I have just stripped the 1kW motor from a Black and Decker 10"
circular saw I inherited. It was running erratic with the symptom
indicating the brushes were making poor contact. Sure enough the
brushes were seized in the brush holders,
As it looks like they had got hot I want to check the armature before
ordering brushes and holders. Am I right to think I need to check the
resistance between opposite commutator contacts? I seem to have about
80 Ohms on the few I have checked so far and a few kOhms between
coils, does that seem reasonable?
I'd expect a small proportion of that to be honest.
In my experience if one of those motors is giving trouble and there is
sparking from the commutator or it has been overheating, the armature is
If you take it to a motor rewinders, they should have a thing called a
growler which they can test it with.
On Sunday, June 22, 2014 9:03:21 PM UTC+1, firstname.lastname@example.org wrote:
Yup, as the brushes connect
if in doubt one can usually bend something to enable old brushes to keep running a while more, and lightly sand the commutator to clean the surface while its running (not on 240v :). Then you can assess whether it runs ok - if it does its worth rebrushing.
On Sun, 22 Jun 2014 21:03:21 +0100, email@example.com wrote:
Those readings rather suggest open circuits between the armature
windings (topologically, the armature is a bunch of electromagnetic
coils all connected in series at each commutator segment forming a
I'd expect a 1HP constant speed universal motor to have the field
connected in shunt, possibly compounded with a series winding to help
stabilise speed with load.
I'd expect the armature DC resistance between opposite segments to
show a reading of around 5 to 7% of the equivilent resistance of a 1KW
heating element (the perfect motor would have zero DC resistance in
the armature windings). In this case, a practical 1HP universal motor,
I'd expect a reading of around 4 ohms with a fraction of an ohm
between adjacent segments.
Hmm. A 1kW motor consumes 1kW of power. The motor will be based on cost
that tends to mean a lot of heat and not so much mechanical power.
Often the motor is speed stabilised by a combination of drag by cooling
fan, and the current lagging due to the inductive stator winding. I've
never seen a shunt winding in addition to a series winding, but I could
be wrong in this instance.
All in all a universal motor is a terrible compromise and best run by
Which is why I used a 1HP rather than a 1.25HP figure. :-)
No, you're probably right. I was just covering the possibility that a
1HP motor _might_ also include a series field winding by using the
phrase "possibly compounded with a series winding". Assuming
(possibly incorrectly) that you'd need constant speed rather the
torque curve speed law of a traction motor for spinning a circular saw
Maybe, and if so, _only_ rectified with no 'smoothing' caps. A pulsed
100Hz DC might imbalance the electrical erosion of the brushess (it
remains balanced out on the commutator segments in either case).
Regardless of the detail, one thing seems clear, the armature appears
to have developed open ciruited coil connections.
The only time I have seen both series and shunt windings are in motors /
generators designed to run at constant speed / voltage with varying load.
I don't understand the assertion, that pulsating DC will be any
different to less pulsating (smoother) DC. The stator's inductance will
go some way to reduce ripple current.
I've not come across electrical erosion of brushes, unless you mean
Still less arcing than in a universal AC motor where there is
considerable induced voltage between adjacent segments due to
transformer action from AC current flowing in the stator.
Thanks Bob and others, I was using a digital meter which gave about 40
Ohms when the leads were shorted so I have now reverted to a moving
coil one and am getting less than an Ohm across the coil and between
coils so I guess it's not worth repairing.
Not at all and thank you again for your trouble.
I think I have a logical problem in that I assumed the adjacent
segments on the commutator should have a higher resistance than
opposite segments, are you suggesting this is not the case?
Only opposite segments are relevant to the motor operation. As others
have said they are often wound in a series ring for convenience.
I would suggest easing the old brushes in the guides and testing the
motor before spending out on spares
On 23/06/2014 20:07, firstname.lastname@example.org wrote:
Adjacent segments should have the lowest resistance. They are
effectively in series when comparing opposite segments.
That way all the copper windings are used all the time independent of
It's not whether it's a pulsating or or ripple free DC that might be
a problem, just the fact that it's DC of any flavour. The problem with
using capacitive smoothing is that it may raise the rms voltage to
somewhere in the region of 300 volts or more if you overdo the
There's always some level of 'arcing' involved with commutator brush
gear. The uni-directional current flow (which won't matter to the
commutator segments) may slow down wear on one brush and accelerate it
on the other brush compared to AC current running.
I don't suppose it would be a major problem since dynamos on vintage
motor vehicles seemed to cope well enough. At least, I never saw any
historic articles regarding 'rotating the dynamo brushes' to even out
the wear but dynamos were a vintage thing even back in the sixties so
I may have simply missed seeing such 'sage advice' being published.
I'm not so sure whether the armature would behave like a secondary
winding of a transformer (at least not to the extent you're
suggesting). I think you'd only see a 100Hz modulation of the inter
segment voltage you'd expect anyway on a pure DC supply of the same
There is no point in dismantling the item.
Free off the brushes and test the continuity by connecting your meter to the
power input rotating the armature.
If the meter jumps about, there is a winding OC or SC.
But you don't need a meter.
So long as the brushes are in order (ie free and in contact) , if there is
lots of sparking on the commutator, IE flashing over several segments, the
armature is shagged. There will likely be burn marks on the commutator too.
Cost of repair (new armature) is likely to be uneconomic even assuming you
can get the part.
"DC" series motors whilst theoretically having an infinite no load speed, in
practice, the cooling fan and the fact they are on AC limits the top speed.
If you ran this motor on DC ,it would go lots faster, mainly be cause the
impedence of the field windings would be much reduced.
In days of yore, some DC motors (for traction) had an additional shunt
winding to limit top speed. But they were on DC. These were "cumulatively
The other possiblity being "differentially compounded machines."
There is in fact no such thing as a DC motor, they are all AC.
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