240v brushed motor - 22 commutator segments

[mark_in_manc]

Folks -

Is it possible to calculate rpm of a brushed motor if you know the supply voltage and number of commutator segments? I also know power - 650W. This is a motor which now smokes, sparks and is not at all happy - I suspect shorted armature windings somewhere. It's on a rather old dust extractor, I have salvaged the remainder of the centrifugal blower, and if I could find a suitable motor to fit (perhaps from something like an angle grinder) I may be able to make a Frankenstein-machine to save it. Something more elegant than current bodge up using a leaf-blower - very useful on the (even more ancient) circ saw.

I guess if I check commutator for shorts I might be able to cut out broken sections and operate at reduced power...but then perhaps it must be shorts to earth, since shorts to other windings might not do much when the brushes are elsewhere. As you can perhaps tell, I'm feeling my way here.


cheers
Mark

[kalee20]

Unfortunately not - the number of commutator segments does not define the speed of the motor, only how ripple-free the rotation is.

Shorted turns are quite likely, and play havoc with a motor, even if the relevant section is completely isolated by disconnecting it from the commutaqtor. That's because, whizzing around in the magnetic field, a considerable voltage is induced - and if it is shorted, big power losses occur.

[Boater Sam]

It's just as bad if there is an open circuit coil which frequently is due to the winding termination at a commutator segment. The back emf induced on every rev with a gap leads to heavy brush sparking.
Now when I was a lad...... we used to rewind armatues. It's not so hard really.

Check it out for open circuits, if all the windings appear to be continuous it may be worth trying a dip in hot shellac or even varnish. Get it warm, not hot enough to set on fire, dunk it in, let it soak till it cools down to hand warm and drain it off. Clean the comm up, cross fingers and try it. Nowt to lose.

Boater Sam.

[mark_in_manc]

It's very nice of yous to hold my hand here.

Opposite comm segments report a resistance of between 2 and 4 ohms. More worryingly, all segments appear connected to all other segments - though nothing shorts to the shaft.

I think I had envisaged discrete windings between opposite comm segments, with no commonality to other segments. But on closer inspection, wires appear to loop in, and then out, of each segment - suggesting my guess at the winding scheme is naive. Do you know more?

<edit to add - now I know more - google was my friend, etc. OK, hot varnish it is...>

thanks
Mark

[Nicklyons2]

Look up ' wave winding ' of DC machine . What you have is an AC/DC universal motor which is very similar; then you' understand why all the segments seem to be interconnected. You should be able to find someone who could rewind your motor locally. Try asking the places who rewind washing m/c motors. A friend got his jewellers lathe motor (c.1940s) by such a place.

[Peter.N.]

Could be short circuit field coils, this was a common fault on old B&D drills, gave the same symptoms as an o/c armature.

Peter

[Boater Sam]

You have discovered how they are wound, there is a coil between every ADJACENT segment, i.e. all are in series with start of one and end of next connected to the same segment.

If it is field coil problem, then the varnish trick may work, but it is easier to rewind the field than armature.

Boater Sam.

[mark_in_manc]

OK great - 'wave winding' reading it is.

Regarding the fix - the hugely improbable happened today, when a partially broken Vax canister vacuum cleaner on my 'come-in-handy' pile yielded a motor-blower which fitted very nearly exactly...only a tiny amount of filing was required to sandwich it into the chamber where it fits on the sawdust extractor. And being 50% higher in power, it works better than the original!

The downside is the huge boost this gives to my inner hoarder, who will use it as ammunition in his war against my more rational self for some time to come

[Kala_12]

So 'keep it, it might be useful' strikes again.
Stuart.

[ex 2 Base]

As the motor is a series type, an open circuit on the field will prevent the motor from operating at all. No Sparking. Take out the armature, lay it carefully on a bench with two pieces of wood either side to prevent it rolling off. Have a good visual look at all the windings, looking for a burnt place. you may be able to solder up a connection and cover the connection with a smear of Araldite, not a lot so as to disturb the balance. Then carry out an insulation test, commutator to shaft, using a 500 volt DC (megger). Next mark one of the commentator segments with a fibre tip pen as reference point. Now using an ohmmeter set on very low ohms, procedure to measure the coil winding resistance connected to each pair of segments all the way around the com, all should be the same. If you know any one in the rewind business they might have a "Growler", which is half of a transformer with the low voltage winding removed and a Vee cut in the laminations, in which to sit an armature. Insert the armature into the Vee, switch on the primary and the armature becomes the low voltage secondary. Now with a thin feeler gauge held just above the armature rotate the armature. If the there is a short circuit a large current will flow and the feeler will be pulled downwards and a vibrating noise made. Ted