Power tool armature repair

If the winding is open and the motor stops with the brushes on that armature segment, the motor cannot start. I have a coupe of auction sale purchases like that. Since you haven't described that condition, the winding must have an internal short.

Have you disassembled the motor and cleaned the armature? There may be something conductive between two armature segments.

Paul

Armature Maintenance (for really big motors) so as to prevent the condition s you describe, include "stoning and undercut". This trues the armature, an d removes sufficient material between the windings such that carbon dust is dispersed. Otherwise, it can accumulate and cause all sorts of problems. A nd if you have a raised edge the brushes will chatter - and that is a serio us sparking cause. So, attached is a YouTube for smaller motors that accomp lishes what you may need.

Peter Wieck Melrose Park, PA

To clarify: the armature has been removed and alive-or-dead tests were done with an ohmmeter. This showed that one segment was disconnected. There are no shorts of any sort - just this one open winding.

Can you clarify if it's really an open commutator segment or an open winding? These two are not the same thing.

Consider the commutator segments as follows:

... A B - D E F G H ...

Let's say segment "C" does not work. What does that really mean? Is "C" completely disconnected or are there just different resistance values when measured between B-C and C-D because one side winding is open?

You can also compare the resistance B-D to some other pair that spans the same number of segments (like D-F, E-G, F-H).

If approximately R(B-D) = R(E-G) then the windings are intact, and only the commutator segment has lost connection.

These windings are usually made with a continuous construction where the same piece of wire goes from one segment through a winding to the next segment, and so on. So the whole rotor is would from one single continuous piece of wire and only the 2 outermost ends are connected together at a single commutator segment somewhere.

With this construction, if the winding is still good then so is the piece of wire that goes to the non-working commutator segment, making the (typically crimped) connection the most likely cause of failure.

If so, consider trying to reconnect the segment by e.g. removing some of the lacquer and soldering through the badly crimped place rather that trying to bridge the broken segment to a neighboring one.

If the winding is no longer still good, then again, first check if there is a loose connection at the segment's crimp joint itself - if you find the loose connection there, try to fix it and only if there is none to be found, consider bridging the commutator segments.

No, you will create shorted turns. This will be worse.

Jon

You're right - I was sloppy in saying "winding", using the word as a synonym for "segment".

My measure was that C was open; "infinite" resistance to everything.

Clever - I wouldn't have thought of doing that. I did do it and found all the adjacent-by-2 resistances to be the same. IOW, the windings are intact, but not connected to this segment.

The wiring end of the segment has a well with a slot & that suggests to me that the winding is routed into the well through the slot & then a plug is pressed in to connect and hold it:

I jammed a needle into the slot, hoping to make a connection to the winding, but did not connect.

The windings at the back of the commutator are wrapped in string & coated. I'm thinking that I can cut back the string & get at the winding.

Thanks for your help. Bob

Well, the one segment isn't connected to anything, so bridging would just create an extra-wide segment.

Yeah that's the way to do it. A lot of times it's the crimp that has failed.

On Fri, 7 Sep 2018 18:22:21 -0400, Bob Engelhardt wrote as underneath :

I would think you would be fine - but I would take the precaution of measuring before and after current with a clamp meter, just to be sure there is no major change in characteristic. Quite a good bodge solution if it works and stops the sparking for an otherwse bin job! C+

The extra-wide segment won't be generating its share of back-EMF so will reduce overall motor impedance and increase current. That might be within acceptable limits or might not.

On 9/7/2018 6:20 PM, Bob Engelhardt wrote: [...]

With a small bit in a Dremel I excavated behind the segment, down to its connecting wire. There was no continuity from that wire to other segments! Then I noticed what had been staring me in the face all along

- the connecting wire exposed and broken:

Its connection to the segment also came loose, so I soldered in a jumper:

It surprised me that the original connecting wire was steel. I don't know why steel would be used, but it made soldering difficult.

winding.

Flux is your friend. I don't solder *anything* anymore without touching th e connection with a bit of liquid flux. Today's solder is garbage, and a b it more flux makes it flow so much better.

Congrats! Now that the connection is done, make sure to touch up the worked area with epoxy to protect it from vibration and from being sandblasted. Since it's going back into a drill and drilling dust is highly abrasive at typical rotor velocities, neither the wire nor the soldering should be exposed. The epoxy coating should be made smooth so that the "bump" over the rework does not stick out too much into the air and dust stream. When applying epoxy, try not to leave any unfilled air pockets underneath as far as practical.

P.S. From the picture it looks like a neighboring wire lead is exposed and already partially abraded down. Make sure to re-protect it as well.

With it "fixed", it still sparked like crazy! It took a while, but I finally found a transformer that I could re-purpose as a growler. With that I discovered that the armature had shorted windings. Sigh.

I took it as an opportunity to do something new: rewind an armature. There's a number of YouTube videos showing it and it doesn't seem too hard.

First, I needed the parameters: size of wire, number of turns in each winding, and the connections of the windings to the commutator segments. It was easy on the videos - you just unwind the old and record the parameters. Not so on mine - the windings were potted with epoxy. With some careful destruction I got some of it unpotted, but not the connections to the commutator. So I trashed it.

Part of the commutator connections problem was that there were jumpers from the commutator to the windings. Most (?) armatures have their windings loop around a lug on a commutator segment and continue. I.e., each lug is the start point of one winding and the end point of another. On mine there were 2 commutator segments per rotor slot; the jumper on one connected to a winding wire, but the jumper on the other connected to _2_ winding wires! How could there be 3 connections per slot? Bizarrely asymmetrical.

I'm disappointed that it wasn't more doable.

along

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motors with partial short can be run in series with a 3kW heater - but not now.

NT