A prototype Lavet stepper motor

I'm pretty certain that they do use the standard Lavet pattern, as these are extremely cheap to make and standardised. This patent is the best I've been able to find that describes how they are driven, also this describes some current measurements that show a pattern of current pulses. The idea seems to be to give the coil a small kick or a few to get it moving and just nudge it over the dynamic equilibrium point, then pulse it in the opposite direction to slow its acceleration towards static equilibrium, then finally stop it briefly at that point so it doesn't overshoot.

You could use a standard stepper, using a processor to do the microstepping so it looks like it's doing equal seconds increments, but of course it takes more current.

Here's a couple of pictures of the Gent's, definitely 4 pole and 1/4 rev per pulse. The white gear is driven by a worm on the motor shaft. It is very quiet

Interesting! I suppose current consumption is an issue for a clock that you expect to run from an AA cell, but as I am using a 12V "wall wart" as the power source, using a conventional stepper motor with microstepping would definitely be an option.

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Edited By Tony Jeffree on 26/03/2021 11:34:59

Is the rotor 2 pole or 4 Duncan?

Another photo gleaned from the web.

Think I have it figured out now - the asymmetry comes from the shape of the pole pieces on the rotor (rather than the shape of the pole pieces on the electromagnet) which is interesting. Potentially one could develop the same approach with any number of poles (but with correspondingly more difficulty in construction).

I suspect it may be time I resurrected my CNC mill, which has been out of service since the move!

Edited By Tony Jeffree on 26/03/2021 14:08:22

4 poles as far as I can see, then 30 teeth on the white wheel gives 1:4*30 = 1:120 reduction to drive minute hand at 30 second intervals.

Tony's explanation sounds about right, I think with a bit of thought at jig to fit on the rotary table could be devised to make the pole pieces. I'm loathe to start dismantling mine to measure it in detail, I might put a posting on the Synchronome site top see if anyone has a scrapper. If nothing else having one which worked off less than 24v would be handy

I'm sure that rolling your own 4 pole Lavet motor would be do-able. I've just received my order of donut-shaped magnets - diametrically magnetised, 6mm diam, 3mm ID, 1mm thick, so my next try will be to re-make a 2-pole motor using one of those - should end up being smaller and quieter than this first attempt. After that I fancy having a go at a 4-pole just to see what can be done.

Breath bated here Tony! This is really interesting as I'm re-planning the next clock and would much prefer to make the movement, but would rather avoid an ordinary bipolar stepper.

I agree John - the Lavet-style of motor has its attractions, not least being the ease of driving it.

Gents [no pun intended] ...

Did you look at the original French patent that I linked yesterday ?

Lines 18 to 20 on page 2 read:

Les fig. 8 à 10 représentent un petit
mouvement de récepteur horaire du type à
émissions inversées toutes les 1/2 minut;

... which translates to:

Figs. 8 to 10 represent a small
clockwise receiver movement of the type to
reverse transmissions every 1/2 minute;

... and I think those figures are worth a look.

MichaelG.

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Edit: __ here’s a bonus : I think this is Gent’s patent describing the essence of Duncan’s motor

https://worldwide.espacenet.com/patent/search?q=pn%3DGB604840A

Edited By Michael Gilligan on 26/03/2021 23:44:38

Duncan - any chance of posting a dimension or two, for example the dimensions of the U-shaped pole piece or the coil, so I can get an idea of the size of the components?

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As the saying goes : You can lead a horse to water ...

MichaelG.

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Edit: __ Yes ... it includes some relevant dimensions

Edited By Michael Gilligan on 27/03/2021 09:11:53

Yes, saw that, thanks, but no dimensions that o could see...?

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Try going to line 10 on page 3, Tony

MichaelG.

The magnet is a disc 5/8" dia and 5/32" thick. Everything should scale from that.

I'll measure it up later on, but if you are in the North West you could borrow it

Thanks Michael - I couldn't get to the text when I looked on my mobile for some reason. Not the easiest site to navigate!

Perfect - thanks.