circular saw armature resistance

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?
AJH
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snipped-for-privacy@sylva.icuklive.co.uk wrote:

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 knackered. If you take it to a motor rewinders, they should have a thing called a growler which they can test it with.
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Bob Minchin wrote:

I've just measured the armature on my Bosch 1600w saw. 1.6 ohms after compensating for the test meter leads.
Either your measurement rig is suspect of the armature is not a happy chappy.
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On 22/06/2014 21:03, snipped-for-privacy@sylva.icuklive.co.uk wrote:

Few Kohms sounds very wrong.
Check resistance between adjacent segments. It should be very constant.
Anything else and its toast.
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On Sunday, June 22, 2014 9:03:21 PM UTC+1, snipped-for-privacy@sylva.icuklive.co.uk 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.
NT
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I'd say the coils should not be connected at all, unless the commutator has graphite between its segments causing it Brian
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Yes, they tend to go shorted turns though not high resistance. Bit odd that one. Brian
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On Sun, 22 Jun 2014 21:03:21 +0100, snipped-for-privacy@sylva.icuklive.co.uk 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 closed loop).
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.
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On 23/06/2014 13:07, Johny B Good wrote:

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 rectified mains!
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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 blade.

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.
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On 23/06/2014 17:06, Johny B Good wrote:

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 arcing?
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.

Yes - very likely.
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On Sun, 22 Jun 2014 21:58:04 +0100, Bob Minchin

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.
AJH
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snipped-for-privacy@sylva.icuklive.co.uk wrote:

That is what it should be!! - 1 to 2 ohms would be about right
Bob
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On Mon, 23 Jun 2014 19:12:46 +0100, Bob Minchin

Should there be that little resistance between adjacent coils?
AJH
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snipped-for-privacy@sylva.icuklive.co.uk wrote:

I took the trouble last night to go and open up and measure my saw motor and gave you the results. Are you saying my measurements are wrong?
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On Mon, 23 Jun 2014 19:54:49 +0100, Bob Minchin

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?
AJH
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snipped-for-privacy@sylva.icuklive.co.uk wrote:

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
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On 23/06/2014 20:07, snipped-for-privacy@sylva.icuklive.co.uk 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 armature position.
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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 smoothing.

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 rms value.
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wrote:

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 compounded machines". 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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