm. If the motor has two leads, it is most likely reversible. If it has more than two leads, it may require additional wiring to reverse the direction.

4. What should I do if my DC motor runs in reverse unintentionally?

If your DC motor is running in reverse unintentionally, it could be due to a faulty switch or wiring. Check all connections and replace any damaged components. If the issue persists, consult a professional.

5. Can I control the direction of a DC motor?

Yes, the direction of a DC motor can be controlled by using a switch, relay, or a motor controller. These devices can reverse the polarity of the motor, thus changing its direction. It is important to use the appropriate equipment and follow proper wiring techniques to avoid damage to the motor.

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