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Magnets, motors and mistake?

donnyb

Plastic
Joined
Dec 6, 2014
Hello to All,

I have a 3000lb capacity 12 volt DC winch used for taking my boat on and off its trailer, ie the motor has forward and reverse action via a remote by cable switch. Chinese made, several years old, but has had very little use.

Recently, the winch slowly died as the boat was being retrieved.

Being a retired mechanic, but with only basic auto electrical knowledge, I pulled the winch apart, expecting to find the brushes as the problem, but also thinking that did not tally with the little use it had been put to.

The problem turned out to be that one of the half moon magnets had become unglued, and was thereby attached firmly to the core. The brushes are OK.

I cleaned everything well after googling how that should and should not be done, glued the magnet in place (?) with high grade epoxy, reassembled the motor and hooked it up on the bench directly by its positive and negative leads to a 12 volt battery. So no relay or switch in the circuit.

It goes well, BUT... only if I give it a nudge to get started, and then it fairly flys.

So back to google, and while I found no direct reference to this, in reading and being reminded how these marvelous things work , I wonder if I have glued the magnet in back to front ? I have tried searching for "can DC motor magnet be glued in back to front?"

If I have by Murphys Law glued it in 180 degrees out,, I dont relish trying to get that magnet out again, as the epoxy I used is heavy duty. But maybe I could get the other one out and turn it around ? But then would I need to reverse the core/commutator to get the orientation of the whole unit as it originally was (which I think would mean Id have to drill and tap new end plate locating screwholes) ?

I did fortunately come across this forum which tells me there are a number of people in the world who know a lot about DC motors !

Hoping someone can fill in my large knowledge gaps.

Regards, Don in Australia
 
Some types of magnet will loose "flux" if pulled away from the magnetic circuit. If you had it out when you epoxied it, maybe it did.

That would be like "field weakening", which loses torque but speeds up the "base speed" of the motor. So it might not start well, but could really take off when not overloaded.
 
hmmmm. perhaps something else is still wrong.... Not sure how you get polarity wrong since the N or S pole is on the curved face, facing the rotor, not end to end. I doubt putting in 180 degrees out could have any effect.

I used to service small 12v DC motors used in a electro hydraulic servo that we made for an indy car I used to work on. I had to mark the magnets if I had to remove them for any reason. They were not glued, they had a plastic spacer between them to hold them in place. If you put them both in the other way around the motor will run backwards. It was amazing, you always had to look at the magnets after a wreck, the unit would show no damage, but many times the magnets would be shattered. In the wall at 220 mph tends to wreck shit.
 
The way the motor is spinning once I give it light nudge suggests what you say is what's happened JST. I think its destined for the bin then unfortunately, which is a pity really, as the gears are in good condition.

The motor has 11 splines, but I reckon it would be hard to find the match on the web, being sight unseen. A similar complete winch can be bought in Oz for $170.00 AU. Im retired, and got time to fix it, which I prefer to do with all my stuff that breaks down, but this one may have me beat.

I did read about a starting stator sometimes in a core, that momentarily tries to spin the motor in the opposite direction, to give it a nudge. I could turn the core around and see what I happens, but is this unfounded wishful thinking? Thanks Don in Oz
 








 
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