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Discussion Starter · #1 ·
Does anyone have advice on rewinding Formula Junior motors? I got a few of these motors in a scrapyard joblot a few years back, I've managed to get three of them running but I'm left with one that is completely dead and one that will only work after a manual spin of the wheels. After poking around with a multimeter I think the connections are completely shot in the former and half there in the latter (connectivity between two of the armature segments). I've tried locating end wires and re-soldering onto armature but no success on these, so I think the only hope is complete re-winds. If anyone knows the right type of wire, number of windings (I suppose I can try to count when I remove the old wiring if I'm able to unwind it fairly cleanly), and any tips on how to do the job (I think it is going to be very fiddly) I'd appreciate it! Also on remagnetising after re-assembling - do they need to be "zapped" or will leaving neodym magnets on each side for a while be enough to reinvigorate ? I know it will be more sensible to buy new running motors but I just fancy a crack at restoring these.
 

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Tony Condon
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If you,ve not done rewinding before this is not a good motor to start on , as they are hundreds of turns of human hair like as not you,ll end up getting within twenty turns of the end and the wire wil snap and so will your patience . They are cheap on e bay treat yourself

cheers tony
 

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Graham Windle
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Another thing the brush holders are made of a substance with the melting point of white chocolate so if you did manage to get the 200 turns of hair onto the armature I'd give it about 5 seconds before it failed . As Tony said buy a working replacement . If however you want to have a go at rewinding then choose a motor like a mabuchi 16d ,13uo or an scx silver can all of which are pretty easy to do . I have an artical on the old forum somewhere which I wrote on the subject or I can send you a PM on doing it if you cant find the artical .

Good luck

Graham.
 

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Discussion Starter · #4 ·
Thanks but actually this is more about having something I fancied trying to repair for the sheer sake of it rather than having an actual need for a formula junior motor. I am fully prepared for failure! I've also had the brush holder melting experience, one of my "working" motors gets very hot (not sure what that issue is, I've given the comm a good clean and tried pepping the magnet with neodyms, maybe that wasn't enough and it needs a full zapping). Does it have to be the same number of turns of same gauge wire, would fewer turns of a thicker wire work?
 

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Graham Windle
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Unless you can make a much stronger magnet ,(Stacking neos may work). I wouldn't go any thicker than 40 swg wire although I suspect the original wind is much thinner ,But I havent one to compare .another solution might be the fitting of an armature from a HO car if the shaft length is suitable . But you would have to re time the com by 90 degrees to make it run .The scalex rx on the other hand is a lot easier to work on and Ive done many of these using 36 swg wire to give a similar performance to an MRRC 3 pole .
 

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Discussion Starter · #6 ·
Thanks Graham, interesting thought, and actually I had trying to use an HO armature but wasn't sure how to proceed. Are you suggesting keeping the original comm ? Is it easy to separate comm from armature ? Or are you suggesting keeping the comm / armature together from the HO motor (I haven't tried taking an HO motor apart, not sure what the comm is like on them). Also would you mind explaining commutator timing? I've heard the term but not sure what it means or how to do it! Meanwhile I'll keep an eye out for FJ original armatures on ebay, I saw one on there but at 15 quid I couldn't justify it when complete motors + axles + wheels seem to go for less than that!
 

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Graham Windle
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I have a bsrt competition arm for a ho motor I'll post some pics tomorrow and show about timing but basicaly it refrers to the position of the com slots in relation to the centre of the arm segment. so with the ho arm you would need to twist the com by about 90 degrees
 

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I rewound one of the sidecar motors because it is so rare. Can't remember which wire but typically I'm using 75 turns of #29 on a 36D . I would try somewhere 30 - 33 whatever Magnet(coated) wire you can find and wrap whatever number you can get on. Zap the magnet and see what happens. At this point you don't have anything to loose and could actually come up with a good runner and gain some experience!
 

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If you are still keen to rewind those motors here's a few tips
Commutator

Clean up the commutator taking car the clean out the slots.

The commutator slots should be orientated at the same angle to the stack as in your working f junior motors. If it's slipped round that could be part of the problem- glue it in the right place so it doesn't slip again.

Beware taking any notice of the commutator from other types of motor - A lot have the brush gear is at a different angle to the magnetic field, that means the comm slots need to be at a different angle.

. Does it have to be the same number of turns of same gauge wire, would fewer turns of a thicker wire work?
It might be best to stick to the standard spec. Measure the wire thickness and buy some enameled copper wire the same size.

Fewer turns have less resistance so takes more current. Other things being equal it'll produce less back emf so will rev faster

Thicker wire has less resistance so takes more current. If the magnetic field is strong enough for an upgrade it'll produce more torque. It's questionable if f junior magnetic field is up to it.

More current means more heating. No problem if the motor's up to it. Doubtful if a f junior can stand more heating, you already know the "good"" ones get pretty hot.

Best to get it rezapped once it's back together

Best of luck
 

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Graham Windle
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ARMATURE.jpg

theses pic illustrate the difference in timing the small bsrt arm is configured to have brush gear oriented as the fj with the com slot near to centre of the wind where as the standard mabuchi has its brushes either side of the motor and has the com slot central to the armature pole
 

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Discussion Starter · #12 ·
Thanks guys for these really informative posts. How do I adjust the commutator? I just twist it on the shaft ? Is there a big risk I'll damage it like that?
 

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Discussion Starter · #13 ·
Just checked my 4 FJ cars...all of them have identical alignment of commutator slots to the edge of the armature segments. Only one of them seems to be running hot though, I have cleaned it but maybe not thoroughly enough (you can get an idea in the picture), I tried pepping the magnet but leaving neodyms either side of the still-assembled motor for a couple of days...not sure what else to try on that one
 

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How do I adjust the commutator? I just twist it on the shaft ? Is there a big risk I'll damage it like that?
Yes twist it on the shaft. They are usually just a press fit. If it's loose enough to have slipped it'll be easy enough to put it back in the right place. If it's too tight to move it's most unlikely to have slipped out of place anyway. As Graham points out, it's best to move the com before rewinding - I should have mentioned that in my last post.

If the com is tight on the shaft there's no need to epoxy it in place, but if it's slipped once it's best to epoxy it in the right place. The epoxy can be added after winding, particularly if you are going the epoxy the rewind in place.
 

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Discussion Starter · #16 ·
Thanks I probably won't attempt that on my existing FJs since they are mostly running fine (except for the one hot one) and all seem to have consistent comm position, I'll take a look though at an HO arm and see if that could be swapped in. I think the shaft is very slightly thinner than on FJ though so that may present issues in the FJ frame.
 
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