A prototype Lavet stepper motor

[John HaineParticipant @johnhaine32865]

A sketch for the purposes of explanation, as implied I think in p2 line 85. I suspect that in manufacture the rotor polepieces were stamped from flat plate then bent in two places, then finally milled to form the tapered circular forms on the actual poles.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by John Haine on 28/03/2021 10:47:15:

A sketch for the purposes of explanation, as implied I think in p2 line 85. I suspect that in manufacture the rotor polepieces were stamped from flat plate then bent in two places, then finally milled to form the tapered circular forms on the actual poles.

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Yes, I agree regarding the purpose, John … but I’m trying to comprehend the three dimensional shape, which [regardless of the actual manufacturing method], appears to be rather like something you might cut from a ferrous equivalent of the plumber’s ‘stop-end’

**LINK**

https://www.jewson.co.uk/p/altech-end-feed-stop-end-22mm-copper-ALTEF068

MichaelG.

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Hopefully: one of the 3D CAD wizards might come to my rescue, or Duncan might be able to provide a close-up photo [unfortunately, the photo that you grabbed doesn’t quite do it].

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Edited By Michael Gilligan on 28/03/2021 11:30:36

[Tony JeffreeParticipant @tonyjeffree56510]

Posted by Michael Gilligan on 28/03/2021 11:22:44:

Posted by John Haine on 28/03/2021 10:47:15:

A sketch for the purposes of explanation, as implied I think in p2 line 85. I suspect that in manufacture the rotor polepieces were stamped from flat plate then bent in two places, then finally milled to form the tapered circular forms on the actual poles.

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Yes, I agree regarding the purpose, John … but I’m trying to comprehend the three dimensional shape, which [regardless of the actual manufacturing method], appears to be rather like something you might cut from a ferrous equivalent of the plumber’s ‘stop-end’

**LINK**

https://www.jewson.co.uk/p/altech-end-feed-stop-end-22mm-copper-ALTEF068

MichaelG.

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Hopefully: one of the 3D CAD wizards might come to my rescue, or Duncan might be able to provide a close-up photo [unfortunately, the photo that you grabbed doesn’t quite do it].

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Edited By Michael Gilligan on 28/03/2021 11:30:36

Michael –

I found Figure 2 from the patent was the best one for visualising what is going on. You are right – the two pole pieces (10 and 11) are best visualised as being cut from a steel version of one of your plumbers' stop-ends. If you look at part 10 in fig 2 and attempt to screen out the "noise", the top surface of 10 looks very much like the profile shown in Fig 4, with the cylindrical portion of the part (which appears from the photo to be essentially rectangular if you were to cut it away and flatten it) has been thinned, so it is full thickness at the end nearest the electromagnet's pole pieces and tapers to almost nothing at the tips. The tips therefore clear the pole pieces of the electromagnet by the minimum clearance plus almost the thickness of the metal. The effect of this is that in its un-energised state (as shown in Fig 2), the permanent magnet will align the rotor so as to minimise the gap between the rotor's pole piece and the electromagnet's pole piece (which is the state shown in Fig 2). However, the thinner tail of the rotor's pole piece is near enough to the next electromagnet pole piece to ensure that when the electromagnet is next energised, with the reverse polarity, the rotor will rotate in the direction of the next pole.

[Michael GilliganParticipant @michaelgilligan61133]

Thanks, Tony

I actually started 'working on' Fig.2 last night … That's when I realised that I seemed to be missing some of the devilish detail; so I paused for thought and advice, in preference to making gross assumptions.

MichaelG.

[duncan webster 1Participant @duncanwebster1]

As I had to mess with it today to change the hours I've taken some more photos. The crescent shaped bits are tapered, see photos. Intuitively I can't see why, I would have guessed that having a simple stepped profile would work. If I start any more projects SWMBO has threatened to lock the workshop door, too many outdoor landscaping projects in the queue, so I'll have to rely on others for the time being. The brass disc is a loose fit, some kind of rattle damper? (see posts from Joe Noci on ELS)

 

Trawling the memory banks brought to mind something from the SMEE archive, so I've searched through it and found a photo of 'experimental silent slave' from a Ron Hough, so I've sent an email to SMEE to try and contact him. Hopefully I won't need a ouija board

Edited By duncan webster on 28/03/2021 13:50:18

Edited By duncan webster on 28/03/2021 13:55:29

[Tony JeffreeParticipant @tonyjeffree56510]

The tapering is all about ensuring that the rotor's rest (un-energised) position is as shown in Fig 2. That is important so that when the next polarity reversal happens, the tapered horns are attracted to the next stator pole, and the rotor will rotate. It controls the direction of rotation of the rotor. If they weren't tapered (or bent inwards to create more clearance at the tips, which would have the same effect), the rest position would be somewhere more nearly half-way along the rotor poles, and it would be uncertain as to which direction the rotor would move on the polarity reversal.

