Arcing universal (brushed AC) electric motors

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Dan Steely

Established Member
28 Jun 2019
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Hi All,

I have had a few mains powered brushed motors fail on me over the years where they start arcing excessivly, loose power and sound horrible.
In cases like these i've examined the motor and found no fault e.g:
1. the commutator and associated rotor windings have no short or open circuits.
2. the stator windings have no short or open circuits.
3. the brushes and commutator are (after a clean up) in generally good condition.

After cleaning up the carbon build-up between the commutator terminals the problem still exsists and the motor has to be replaced.

Can anyone explain what causes this fault and if it is fixable by the likes of me?

Many thanks.
If I recall, universal motors with an armature, commutator and brushes, the brush contact faces need to be curved to the same arc as the commutator, clean and of the correct dimensional widths so that no more than two adjacent segments are bridged at any one time. Often overlooked when brushes are replaced..... a bit of judicious sandpaper, usually.

The comm segments also need to have a small recessed gap between them. Look closely with a mag glass. Sometimes, over use, this gap fills with powdered waste carbon and this can add to the resistance of the contact and produce an arc. In extreme cases it can lift the brush clear of the comm surface so that there is effectively an open circuit. Excessive arcing can produce high frequency interference, which you'll hear on radios etc.

You don't see big DC motors much, these days, if at all but on bigger DC motors with large armatures, it was a customary practice to get a piece of pointed wood to scrape all this kratzenkrud out of the inter-grooves, however this may be difficult on very small diameter comms., but a stiff brush stroke may do the trick. It's essential to stay away from any varnished windings behind the comm with sharp bits, though! Be careful.

Basic stuff, but hope that it helps.
You don't see big DC motors much, these days,
What about starter motors, maybe lightweight on modern cars but you still find 5Kw + on commercial / Agricultural vehicles.

With any motor when being overhauled you always clean out the gaps between the segments, a piece of sharpened hacksaw blade is often ideal and to ensure electrical integrity we used to use a growler.
First post is a bit heavy but this:
Gets the point across that a low value capacitor is not uncommon for arc supression in universal (commutator/brushed) motors.
Try changing the capacitor in case it has failed.
These are small capacitors and unlike the capacitor(s) ona single phase AC induction motor aren't needed to make the motor run. They are there to reduce sparking and the radio frequency interference generated by the motor.
A line of small sparks parallel to the axis of rotation along the edge of the brush are to be expected but rings of sparks around the commutator is normally indicative shorted turns on the armature or possibly open circuit in which case it needs to be replaced or in extremeis re-wound.
Many thanks for your help on this. I've seen this happen to quite a few brushed motors ranging from a DeWalt mitre saw to a Henry. In both of these cases I don't recall a starter capacitor. When the next one turns up I'll do the cleaning between the commutator pads trick. I have one of those, short stubby (glass fibre?) brushes I normally use to clean the commutaor when it's spinning.
Many thanks again.
what is the application? Woodwork dust is a pain and gets clogged in motors. Routers and saws, I had 80v leaking from field to casing causing a shock. Stripped it down and the housing was packed with dust. I blow out all my motors now with an air line after use. Excess heat generated from that and in turn caused some sparking. I too use the fibre glass pencils for cleaning.
Universal motor armatures can (and usually do) fail with faults that are almost impossible to measure with conventional electrical equipment.
There is a machine designed specifically for the job called a "growler".
It is very rare to find anyone who will rewind an armature especially with todays labour costs and lack of specialist skills. Some you tube videos may help.
It may be worth the cost of OEM armature spares if the rest of the machine is near perfect, otherwise keep it as a donor for other parts and buy a replacement machine.
You are bringing back some memories from must be forty years ago, we had two growlers that we used for testing the armatures from PSV & HGV starter motors, even then they were either cleaned and re-used or scraped with a few going back to the OEM. In todays throwaway world I doubt they even look inside, it could be just a stuck brush or something simple but then it is not there money that is being wasted.

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