Douglas B31 - BTH pancage dynamo

[Edin]

Hej all
I need some help and advise please,- about electrik polarize on my B31
Can anyone please inform me,- if they now that B31 is born with positive polarize to ground/frame.
My BTH pancagedynamo rotate in correkt direction (according to direction arrow) but the outcome is positive to frame and negative to battery.
Can I maybe have made and error when fitting parts together from several old BTH dynamos?
Forwards thank you for any help and advise.
Regards Edin DK.

[Chris]

Hi Edin
   What a coincidence!! I have been struggling with the same problem for the last week or so. I have ascertained the following which I believe to be true from various sources of literature. The pancake dynamos should all be negative earth. The 500 and 600cc machines have the dynamo revolving in the clockwise direction facing the dynamo cover as they are driven from an idler between the upper cam wheel and an anticlockwise magneto. \The 350cc machines have a clockwise magneto with no idler so I assume that the armature of the dynamo armature in the anticlockwise direction. The dynamo for my S6 is also positive earth and I have compared it with two other pancake dynamos from S6 machines and it appears from all external aspects to be identical but they both have negative earth. I have shown that reversal of the correct direction of rotation reverses the polarity so the only conclusion I have come to is that my pancake has the armature from a 350cc machine. I would welcome any comments or advice on these devices and also whether anyone has yet found a source for re-magnetising the magnets. Using a compass indicates that the two separate magnets in the dynamos of the larger machines are South pole at each end and North pole in the centre. Normal bar or horseshoe magnets have different poles at each end. I cannot imagine how they have managed to magnetise them in this way. The 350cc machines have a single circular magnet. I do not know how the poles are formed on these but clearly BTH had some very fancy machines for magnetising these devices. Chris.

[oily bloke]

I am not familliar with the pancake dynamo but can it be "flashed" to reverse the polarity like a regular lucas dynamo?

[Chris]

Hi Oily bloke
   The reason that the Lucas dynamo can be flashed is because it has field windings which provide the magnetism generated when the dynamo armature is rotating. When starting it relies upon residual magnetism to ensure that it starts to provide current of the correct polarity. If the polarity needs to be changed it can be reversed by "flashing" the field connection with a dc supply to change the polarity. The pancake dynamos have a permanent magnet so there is nothing to flash. Since posting the last message I have seen pictures of the two different dynamos used on 350s PA Generator Form A and 600s PA Generator Form C and it looks as though the drive ends  of the shafts are different which would suggest that my theory of possible swapping of the armatures would not be possible which again raises the question of how the polarity of my and Edin's dynamos have become reversed. Chris.

[eddie]

Chris,
          It should be possible to remagnetise the magnets of a pancake dynamo on an ordinary remagnetising rig. You should be able to set the magnet up with the centre against one pole piece, then bridge the 2 ends with a soft iron strap and attach the other pole piece. Likewise, with the full circle one piece magnet, it should be possible to fit straps to either side of the magnet - one positioned 6 o'clock/12 o'clock, and the other 3 o'clock/ 9 o'clock. The main problem with the full circle magnet would probably be the amount of power needed as you are effectively trying to remagnetise 4 magnets at once.

      Eddie.

[Doug]

Chris,

The BTH pancake dynos with the single magnet are magnetized the same as the ones with the two crescent magnets; alternating poles every 90 degrees.

For everyone else, if you are not aware of it, there are pole pieces between the magnet and the armature that conduct the permanent flux of the field (see picture).



There are two types of drive that I am aware of. The type used during the EW era, with a long 'snout' and a small pinion and then a shorter 'snout' and a drive using a gear coupling as seen on the S6 types. The EW (and lightweights) and the S6 type have the dynamo rotating in opposite directions as Chris mentions.

-Doug

[Chris]

Hi all
Does anyone know if the polarity would be changed if the magnets had been re-magnetised as per Eddie's suggestion but with NSN instead of SNS which is the configuration of my S6 dynamo with the two separate curved magnets? Perhaps someone can check the magnet poles with a compass on their dynamo. I assume that Edin's dynamo has the circular magnet as the B31 is a 350cc model. Chris.

[Chris]

Hi
Thanks to a friend I got my hands on another three pancake dynamos this afternoon. Two with circular magnets from 350cc models and one from an S6. All three are negative earth and the end plates with brush holders are all in an identical position relative to the body mounting holes. In the case of the S6 unit the magnetism of the two magnets were also identical to mine being SNS and the end plate was in an identical position relative to the directional arrow on the rear of the body. No closer to discovering why my dynamo is positve earth. Chris.

[Edin]

Thanks to Chris,- Eddie,- Oily Bloke and Doug for reply.
After spending a lot of time with investigation,- talk with elctronik Guru and practical work with remagnetize the BTH dynamo - I am now in that situation,- that my
BTH dynamo runs very well,- with an output of 7,2volt on 3000 rev/min - but with positive to ground/frame.
I can assure you,- that it is not an easy job to fit four pole in the right possition.
If not an easy way to change the polarity comes up in a few days,- I think I run the bike with positiv to frame.
Thanks for help and interest about my problem.
Regards Edin DK