You could achieve the same effect by keeping the thickness constant and the clearance between the rotor and stator poles constant, but making the "flying" part of the rotor poles taper to a point in the axial direction.

[duncan webster 1Participant @duncanwebster1]

Posted by Tony Jeffree on 28/03/2021 14:15:20:

……. If they weren't tapered (or bent inwards to create more clearance at the tips, which would have the same effect), the rest position would be somewhere more nearly half-way along the rotor poles, and it would be uncertain as to which direction the rotor would move on the polarity reversal.

………

constant thickness but varying gap seems a lot easier to achieve, in mass production a press tool, or as a one off just machine a cup , fret out the arms and tweak the rim. Or as I said before, a stepped profile.

[John HaineParticipant @johnhaine32865]

Some of the details which seem odd may have been added to differentiate this from other patents…

[Michael GilliganParticipant @michaelgilligan61133]

Pretty obvious, but I might as well post it for reference:

Fig.2 in the Gent’s patent is too sketchy to be be directly scaled.

I have tweaked the proportions a little, to make it a reasonable fit in a square, with the smaller circles fitting quite nicely … But: the all-important ‘rotor arms’ are then distorted.

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Having ‘got the general idea’ … I can now concentrate on the shape of the rotor arms.

MichaelG.

[Michael GilliganParticipant @michaelgilligan61133]

Update:

I may be wrong [ it has been known! ] … but :

I think I now understand the elegant geometry of the tapered ‘horns’ on those rotor-arms

Although it looks like the production items might have been stamped from sheet; it should be a relatively simple matter to machine them from solid.

I will leave you guessing for a while

MichaelG.

Edited By Michael Gilligan on 29/03/2021 08:32:36

[Tony JeffreeParticipant @tonyjeffree56510]

Posted by Michael Gilligan on 29/03/2021 08:22:06:

Update:

I may be wrong [ it has been known! ] … but :

I think I now understand the elegant geometry of the tapered ‘horns’ on those rotor-arms

Although it looks like the production items might have been stamped from sheet; it should be a relatively simple matter to machine them from solid.

I will leave you guessing for a while

MichaelG.

Edited By Michael Gilligan on 29/03/2021 08:32:36

What a tease, Michael c'mon…spit it out!

[Tony JeffreeParticipant @tonyjeffree56510]

I've made a new rotor for the motor. The original rotor looked like this:

The magnet (in the middle) is 6mm diameter 4mm long. Had to be made in 3 sections because the magnet is a solid cylinder. The new rotor:

The magnet is 6mm OD, 3mm ID, 1mm thick and the brass bobbin that it is mounted on can therefore be machined in one piece. Short video of the motor with its new rotor:

The motor runs much quieter than with the original rotor, as the magnet is significantly lower power. Next stage will be to re-make the electromagnet/armatures/pole pieces to scale them down to a more reasonable size.

[John HaineParticipant @johnhaine32865]

Nice! What is the diameter of the shaft please? It looks like about 2mm. Is the shaft silver steel?

I've ordered some of the magnets of the same size. I was wondering, if the shaft was the same nominal size as the hole, would it "short out" some or all of the flux? Can you tell if there is significant flux in the hole?

I'm thinking of trying the configuration where the stator is a solid strip, with a pair of "notches" in its periphery at 45 degrees, as in the wikipedia illustration (and patent I think).

How many turns on the coil? I've gone back to the coil winder project, have to get that working to make the coil.

Now where did I leave that bucket that needed mending.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Tony Jeffree on 29/03/2021 11:17:23:

Posted by Michael Gilligan on 29/03/2021 08:22:06:

[…]

I will leave you guessing for a while

MichaelG.

What a tease, Michael c'mon…spit it out!

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… it’s sheer poetry :

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MichaelG.

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Edit: Just noticed that I cropped a little too much off the bottom of the screen-shot

… The purple radius line starts at the 6 o’clock point.

Edited By Michael Gilligan on 29/03/2021 18:25:58

[Tony JeffreeParticipant @tonyjeffree56510]

Posted by John Haine on 29/03/2021 17:58:17:

Nice! What is the diameter of the shaft please? It looks like about 2mm. Is the shaft silver steel?

I've ordered some of the magnets of the same size. I was wondering, if the shaft was the same nominal size as the hole, would it "short out" some or all of the flux? Can you tell if there is significant flux in the hole?

