A prototype Lavet stepper motor

See Youtube video here:

Excellent, are you going to write it up? The video seems to show it doing 3/4 turn followed by 1/4 turn, is this an optical illusion? I have a Gents silent slave with a Lavet motor, but it does 1/4 turn per pulse, which helps with the gearing

Thanks! Yes, it's an optical illusion - the blob of white tac is actually slipping on the shaft because the motor's acceleration from rest is quite high. I will be writing it up in due course, but it is early days!

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That’s a relief ... I thought my eyes were going funny again

The write-up will be most welcome.

MichaelG.

Fooled me to start with too!

Whilst not wishing to steal your thunder, Tony ...

For anyone who is unaware of the general design of Marius Lavet's motor :

This is worth watching : **LINK**

https://youtu.be/cGN0ueVhNEY

MichaelG.

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Edit: __ See also : https://en.wikipedia.org/wiki/Lavet-type_stepping_motor

Edited By Michael Gilligan on 24/03/2021 22:18:25

PostScript

The collection of drawings in the patent is wonderful ... but sadly let-down by the scanning

**LINK**

https://worldwide.espacenet.com/patent/search?q=pn%3DFR823395A

... The hunt is on for a better copy.

MichaelG.

I've removed the white blob and marked the rotor with red/black pen to make the movement more obvious.

Hard to see from the vid, but the necessary asymmetry in the magnetic circuit was achieved by making the two pole pieces moveable - they are offset slightly so the rotor's rest position is not straight across. The pole pieces started life as a 3/8" X 1/8" steel strip with an 8mm hole in it, held in slots in the ends of the armature side pieces. They are each offset by about 1/2mm to get the asymmetry. The shaft is pivoted in tiny ball races, but that is probably overkill.

A nice feature of these motors is that you don't have to worry too much about end float in the rotor shaft - the PM in the rotor centres itself on the pole pieces and holds the rotor in place with significant force.

Excellent Tony, I have been thinking about the same thing, you beat me to it! Do you use a short "bar" magnet in a brass (or ali) cylinder?

I've also been trying to find out how those clocks with apparently continuous motion work. I believe that they use the identical type of motor but drive them with trick pulse trains, that's the next challenge, should make them a lot quieter.

I get magnets from these people:

**LINK**

Good range of small rod magnets, also have various materials and diametrically magnetised rods.

Thanks John!

It is a short cylindrical (but diametrically magnetised) magnet sandwiched between two brass cylinders of the same dimensions, with axial 1.5mm holes to take the steel shafts. Not the easiest way to go and a pig to align, even badly - but I have found some donut shaped magnets that will do the trick and will be easier to use. I've sourced some from Ebay, some from Amazon; the donut shaped ones are these:

LINK

Supplied by Magnet Experts - haven't been delivered yet though!

Thanks John!

It is a short cylindrical (but diametrically magnetised) magnet sandwiched between two brass cylinders of the same dimensions, with axial 1.5mm holes to take the steel shafts. Not the easiest way to go and a pig to align, even badly - but I have found some donut shaped magnets that will do the trick and will be easier to use. I've sourced some from Ebay, some from Amazon; the donut shaped ones are these:

LINK

Supplied by Magnet Experts - haven't been delivered yet though!

Yes, getting them to work quietly is an issue - even the tiny ones in the quartz movements generate a significant "tick". With the relatively large magnet I am using the tick is very loud indeed, especially when attached to a sounding board. The smaller ring magnets should be better from that point of view.

I'm wondering what happens if you simply drive the Lavet motor using a Sine wave...? Might be worth a go.

Of course, I had to try it...I have a crude sig gen that will give a ~1 Hz sine wave and a cheap Class D amp, so this is the motor driven from the output of the amp:

LINK

Not very smooth, some of which may be down to the quality of the signal of course, but interesting!

Edited By Tony Jeffree on 25/03/2021 16:52:33

Not easy to see without taking it apart but I suspect the gents has an axial magnet with steel pole pieces. This could be achieved using a number of small magnets rather than one polo mint. Can also give more than one pair of poles. A non magnetic axle through the middle would avoid short circuiting the field 

Edited By duncan webster on 25/03/2021 16:55:36

Managed to delete one of the earlier videos - here it is again:

LINK

More poles would certainly help with smooth running of course.

Nice to see it works on a sine wave too. I thought the Lavet motor was limited to 2 poles? You would need a more complex stator structure with more rotor poles. From what I can glean the "continuous motion" quartz clocks have a conventional motor but a clever drive waveform with multiple pulses of varying width - I'll try to dig out what little documentation I could find (mainly a patent).

Looking at the patent drawings that Duncan posted, it seems that he built variants with more than 2 poles, but I haven't (yet) got my head round how that works in detail.

The continuous Quartz clocks could well be using a fairly conventional 2-phase bipolar stepper - if you apply sine waves (one phase-shifted) to the two phases you get a close approximation to continuous rotation. The tricky thing then (for a pendulum clock) is generating the sine waves at the exact frequency of the pendulum.

Edited By Tony Jeffree on 26/03/2021 10:04:47