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Old 23rd Feb 2011, 2:21 pm   #1
ben
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Default electric drill sparking badly

I have here an electric drill, probably not more than 15 years old, motor turns Ok but there was considerable sparking and a burning smell coming from the vents.

The conmutator was very dirty and so I cleaned off the black resdue with alcohol. The carbon brushes looked a bit worn so I reversed the little pieces of carbon in their spring loaded holders.

Tried again and still got the same problem. I did notice that the plastic/bakelite surround of the brush holders was a bit burned.
Basically I'm not sure whether it is a case of replacing the carbon brushes, or whether the holders also need changing. I presume the damage to these mean the brush is not held steadily enough to allow even contact.

Any tips?
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Old 23rd Feb 2011, 2:41 pm   #2
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Default Re: electric drill sparking badly

In my experience, faults of this type are usually caused by shorted turns in the armature windings, check for overheated or discoloured windings. Not normally worth the effort of fixing unless you can cannibalise the bits from another motor.

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Old 23rd Feb 2011, 2:55 pm   #3
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Default Re: electric drill sparking badly

thanks Mick. Could be the case. Someone gave me this drill and I have no idea of its history.
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Old 23rd Feb 2011, 3:02 pm   #4
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Default Re: electric drill sparking badly

A certain amount of sparking is normal with some drill motors, but obviously it shouldn't be flashing and banging with visible smoke.
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Old 23rd Feb 2011, 3:36 pm   #5
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Default Re: electric drill sparking badly

The burning smell is a bit of a give away. I think it's a goner.

If you replace the brushes you have to get the right grade, or chance on the ones you've got happening to be the right grade. If they are not you get all sorts of horrible coloured sparks and it obviously isn't working properly.

There's a place on the WWW which specialises in brushes for power tools, particularly the B&Q stuff with no spares support. I haven't tried them.

http://www.solenttools.co.uk/

The top drawer stuff has pretty good spares support from other places often for models going back years and apart from main motor parts, the prices are pretty reasonable.

Given the price of electric drills these days, even top brand ones, if you can't fix it with a bit of cleaning, I'd say it wasn't worth pursuing.


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Old 23rd Feb 2011, 4:03 pm   #6
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Default Re: electric drill sparking badly

There might be a suppression capacitor on the drill. That might have gone?

I know DC motors have them - not sure if this is AC or not.

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Old 23rd Feb 2011, 6:33 pm   #7
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Default Re: electric drill sparking badly

Quote:
Originally Posted by mickjjo View Post
In my experience, faults of this type are usually caused by shorted turns in the armature
And in mine, too. The motor can appear to run normally, but will quickly overheat.

Suppression capacitors wouldn't aggravate sparking, after all, they are upstream of the brushes.

It's worth checking that the brushes slide in their holders, are properly bedded in to the commutator's curvature, and the commutator itself is clean and smooth. But assuming these are OK, I think your motor's a goner.
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Old 23rd Feb 2011, 7:15 pm   #8
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Default Re: electric drill sparking badly

Almost certainly shorted turns on the armature, I've seen dozens of these particularly on the B&D consumer range and cheap Chinese drills the burning smell rules out the capacitor and brushes, flashing and burning together with much reduced power, i.e. stalls easily means shorted turns. Unless you have another to rob some spares or most certainly beyond economic repair. Chris
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Old 23rd Feb 2011, 9:42 pm   #9
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Default Re: electric drill sparking badly

Hi Ben, if some comm segments appear more burnt than others, or some of the riser wires from the armature to the comm are discoloured it is pretty certain the armature has shorted turns.

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Old 25th Feb 2011, 12:25 am   #10
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Default Re: electric drill sparking badly

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Originally Posted by kalee20 View Post

It's worth checking that the brushes slide in their holders, are properly bedded in to the commutator's curvature, and the commutator itself is clean and smooth. But assuming these are OK, I think your motor's a goner.
That makes sense. There was plenty of life left in the brushes, tried 'em both ways round, and I cleaned the conmutator. The motor windings /riser wires did look a bit iffy.
Ah well, worth a try...Will strip it for parts. I have two decent drills anyway!
Thanks to you all for the suggestions.
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Old 18th Mar 2011, 4:33 pm   #11
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Default Re: electric drill sparking badly