I'm thinking of trying the configuration where the stator is a solid strip, with a pair of "notches" in its periphery at 45 degrees, as in the wikipedia illustration (and patent I think).

How many turns on the coil? I've gone back to the coil winder project, have to get that working to make the coil.

Now where did I leave that bucket that needed mending.

The shaft is 1.5mm diameter and is the plain part of a 1.5mm twist drill. The bearings had an ID of 1.5mm and I happened to have a pack of cheap drills to hand so that was the obvious solution.

There is certainly some flux in the hole, but I suspect it wouldn't make much difference to the peripheral flux – might actually increase it. Worth some experimentation there.

I'm also considering the solid-strip-with-holes-at-45-degrees variant for the next (final) attempt as it is potentially easier to make.

The coil is 20mm long wound on a piece of 3/8" steel bar. The OD of the winding is a gnats less than 14mm but I couldn't tell you how many turns. The wire is 38 SWG. I'm planning to try a slightly different configuration for the magnet – I have 4 plastic sewing machine bobbins that I wound with rather thinner wire (50 SWG I believe) for another project that never completed; the bobbins are around 6mm ID. One or possibly two of those may be about right to drive a rather scaled down motor. We'll see.

Edited By Tony Jeffree on 29/03/2021 18:34:50

[Tony JeffreeParticipant @tonyjeffree56510]

Posted by Michael Gilligan on 29/03/2021 18:04:57:

Posted by Tony Jeffree on 29/03/2021 11:17:23:

Posted by Michael Gilligan on 29/03/2021 08:22:06:

[…]

I will leave you guessing for a while

MichaelG.

What a tease, Michael c'mon…spit it out!

.

… it’s sheer poetry :

.

.

MichaelG.

.

Edit: Just noticed that I cropped a little too much off the bottom of the screen-shot

… The purple radius line starts at the 6 o’clock point.

Edited By Michael Gilligan on 29/03/2021 18:23:35

Hmmm…still not seeing where you are going with this…

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Tony Jeffree on 29/03/2021 18:26:16:

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Hmmm…still not seeing where you are going with this…

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Just that it’s a very simple milling job

Oh well … probably best if I stop intruding.

MichaelG.

[Tony JeffreeParticipant @tonyjeffree56510]

Posted by Michael Gilligan on 29/03/2021 18:33:47:

Posted by Tony Jeffree on 29/03/2021 18:26:16:

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Hmmm…still not seeing where you are going with this…

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Just that it’s a very simple milling job

Oh well … probably best if I stop intruding.

MichaelG.

Ah…yes indeed. Mind you, my head tends to take me in the direction of CNC for that kind of milling, so much less of an issue.

[Tony JeffreeParticipant @tonyjeffree56510]

There may be a short (or long!) delay while I get my Taig CNC Mill back up and running – looks like I will need to do some major surgery on the controller side as my Desk CNC software/hardware doesn't seem to want to play with a modern-spec PC. So currently looking at alternatives…

[John HaineParticipant @johnhaine32865]

This is the stepper. Has the advantage that it can do more steps per rev – the design shown has a 20 tooth wheel and 3 phase drive so 60 steps per rev. And should be able to microstep too! Also shows the clock – the wheels are aluminium with small neodymium magnets for "teeth".

This the book – highly recommended. Available from Camden.

[John HaineParticipant @johnhaine32865]

Sorry, lost a bit at the start of last post! I should have said that this chap, Weston Bye who writes in the Digital Machinist mag, has designed an interesting stepper that could be a better bet than the Lavet type.

[duncan webster 1Participant @duncanwebster1]

At £43 I'll give that one a miss, but instead of 3 phase 20 teeth it could be 2 phase 30 teeth?

[Tony JeffreeParticipant @tonyjeffree56510]

Posted by John Haine on 25/04/2021 10:44:48:

Sorry, lost a bit at the start of last post! I should have said that this chap, Weston Bye who writes in the Digital Machinist mag, has designed an interesting stepper that could be a better bet than the Lavet type.

Interesting – thanks. I had already started experimenting with magnetic gears…

[Michael GilliganParticipant @michaelgilligan61133]

Posted by John Haine on 25/04/2021 10:44:48:

Sorry, lost a bit at the start of last post! I should have said that this chap, Weston Bye who writes in the Digital Machinist mag, has designed an interesting stepper that could be a better bet than the Lavet type.

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Brief video here: **LINK**

… seems to have rather a lot of recoil [bounce] on that seconds hand.

MichaelG.

[John HaineParticipant @johnhaine32865]

Thanks Michael, I hadn't seen that. Looks like one of the versions made from Corian. Bye's design has an eddy current damper on the seconds arbor, I wonder if the one in the video has that?

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