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Dynamo polarity question

Hopefully one of you gents will be able to explain this to me. In the Tech section, it states that the (quoted) dynamo is suitable for positive earth and can be switched to negative earth by connecting the 'flash' wire from the positive pole of the battery to the F point on the dynamo. Why the positive pole when the intention is to make it negative. ..and how would you then switch it back to positive, if you needed to, at a later date?

I'm asking this as I'm of the opinion that my refurbed dynamo is wrongly set up for 'pos', as when I tested the voltage across the two F&D points the voltage appears to drop when I increase the speed. Any suggestions guys??

Re: Dynamo polarity question

If you feel the need to change to negative earth just connect the negative terminal to the frame. Once done repolarise the dynamo by flashing the field wire on the positive terminal. If positive earth is required connect the positive terminal to the frame and flash the field wire onto the negative terminal to repolarise the dynamo.

Re: Dynamo polarity question

Hello btb,

If voltage appears to drop when you increase the speed,
Then the clutch of the dynamo might be slipping,
Though it usually happens under load.

If you connect a Voltmeter, when the negative side of the meter
Is connected to the chassis, and the positive side of it touches the 2 wires,
Shorten between the 2 terminals,and the reading is positive,
[No (-) sign on the display on a digital Voltmeter,
Or in an analog Voltmeter, the needle does not want to go to the 0 side]
Then you got the polarity set to a "Negative chassis".

Hope that helps.

Re: Dynamo polarity question

OK Ian that makes sense now. I should have mentioned I suffered a stroke not too long back and I'm having to re-learn a lot of basic stuff - I feel like a bit of a numpty but I'm getting there.

Noam, thinks for the further suggestions regarding tracking down the pos/neg problem, I will take a look this morning. What struck me about my problem is that when the bike was ticking over the (analogue) voltmeter shows a healthy 6volts. However when I increase the speed the voltmeter dropped with a matching amount until it is at the bottom of the scale. Hence my confusion over the polarity.

Re: Dynamo polarity question

You will have lost a lot of information after a stroke but it will slowly come back especially with repeated tasks.

Re: Dynamo polarity question

Hi btb,

First of all I want to wish you a quick recovery.
If you are already plying with bike, it is a good sign..
I had an 8 hours long major surgery, and after recovering,
I found some things missing that I just do not remember.
Most of it came back as time passes.

Regarding the voltage measuring-
Just to make sure-
You did it on the DC position, right? not the AC.


Re: Dynamo polarity question

Yes Noam, I made the checks on the DC scale. Checked again today but I still have the same problem. Further fiddling around with the dynamo suggests that the fault might lie in the brushes, in that applying pressure on the them while it was running made the scale drop to zero. Which was just the opposite of what I expected ! :-) The brushes have plenty of length to them so I wasn't expecting anything faulty there - assuming it is.

Re: Dynamo polarity question

thought this may be the thread to post my dynamo question in :

the two field coil wires that poke out of the internal fibre plate.i have the single vertical slot for the two wires to poke through

the manual says one has a sleeve which goes to the brush .(other to F terminal)

well on mine there is no sleeve .

does it matter which of the field coil wires goes where ?

and if so -how would you work out which is which ... preferably without dissembling the dynamo....

if it also matters I have a positive earth regulator


Re: Dynamo polarity question

Hello 9triton,

It does not really matters which of the field leads goes to the brush, or to which brush,
But it does affect the polarity of the dynamo,
So you can connect it in any random way and re-polarize the dynamo,
Or connect, check the polarity, and if it is not what you need,
(in this case, you get a positive earth) switch between the field wires.
(Or move the same field wire which is connected-> to the other brush.
I guess it would also change the polarity)


Re: Dynamo polarity question

Noam - thanks

will give it a go and re polarise

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