My experience is that sparking on the commutator means the armature windings have gone. This applies for small series wound AC motors as used in drills, Hoovers etc. Only solution is to scrap the whole thing.
If you want to be sure, take out the armature, and check the resistance between adjacent armature segments, going all the way round. Start at the segment where the edges of two adjoining commutator segments are eroded and blackened by the sparking. That can usually be done without dismantling the unit. Remove the brushes of course.
If the brushes are worn down or the commutator dirty, that is also a warning of trouble, caused by erosion due to the sparking.
It is a good test if you are buying a second hand unit to plug it in and see if it sparks. On no load, there should be virtually nothing. Bill m0wpn
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Old 18th Mar 2011, 10:08 pm   #12
Lucien Nunes
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Default Re: electric drill sparking badly

Quote:
Only solution is to scrap the whole thing.
Sometimes you can get lucky! Obviously you wouldn't spend much time on a modern drill but occasionally this kind of fault on a vintage universal motor can be fixed or worked around without rewinding.

I had a 1920s Tellus vacuum cleaner with excessive arcing and streamers around the commutator. A bar-bar test identified an open coil, a tone test showed that the fault was near the comm. Carefully stripping away the binding revealed a breakage where the coil end had been put under too much tension during assembly. Only 1/32" of wire was left projecting from the slot but it was enough to solder on a tail to reach the comm. After refixing the insulation and varnishing it works perfectly.

Arcing and hot bars on the comm of a 1950's Stuart water pump clearly indicated an armature short. The motor was immediately stopped and examined before further winding damage occurred. The short was found to be from one coil to the next rather than within a coil, therefore once one of the two affected coils was isolated at the ends there were no remaining shorted turns. The circuit was completed by linking the commutator segments together, resulting in a motor which works albeit at slightly reduced power.

For these and other experiences I would say that universal motor armatures are not always irreparable when they fail, so worth a closer look on anything rare or valuable.

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Old 18th Mar 2011, 11:56 pm   #13
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Default Re: electric drill sparking badly

I have known it be caused by s/c field coils but that's quite rare.

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Old 19th Mar 2011, 1:22 pm   #14
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Default Re: electric drill sparking badly

Bar-bar and tone tests? Please tell me more!

Quote:
Originally Posted by Lucien Nunes View Post
I had a 1920s Tellus vacuum cleaner with excessive arcing and streamers around the commutator. A bar-bar test identified an open coil, a tone test showed that the fault was near the comm.

Last edited by Dave Moll; 19th Mar 2011 at 2:31 pm. Reason: quote pruned to the relevant section
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Old 21st Mar 2011, 1:08 am   #15
Lucien Nunes
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Default Re: electric drill sparking badly

One of the most fundamental tests you can do on an armature is to measure the resistance between adjacent comm bars. Extreme variation in readings around the armature indicate something amiss, although it can then be rather harder to identify exactly what. On small universal motors in household appliances, the resistances can usually be read with an ordinary ohmmeter. On larger motors with low resistance coils it is necessary to use 4-wire measurement, or the alternative of injecting a current diametrically across the winding from which voltage readings can be taken between any two desired points. On small motors there may be some inherent variations due to unequal coil spans, which can conceal small numbers of shorted turns. Since there are two parallel circuits around such armatures, the differences manifest as a succession of unequal readings.

If a short is suspected, a growler test is fairly conclusive. By passing an alternating field through the armature, a current is set up in the shorted coil that causes a local field through that particular coil. This can be detected with a ferrous feeler blade which vibrates or 'growls' when passed over the shorted coil. Shorts often occur due to overheating which affects the innermost and generally inaccessible windings first, hence usually require rewinding to repair.

If an open coil is suspected (which is rare except on very small motors with fine windings that can be destroyed by corrosion at the comm risers etc) then unlike a short it is more likely to be outside the slot and thus repairable. If it can't be seen or felt, the fault can sometimes be located capacitively because the stray capacitance gives an indication of the length of wire between the comm and the break. I have a close working relationship with my tone oscillator and tracer, so tend to use them on all sorts of things, such as that vac armature. You can clip the oscillator on to a wire and 'sniff it out' with the tracer probe. Careful adjustment of the sensitivity can localise a hidden break to within a few mm.

I have a couple of arcing motors waiting to be sorted out. One is a 1920s DC lift motor with a shorted coil that I was discussing with a winding expert last week. We are hoping to isolate the affected turns surgically, which stands a fair chance of success on a motor of this size. Another is a repulsion-start induction motor with so much comm wear that the brushes hardly touch the copper for all the mica poking out. That hopefully just needs a skim in the lathe

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