Marvil Pygmylyte - info sought

"Kim Siddorn" wrote

I suspect it will be a simple three brush dynamo and cut-out set up as used on a myriad of vehicles from the 1930's onwards. This is the first useful looking ref I came across on the internet, but I'm sure there are plenty of others:-

Very useful - thanks Nick, copy & pasted in a flash!

regards,

Kim Siddorn

I've got the Pygmylyte in big bits now - the dynamo took some driving out too! - and it all appears to be in reasonable order, the comm having continuity. The three brush system is carried on two plates with rings that fit over the central end bearing boss. They are secured by screws that move in long slots enabling a wide range of movement around the comm. All clean now and awaiting the satin black spray can! The cowling will be polished, probably over Easter.

I'm inclined to rebuild it as a two brush device & fit a Zener or other electronic control device in a concealed position as the regulator has long since departed this life & the generator had been modified with a simple knife switch mounted on top of the cowling. This literally fell to bits when its screws were removed. Deep inside is a label on the dynamo. 100 watts, 12 volts.

I managed to scrounge a bit of stainless pipe that exactly fitted the Marvil silencer as the original baffle had been hacked to bits and had to be destroyed to get it out. A two pence piece is exactly the right size to silver solder over the end & with the addition of a few holes, it will be perhaps better than new. I'd assumed the unit was built post WW2, but they were certainly in production in 1935 - any serial number records anyone?.

In other news, I've caused to be made a number of flanged copper pipes, two for the WM2 C-V & no less than eight for the ABC Mk I I'm restoring. On the latter unit, I'm moving the bronze flywheel back to the rear of the engine along with the rope starting flange. This will enable me to use the original front-mounted water pump which had been removed when the flywheel was mounted on the front of the engine. This necessitated the manufacture of an ali spacer to bring it rearwards by about an inch to clear the heads. My plan is to utilise a small car heater radiator fan cooled and mounted above the long sump. The rear compartment of the sump originally contained oil for the ancillary gearbox, but this was not with the engine when I got it. I intend to use it as a water reservoir as there is a 1/4" cast in ali divider separating it from the front (engine) sump.

More news as it happens ;o))

Kim Siddorn

Nick H said -

The dynamo cleaned, tarted up & checked statically for continuity & insulation, I settled down yesterday to rebuild it .

It is a three brush 12 volt 100 watt unit. Two of the brush holders are brass and insulated from the frame, the other is steel and earthed. I don't get that at all, but never mind, I intend to rebuild it as a two brush constant output device & regulate the flow of current via a diode in an out of the way place. This is mainly because the regulator/cut out is effectively missing. The two brass brush holders cannot lie exactly 180o apart from each other, but I'm not after maximum output.

Cleaning & assembling the end cap, I found that one of the brushes had no lead connected to it. After a little thought, I drilled a small hole in the top of the carbon & fiddled about with some copper wire to get a good, tight, push fit. I then cleaned it all up, Araldited over the top of the brush & put it under a table spotlamp to cure which it did most obligingly in twenty minutes. Whilst I waited, I took a bit of mains two core flex and made a spiral cut down it, thus replicating the pigtail spiral sleeve found on brushes. Should be OK if it doesn't get too hot. I checked it with a meter & got continuity.

Finally, this Mar Vil sold for £84. I'd say that was more than it was worth meself ..........

Kim Siddorn

Would you pop up a pic or two before it all goes back together, can't quite get my head around the brush layout - I wonder which one fed the field?

Nick H

Back together now Nick, but the wires are not yet connected. Two field leads appear next to the comm & another (possibly two) make their way out of the case & into the remains of the regulator where they connect to the resistance coil. The screws that retain it are so rusty & butchered I may have to reluctantly destroy it to access the ends of the wire, there being no room to manoeuvre.

I'd welcome any ideas of how to bring them together in order to have 12 volts positive & negative leads emerging from the dynamo! Otherwise, I was going to use the Lucas motorcycle dynamo wiring as a guide.

regards,

Kim Siddorn

(snip)

KS> I'd welcome any ideas of how to bring them together in order to have KS> 12 volts positive & negative leads emerging from the dynamo! KS> Otherwise, I was going to use the Lucas motorcycle dynamo wiring as a KS> guide.

KS> regards,

KS> Kim Siddorn

Re-reading; the earthed brush must be the negative output, there should be another brush pretty much 180 degrees away which is the positive output. The other (which may or may not be thinner and/or independently adjustable) will be the field brush. This may be connected directly to one end of the field coil or via some sort of regulator (automatic or manual as in a 'half charge' switch). The other end of the field coil will be grounded either directly or again via a regulator. If you don't want to use the three brush system, you will need to identify the field coil connections and use and feed with a rheostat or fixed resistor. As you say, a diode will stand in perfectly well for the cut-out.

Rather a lot of maybe's I know, but ten minutes of intelligent dibbling about with an AVO should sort out what is w]hat.

nickh=== Posted with Qusnetsoft NewsReader 2.2.0.8

ISTR that three-brush dynamos were set by up adjusting the position of the third brush around the commutator in relation to the output brushes.

There is an interesting bit about the coming of the controlled dynamo in the Lucad history books.

The advent of the control box and field cut-out relay did away with all that stuff.

Peter

-- Peter & Rita Forbes Email: snipped-for-privacy@easynet.co.uk

"Peter A Forbes" wrote (snip)

Feeding the field from a third brush works with armature reaction to give some self-regulation. Armature (ie output) current distorts the field which reduces the voltage at the third brush and hence field current. This negative feedback loop gives the dynamo very approximate constant current characteristics and moving the third bush around the commutator changes that current.

Nick H