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BTH pancake dyno, Type PA

Started by Doug, 16 Dec 2007 at 06:23

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This thread is for the BTH type PA dynamo, also known as the BTH pancake dyno.

To kick it off, here are three types known to be used on Douglases-

The dynamo was identified as the Type PA, and then by Form A, B, or C

This first is missing its brass tag, and is shown with the cover off, but seems to be the oldest style.
There are two brushes fixed at 90 degrees.

Next we see it with the end cover removed, showing the armature. Also seen is the ring magnet, which is magnetized with four poles.

And next the inside of the end cover, showing the tail bearing for the armature.

Next up is a PA Form B. According to spares and service manuals, this was used on the 350 and 600EW. This one is off a 1935 250cc (as clearly marked!) Like the previous example, it has an extended snout with a small pinion drive. This snout is eccentric to the housing by a 1/64 inch, so that by rotating the dyno, the mesh of the gear can be adjusted.

Inside, the end plate no longer covers the magnet, which is still of the ring type. They probably realized the several screws and the press fit onto the register of the aluminum housing was more than enough to hold it in place. Also the end plate could now be removed without unbolting the dyno from the machine.

And with the cover on.

Next is a PA Form C. These were used on the 1930 S6 an related models. There is no extended snout, and the housing is not machined eccentric. What looks like a gear drive is in fact a gear coupling, and the coupling sleeve is shown adjacent.

Inside, the ring magnet has been superceded by a pair of short two-pole segmental magnets. This probably could be recharged on conventional equipment with out much difficulty compared to the full ring magnet. Yet the ring magnet continued to show up on later models like the Aero 600 and the 250cc shown above. Again the tag is missing, but I compared it to another identical dyno on a S6 that did have its tag.

Inside, pretty much the same armature, except the shaft is shorter.

And inside the cover, much the same as the others.

Not shown, but these had a cover supplied by Douglas spun in heavy brass, which are often stress cracked. They wanted a smooth surface without the BTH embossed logo as they affixed various varnish transfers to the face of the cover.

The Douglas handbooks do not always state which Form of PA dynamo was used. The A32, B32, C32, K32, and M32 booklet says a PA Form A was used, a step backwards from the PA Form B used on the older EWs! And it is known Form B continued to be used after Form C. The other problem muddying the waters is the pressed end housing holding the brushes appears to be completely interchangeable between all Forms. So service, repairs, or plain old part swapping could place the wrong brass tag with the wrong dyno. They have had over seventy years to get mixed up!

For a discussion on alternator conversions, see this thread-,2386.0.html



Many years ago I started collecting bits to build an S6 and one of the first bits of advice that came my way was: "You'll need to do something to improve the output of the standard BTH generator!" The advice went on to suggest there was a similar armature fitted to the early LE Velocette - so this had to be investigated. The armature was dimensionally almost identical but needed a drive shaft to fit it to the 'Douglas' generator. Also the coils looked to be of heavier guage wire and there were less segments to the commutator. Eventually, it was rigged up and tested - only to find the output was less than the original. Not ready to give up at this point, I then experimented with the orientation of the brushes, and eventually got enough output at about 800 rpm (the top speed of my old Myford ML7 lathe) to give a good light from a 21 watt bulb. I guess, the proof of the pudding will be to see if it is capable of keeping the battery charged when running at night!
        Another problem which arose recently with a similar generator on a 500 Aero was that although everything looked to be in good condition, very little output could be obtained. Electrical tests showed the windings to be O.K., the magnets had a good 'pull' and it didn't look to have been bodged. Eventually, one of our 'wise men' in the Douglas club suggested that it might be worth changing the magnet. "I've got one hanging up in the shed that I stick odds and sods to, so that they don't get lost! I'll bring it along". With the 'new' magnet fitted, miraculously, the output was restored. Then another 'wise man' suggested we should check out the original magnet using a compass, and what did we find - although the magnet was physically in one piece, it had two cracks which effectively made it into 3 two pole magnets of differing lengths - thus destroying the overall output.
                Good luck with the tinkering - keep those old Douggies going, but remember you don't get anything for nothing in this world. If you upgrade the lighting, it still has to be powered and that power has to come from somewhere!


Great stuff! I'm fascinated by the ring magnet: I wonder how it is made? Is it really two magnets somehow bonded in non-magnetic material, or really just one continuous piece of magnet iron? I suppose that the magnetic field configuration is the same as, or very similar to,  the two-magnet "form C".

To remagnetise the ring, it's important to know how it's made. Do you remember plotting magnetic fields with iron filings at school? Any chance of putting a sheet of cardboard over the magnet, scattering some iron filings and taking a snap?

Eddie it would be interesting to do this on the cracked one - I suspect you'd see straight away why it had little output!



Article on the BTH PA dyno form the original ConRod magazine, circa 1926.


[edit image links from http to https, Doug, Site Moderator, 04Dec18]