Friday, December 23, 2011

How to Re-wire a Potted Motor, Part 12: Detaching the Field Core

Now we need to access the motor leads so we can replace them. To get to the motor leads, we need to detach the motor’s core, or field core, from the motor housing. As you’ll see, we cannot detach it completely; although we will physically separate it from the housing, it will remain attached via two wires. (In the photo up above, the two wires in question look like water mocassins.) We must be careful not to disturb those two wires.

The first thing you’ll need to do is remove these two screws.

The screw on the top side of the motor--which appears on the right here, since the motor is on its side--has a metal plate attached to it with the motor’s specifics written on it. This is like a laundry-care tag for electric motors. In any case, it comes off with the screw.

Put the screws and the metal plate somewhere safe. Now, looking down on the motor from this angle, look into the dark gap between the core and the housing.

In the center you’ll see a little nub protruding from the core.

Carefully stick a flat-bladed screwdriver--a regular one will do, it needn’t be hollow-ground--into the gap so that the flat part of the blade is fully contacting the nub. (Don’t place the screwdriver anywhere else in this gap, as you don’t want to damage the enamel-wrapped windings.)

What you need to do next is super-easy, but I found it was easier to show it in video rather than photos, so you can see the motions I’m using and get a sense of how gently you must open this part:

If for some reason what I just showed you in the video doesn’t work, flip the motor over, repeat the screwdriver trick on the other side, and you should be all set.

From here on in, remember that we must handle these two parts (the core and housing) with care so as not to disturb the two wires that remain connected. Those are the brush leads, they connect the core and the brush tubes, and I have never had to replace them on a potted motor yet. These are not the wires we’re concerned with in this tutorial, so unless you see bad insulation on them, leave them be.

Now I’m going to show you the Worst-Case and Best-Case Scenarios for what you might see when you crack the motor open as we just have. Sadly, a small percentage of you may have the Worst-Case Scenario, which may not be fixable.

The Worst-Case Scenario

This is the machine that had grease and possibly oil inserted into the brush tubes by a previous owner. Unsurprisingly the inside looks like hell.

Zooming in, we see the wiring is totally fried, and there is dark grit all over the place. This is bad.

I’ll flip the motor over so you can see the wiring from the other side, and get a sense of just how bad:

The insulation has melted, either as a result of a short somewhere within the motor, or due to coming into contact with the grease and oil it was never meant to endure. The wiring insulation looks like melted cheese that appears to have seeped between the individual strands of wire. I don’t think I can strip this, and if I can’t strip it, I can’t solder a new connection on. For those of you with Worst-Case Scenario motors like this, I’ll continue searching for a solution.

Stuck Brushes

Before we get to the Best-Case Scenario motors I’m hoping most of you have, we need to deal with the brush tubes.

On some of your motors the brushes may not have been removable from the outside. So we’ll deal with them now.

Looking inside the Worst-Case Scenario machine, it’s easy to see why the brushes were stuck: The ends of them have melted out of shape due to the grease or oil that got into the motor.

To remove these brushes, we push from the outside.

Stick a Q-tip into the brush tube…

…and the brush pops out inside.

Now do the same to the other side.

Finally, remove both brushes and set them aside. Even if they're covered in oil and grease like this, we'll still be able to salvage the springs, which will come in handy as some places sell motor brushes without springs.

The Brush Tubes

Whether you have a Best- or Worst-Case motor, clean your brush tubes out with dry, clean Q-Tips.

Yep, same photo from above, but this time we're swabbing.

You can visually inspect the brush tubes with a flashlight to see if they’re clean.

If there’s a little graphite dust from the brushes, that’s fine; but if there’s any oil or grease in there, it must be removed. For that you can use a Q-tip dipped in rubbing alcohol. Be sure to blot the Q-tip well so you don’t send alcohol sloshing down the tube and onto the brush lead wires.

The Best-Case Scenario

Here we’ve cracked open a motor where the wiring clearly needs replacing; the insulation is dry, cracked and rotting.

But it’s still a Best-Case Motor, because the key word is “dry.” This motor was properly maintained and there is no grease or oil inside and on the wiring.

So if your motor looks like that, breathe a sigh of relief, because our job here will be relatively straightforward.

Go on to Part 13: Cutting the Motor Leads


  1. Oh my goodness! Thank you so much for this! Exactly what I've been looking for! But quite question... what if those two wires that you're supposed to be so careful with are exposed? Sadly, one of the brush leads is exposed and I'm not sure what to do with it. Any help would be appreciated!

    1. Hmm, not sure about this one; you'd have to ask an electrician or someone experienced with that type of repair. If you find the answer, please be sure to let the rest of us know.

  2. Hello, unfortunately one of my "water moccasin" wires have snapped and I'm having no luck trying to reattach it the the cavity where the carbon brushes reside. Any ideas? Thanks!!

    1. I make it my policy to only give advice/offer tutorials for things that I've actually done with my own two hands, preferably more than once. I've never had to deal with snapped brush lead wires, so at this point, your guess is as good as mine.

      If you do manage to sort it out, please be sure to let the rest of us know how you fixed it!

  3. I forgot that the motors specs go inside the case..thanks!!!

  4. I have a motor which the leads have detatched from the coils.
    can they be reconnected by soldering?

    1. I don't know, is there enough of a "tail" for you to solder the wire to?

  5. Your photography is stunning! Seriously, I would love large prints of some of these on my walls. Very artistic.

  6. Can the brush tubes be removed from the housing? I prefer to ultrsonicaly clean greasy parts.

  7. I have worst-case scenario motor! When I open up my motor the housing and copper winding were covered in black soot and some grease. I am pretty sure it will need to be rewinding but is it salvageable? If I rewind the (armature?) will it be a waste of my money? Should I just but a need motor?

  8. I have a motor where there is no gap between the core and the housing.In your photos the housing is curved where the little nub is protruding from the core,but in my case there is no gap at all and I tried pulling the core to get some gap,but its so tight. I dnt want to apply a lot of force either .Iam not sure how to get it apart.Any help would be appreciated.

  9. Hi, I just got my 15-91 and the motor was getting hot so I took it apart. There was grease inside but not on the brush springs.
    I think the grease made its way down the shaft and got by the bushing. There isn't a seal there to keep the grease out. I'm soaking the armature in mineral spirits and will let it dry out for a week. It will short out if its not dry.