SLOT CAR BUILDER

 

Armature rewinding
A beginner’s guide


Every one at some time has looked for that bit of extra performance but how to get it, in the past it was easy as small firms and individuals often offered rewound motors and armature at affordable prices, sadly those days have gone and performance sometimes can be costly. However there are people out there with boxes of old motors craving to learn the lost black arts of motor winding, so for those out there as mad as I am, read on.
For the purpose of this article I will use an old Johnson 111, as this is a motor often in need of a boost and the technology here can also be applied to the scx and team slot can motors. S can Mabuchis are best left alone at this stage as the poor brush gear can lead to problems
Requirements.
Motor suitable for rewinding (in good condition, good bearing and brushes etc, worn out bearings and shafts will only lead to vibration and poor performance)
Wire to replace original, this is available from good electronics shops such as Maplin.
For this article I will be using 36 SWG enamelled copper wire.
Epoxy resin
Linen thread
Solder, and a soldering iron
A sharp knife
2 razor blades
A piece of modelling clay and a selection of small drills

Step 1
First thing we have to do is strip the motor to its component form very carefully making sure we don’t loose ant of the springs or tiny washers .If you are using a Johnson motor carefully file off the serrations on the shaft to allow it to slide past the bearing with out damaging it. Check all the parts of the motor are in good order and the brushes and bearing are not worn.



Step 2
Carefully remove the old wire from the armature; this is best done by slicing through the old wire with a sharp knife.



When you have done this inspect the armature making sure none of the old wire is left on and the insulators at either end of the armature stack are in good condition, this is important as if there are any sharp edges the new wire can be damaged when winding. As you get more experienced you can trim these insulators slightly to give more room to pack the wire in but at first leave alone as it’s just something else to go wrong.
Next we need to determine which way the motor is going to run by timing the commutator. On the standard motor the com is set with the slots in the centre of the web, by turning in the direction of rotation will determine which way the motor runs fastest (see diag). With a motor like this I would not use more than 10 degrees advance on the com as you may loose brakes and generate heat due to the poor quality magnets in these motors but, as we are just doing a simple wind, I don’t propose to change the magnets at this stage.



Once you have determined the amount of advance, a spot of super glue will hold the com in place.

Step 3
Winding the armature.
For this motor I have chosen 120 turns of 36 swg with 5 degrees advance which will give us a fairly high revving motor some where in the region of 35 – 40K with a good spread of torque and reasonable brakes.
Clean the enamel off the end of the wire and solder on to the first tab making sure you have a good joint. Ordinary multi core solder is ok with a wind of this type, as it should not get too hot when running.



Now start winding onto the first pole in a clockwise direction, (some say you should wind in the same direction as you advance, at this level it doesn’t matter to much, the motor will run fastest in the direction of advance, as I am right handed I wind clock wise). Try to get the wire to go on tight and even laying the wires on evenly in rows, stopping every 5-10 turns if necessary to pack them down with a match or a popsicle stick until you have reached the required amount, Take care not to over stretch the wire as it is easy to snap when using thin gauges such as these.



When you have completed the first pole loop the wire over the next comm. tab, clean the wire and solder before moving on the next pole.



Repeat the process and finish off by soldering the remaining wire onto the first tab.
You should now have a rewound armature, If you are just practicing winding on an old motor I would say just wrap a couple of layers of tape round the stack, put it in the can and stick 12 v through it to see how it runs. But if you are happy then continue to the next step

