Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems.

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Old September 7th 18, 05:50 PM posted to sci.electronics.repair
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Default Power tool armature repair

Not "electronics", but probably in the S-E-R realm.

I have a really old electric demolition hammer with a bad motor
armature. The fault is an open on one of the commutator segments.
Resulting in a lot of sparking when running & eventual erosion of the
brushes and/or adjacent commutator segment.

The tool is so old that Google doesn't find the model, let alone a
replacement part. Rewinding the armature would be $125. Not worth it
to me.

My intuition about the sparking is that the armature current is normally
continuous because the brushes overlap adjacent commutator segments, but
with one segment open, the current is interrupted and sparking results.

If so, my idea is to short the open segment to the adjacent one & allow
continuous current. The phasing would be affected a little, but there
are 16 windings, so the effect should be small. And a 1/16th (?) loss
of power, but that's already the case.

Does this make sense?

Thanks,
Bob

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Old September 7th 18, 06:02 PM posted to sci.electronics.repair
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Default Power tool armature repair

On 9/7/2018 9:50 AM, Bob Engelhardt wrote:
Not "electronics", but probably in the S-E-R realm.

I have a really old electric demolition hammer with a bad motor
armature.* The fault is an open on one of the commutator segments.
Resulting in a lot of sparking when running & eventual erosion of the
brushes and/or adjacent commutator segment.

The tool is so old that Google doesn't find the model, let alone a
replacement part.* Rewinding the armature would be $125.* Not worth it
to me.

My intuition about the sparking is that the armature current is normally
continuous because the brushes overlap adjacent commutator segments, but
with one segment open, the current is interrupted and sparking results.

If so, my idea is to short the open segment to the adjacent one & allow
continuous current.* The phasing would be affected a little, but there
are 16 windings, so the effect should be small.* And a 1/16th (?) loss
of power, but that's already the case.

Does this make sense?

Thanks,
Bob

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
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Old September 7th 18, 06:32 PM posted to sci.electronics.repair
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Default Power tool armature repair

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

https://www.youtube.com/watch?v=oB-h-vZc6FY

Peter Wieck
Melrose Park, PA
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Old September 7th 18, 07:53 PM posted to sci.electronics.repair
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Default Power tool armature repair

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.
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Old September 7th 18, 08:49 PM posted to sci.electronics.repair
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Default Power tool armature repair

On 2018-09-07 20:53, Bob Engelhardt wrote:
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.


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Old September 7th 18, 10:36 PM posted to sci.electronics.repair
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Default Power tool armature repair

On Fri, 07 Sep 2018 12:50:34 -0400, Bob Engelhardt wrote:


If so, my idea is to short the open segment to the adjacent one & allow
continuous current. The phasing would be affected a little, but there
are 16 windings, so the effect should be small. And a 1/16th (?) loss
of power, but that's already the case.

Does this make sense?

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

Jon
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Old September 7th 18, 11:20 PM posted to sci.electronics.repair
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Default Power tool armature repair

On 9/7/2018 3:49 PM, Dimitrij Klingbeil wrote:
Can you clarify if it's really an open commutator segment or an open
winding? These two are not the same thing.

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

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?


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

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.


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.

[...]
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.

[...]

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:
https://imgur.com/a/XeQhLCw

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
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Old September 7th 18, 11:22 PM posted to sci.electronics.repair
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Default Power tool armature repair

On 9/7/2018 5:36 PM, Jon Elson wrote:

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


Well, the one segment isn't connected to anything, so bridging would
just create an extra-wide segment.
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Old September 8th 18, 12:56 AM posted to sci.electronics.repair
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Default Power tool armature repair

On 8/09/2018 6:20 AM, Bob Engelhardt wrote:
On 9/7/2018 3:49 PM, Dimitrij Klingbeil wrote:
Can you clarify if it's really an open commutator segment or an open
winding? These two are not the same thing.

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

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?


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

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.


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.

[...]
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.

[...]

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:
https://imgur.com/a/XeQhLCw

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

Yeah that's the way to do it. A lot of times it's the crimp that has failed.
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Old September 8th 18, 08:10 AM posted to sci.electronics.repair
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Default Power tool armature repair

On 08/09/18 08:22, Bob Engelhardt wrote:
On 9/7/2018 5:36 PM, Jon Elson wrote:

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


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


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.


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