The BSA A7-A10 Forum
Technical (Descriptive Topic Titles - Stay on Topic) => Lucas, Ignition, Charging, Electrical => Topic started by: RoyC on 26.05. 2018 20:10
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My dynamo is (+ve earth) showing no charging, I blew a fuse a while back, could it be that I have somehow reversed the polarity ?
How do you flash a dynamo ?
I have purchased this dynamo but don't know what polarity it is https://www.ebay.co.uk/itm/BSA-A10-A7-LUCAS-DYNAMO-/312136396983?_trksid=p2047675.l2557&ssPageName=STRK%3AMEBIDX%3AIT&nma=true&si=%252FlYe%252BCp%252B5UwNRURx2fojA0dVdd0%253D&orig_cvip=true&rt=nc
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If you're + earth just run a wire between the neg of the battery and the field terminal of the dynamo ( normally the one with a green wire). Engine doesn't need to be running. With you're other dynamo, join both terminals together and conect them to neg of a battery and the pos of the battery to the dynamo body. It should run as a motor. Check its running the correct rotation for the bike otherwise you will have to reverse the field coil wire inside. This test is not 100% but should show if it's worth playing with.
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If you're + earth just run a wire between the neg of the battery and the field terminal of the dynamo ( normally the one with a green wire). Engine doesn't need to be running.
So I just have to connect the green wire on the dynamo to -ve on the battery, this will make it +ve earth ?
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If you're + earth just run a wire between the neg of the battery and the field terminal of the dynamo ( normally the one with a green wire). Engine doesn't need to be running.
So I just have to connect the green wire on the dynamo to -ve on the battery, this will make it +ve earth ?
This is best done with no wires connected to the field connection on the dynamo, it might damage a regulator if it’s also connected to the field connection and you apply battery voltage to it, probably not, but it’s best not to risk it.
Without want to step on bsareg toes, yes a positive earth dynamo is flashed by connecting the negative of the battery to the field coil. All this is doing is passing current through it in the same direction as the regulator does, the little bit of magnetism left in the field coil core ensures the dynamo will start up generating in the correct polarity.
I’m not aware of any need to swap leads inside the dynamo to make a dynamo generate in the correct polarity.
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And just to add you only need to "flash" it meaning just briefly touch the wire to the -pole giving a tiny flash, no need to hold the wire on there.
Cheers
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If you're + earth just run a wire between the neg of the battery and the field terminal of the dynamo ( normally the one with a green wire). Engine doesn't need to be running.
So I just have to connect the green wire on the dynamo to -ve on the battery, this will make it +ve earth ?
This is best done with no wires connected to the field connection on the dynamo, it might damage a regulator if it’s also connected to the field connection and you apply battery voltage to it, probably not, but it’s best not to risk it.
Without want to step on bsareg toes, yes a positive earth dynamo is flashed by connecting the negative of the battery to the field coil. All this is doing is passing current through it in the same direction as the regulator does, the little bit of magnetism left in the field coil core ensures the dynamo will start up generating in the correct polarity.
I’m not aware of any need to swap leads inside the dynamo to make a dynamo generate in the correct polarity.
So, disconnect the F=Green and D=Yellow from the DVR2 before doing this flash ?
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Yes. I disconnect at the dynamo and then just hook up a wire to F on the dynamo and flash the other end to the battery (whichever pole you want as feed/power not earth).
Cheers
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Thanks for all that info lads, much appreciated.
Roy.
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You don't need to swap any leads to set the polarity as kiwif said - but if the direction of rotation were wrong, you would need to swap the field coil wires over as BSAreg said (or the brush tails). Only motoring it or running it on the bench will tell if rotation is correct, as the arrow on the body may mean norralot.
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You don't need to swap any leads to set the polarity as kiwif said - but if the direction of rotation were wrong, you would need to swap the field coil wires over as BSAreg said (or the brush tails). Only motoring it or running it on the bench will tell if rotation is correct, as the arrow on the body may mean norralot.
I’ve never checked which way a dynamo turns when it’s run as a motor. Have I missed something and just been lucky?
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Bit of both probably kiwigf!
Not a big deal, but as the things can run either way, with either 'earth', it's a pita to think something doesn't work when all it needs is a shove in the opposite direction! And then maybe reversing. If you're starting from scratch with an unknown item, it's probably 50:50 which way it's set to go.
Motoring will confirm it - if it will go, that is - and will also set the polarity depending which way you hook up the battery.
Dyning will confirm direction (if it will charge), as long you have the means to spin it both ways if it won't play going the first way. But polarity may still have to be chosen. A meter will show whether the volts are + or - and then if it's not what you want, it can be swapped as described in previous posts.
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Bit of both probably kiwigf!
Not a big deal, but as the things can run either way, with either 'earth', it's a pita to think something doesn't work when all it needs is a shove in the opposite direction! And then maybe reversing. If you're starting from scratch with an unknown item, it's probably 50:50 which way it's set to go.
Motoring will confirm it - if it will go, that is - and will also set the polarity depending which way you hook up the battery.
Dyning will confirm direction (if it will charge), as long you have the means to spin it both ways if it won't play going the first way. But polarity may still have to be chosen. A meter will show whether the volts are + or - and then if it's not what you want, it can be swapped as described in previous posts.
So as far as I can see then there is no need to change the way a dynamo motors (by swapping wires) except as a last resort if all else fails?
I don’t understand the technical reason why swapping wires would make a non working dynamo (that has been flashed) start working *dunno*
Having said that, why did Lucas put a direction arrow on? Maybe because it affects brush wear?
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Swopping wires won't repair a nonworking dynamo but for a dynamo to charge a battery it must produce the correct polarity. Reversing it's rotation will also reverse the polarity of the charge so you must have the same rotation when motoring the dynamo as when it's being driven by the engine.
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Indeed there is no reason at all to change anything kiwigf, if it has been working fine and if nothing has been touched. But it is always worth testing both ways before condemning anything that's been played with, unless you KNOW it's gone back together exactly as it came apart. The unmarked field coil wires are commonly hooked up t'other way round after fettling.
So reversal may make a 'non-worker' work if there has been an inadvertent change in the wiring during maintenance, was my point.
With an unknown dynamo picked up from an auto-jumble or whatever, you can't tell which way it has been set to run. The arrow indicates how Lucas had it in the beginning - but loads of dynamos have been set to run against the arrow, and pretty well all two brush ones can work equally well either way. It's one of the nice things about them!
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Yes but, when I blew the fuse, could I have accidentally changes from +ve earth to -ve earth ?
I was rewiring the headlight switch with battery connected.