Step 4
Epoxying

Epoxying and tying is an important part of a rewind because it is unlikely that your wind will be as tight as a factory wind and there is the chance that centrifugal force will throw the wires off, so in order to prevent this we must stick the wire to the armature with epoxy resin also to prevent the com letting go at high revs we tie a bit of linen thread around it and epoxy that also.
There is a knack to getting an even coat of epoxy to the armature the trick is to get it to flow into the winding and penetrate not just sit on top to do this first warm the armature under a grill on a low heat setting for a few minutes, be careful not to burn your hands or use too much heat as to damage the arm, mix a little epoxy 5min araldite is ok for this and carefully apply to the hot arm, the araldite will thin and flow into the winding and around the thread which you have wrapped around the com ,take care not to get any on the com segments as it will insulate the electrical contact once set, just use enough to cover the wires don’t build up a great covering you wont gain any thing and it will just make it harder to balance later, Stick the epoxied arm back under the grill for a couple of mins while it hardens then let it cool before handling .
At this stage we have a usable armature just in need of the comutator polishing, do this by putting it in a drill and touching the com with a hard eraser this will put a shine on it and help the brushes to seat properly, with poorly made /mass produced coms like these it is unlikely you would be able to true them up using a com truer, It is certainly impossible with the scx and Mabuchi s as these will disintegrate at the thought of it The Parma /mura and slot-it can be done with out a problem



Step 5.
The next step is to balance the armature in order to damp down some of the vibration
Commercially produced armatures such as mura, proslot, koford are dynamically balanced on a vibration analyser, we cant do this on the kitchen table but what we can do is a static balance which involves finding out which pole is the heaviest and reducing its weight by drilling bits out of it until it is equal to the others, It may be necessary to remove weight from 2 pole in order to obtain a god balance.
The easiest way to do this is to set 2 level razor blades in some modelling clay and let the armature roll down the edges it will come to rest with the heavy side down. Using a small dill bit remove metal from the pole face and repeat the process until all three poles weigh the same and the armature does not rest in any one position when allowed to roll down the blades. If done properly you will get a much smoother and higher revving motor, but if you are unsure of this part then leave it out and run the arm unbalanced you will still end up with a quicker motor.



Step 6

Magnets and motor limitations

I wont harp on about magnets much in this article only to say that in any electric motor the main limiting factor for performance are the magnets, you can have the best armature in the world but if the magnets are poor the motor will just overheat and die .You will also have poor brakes and acceleration
Chose your wind to suit the magnets I have made a list at the end of suitable winds for the motor in this article and for the scx motor
The scx magnets are better than the Johnson but in both cases I would get some very thin steel shin and place it behind the magnets to reduce the air gap This will improve the magnetic field and increase the torque, but it will reduce the rpm but the motor will respond better and run cooler.
The other limiting factor is the brush gear, the end bells of the Johnson and scx are made have quite easily melt able plastic so if you wind with too thick a wire you will melt the brush gear due to the heat generated.
The scx motor has probably the worst comm. I have ever seen but the rest of the motor is good so when I wind these I usually take a com off an old slot it /parma or Johnson and replace it before I start.

Step 7

Rebuilding the motor
We are now ready to reassemble the motor this is basically a reversal of the disassembly, make sure the armature spins free in the can with about 5thou end float no more especially if you have shimmed the magnets.
To get the best out of your motor you may find it necessary to increase or decrease the brush pressure, Run the motor for ½ an hour on 3volts to bed the brushes in and generally check every thing is ok using a light lubricant on the bearings Then you can experiment with the brush pressure, increasing it slightly by bending the springs will usually yield a bit more rpm

Step 8
Installation
You may be shocked when you put the motor into your once docile car I have had tyres flung off the rims so glue and true every thing
Also you will probably need to up the gear ratio A good start would be a slot it 8 tooth pinion but it depends on car and track

Results
The motor used was dyno checked before at 26300rpm
After the rewind it gave 36300rpm an increase of 10,000 RPM

Usable winds

Johnson/Mabuchi 111/13uo standard mags

120-140 turns 38 SWG 5 degree advance, home use
100-120 turns 36 SWG 5 –10 degree advance club /scalex use
70-80 turns 34 SWG 5-10 degree advance, club, scratch build use

SCX standard com
120-140 turns 38 SWG 5 degree advance

SCX with better com
100-120 turns 36 SWG 5 –10 degree advance, club /scalex use
70-80 turns 34 SWG 5-10 degree advance, club, scratch build use

These winds all have worked for me over the past 40 yrs on various motors ,and use UK wire sizes
Rewinding is not such a black art give it a try

Sorry about the photos but my camera is giving focus problems I think it needs a rewind