Building the Minimag

[Bogstandard]

I just knew you lot would have me dancing about with curly hair (what I have left of it) and the a**e of my pants on fire.

Just to answer Jack's question.

The 'sweet spot', depending on your required direction of rotation is just as the rotor 'goes over the top', you can feel it being forced away forwards. The spot is that the points should just be starting to open somewhere between then and about ten degrees after, you need to tweak to find the point of best spark. It took me no more than 10 minutes playing about to find the optimum position.

During my dark session, I was driving it with a portable drill, and noticed that the contact points did flash occasionally, about once every five or six seconds, suggesting that a condensor might be needed to suppress that, but I can attest that none is required for normal running, there were plenty of sparks flying about on the plug tip with a 0.040" (1mm) gap on the plug and 0.010" (0.25mm) gap on the points.

I hope that answers most of your questions.

I hope to start back on the flame licker tomorrow.


John

[madjackghengis]

Hi John, thanks much for clarifying all of that, glad to hear it sparks across forty thousandths, that should fire well under compression.  I'm looking forward to your flame sucker getting back on schedule.  This has been a very enlightening build log, thanks much.  jack

[Dean W]

Was an informative build thread, John.  The finished item looks the part, too.  Very nice.
Thanks!

[John Stevenson]

I like the nice touch of the strain relief.

John S.

[krv3000]

WELL dun brill work 

[lordedmond]

Sorry this final post has taken so long, I had done a little bit too much , so I just took a nice steady rest.


The plate that covers the rotor was made to drawings, and as you can see, it had to have a big hole in it to give the rotor somewhere to stick out into.  I still thought it wasn't really swarf proof, so I turned up a disc with a very shallow spigot on it to fit into the hole and duly soft soldered it in.




Unfortunately, I had to a bit of bling as well. So my trademark of concentric circles and a bit of playing about with my latest digital toy, my rotary table, I came up with this.




The cable strain reliefs were made and fitted, just to stop the cable from flopping about all over the place due to vibration, and maybe later on causing a breakdown due to fracturing of the inner strands.

The test rig was then made up and the Minimag given a turning over. I tried to get a vid of it showing the sparks on the plug, but underneath the lights you couldn't see them hardly at all, and in the pitch black my camera wouldn't work, so my word will have to do.




So the unit has been built and tested, I will just have to wait until my big engine is built to see if it works OK on that.

I'm sorry I can't go into more detail, but this post has over run by over a week, so I must now get back to the flamelicker project, otherwise that will end up in a box somewhere.

I hope you have enjoyed the journey.


Bogs

John
As you comment on a low output have you tried it without the new cover ?

as I am an Electrical Eng ( when I when to work 14 years ago ) consider that the brass cover plate could be presenting  a single shorted turn to the magnet

maybe try without the cover , if this works then make the cover out of a none conductor

Stuart

[Bogstandard]

Stuart,

I never commented about low output, just that under bright lights the camera couldn't pick up the spark too well. In darkness there was enough sparks for everyone and his dog.

The back plate is in fact part of the original build sequence, and there is no difference to the sparks whether it is there or not.


John

[lordedmond]

John

Sorry I misunderstood your comment

It was just a comment as I have come across similar problems in the past with brass plates and magnetic devices


In one case some cables had been installed with brass bushes round each core , result brass got so hot it melted the insulation on the cables


Stuart

[Bogstandard]

I have a few apoligies to do on this post.

All those that queried why this unit didn't have a condensor and I replied that none was supplied and as far as I knew it didn't require one.

Anyhow, talking thru things with Julian, he did mention that even though there is no external condensor with the Minimag, the coil actually has one built into it and is all contained within the potting shell.

Anyway that bit is now explained, and for all you doubting Thomases, and especially of interest to the Webster builders.

Because I don't have an engine to try my Minimag out on, Julian very kindly cobbled together a mounting and has done a video of his Webster running with my Minimag doing it's own thing.

The extra 'clicking' towards the end is radio interferance.

Bogs

[sbwhart]

Love that crazey beat 

Very nice

Stew

[Stilldrillin]

Poetry! 

David D

[NickG]

Brilliant! 

[Bogstandard]

I thought that this post was done and dusted, but it seems not quite.

Julian, the supplier called me today, just for a general chat to tell me that sales of the Minimag are now climbing, and that he is returning my unit, and with it, he will be sending a new neo magnet that he is going to be trying out, to see if it will give even bigger sparks at much lower revs. I will give an update when I can get around to it.

We also discussed a little about the chain drive, and he stated that he is using an 8mm wide one, plus also, because of the slight slackness in the chain drive, it somehow gives a spark boost as it comes up to the generation stage, the chain sort of lags a little as the pressure from the magnet is felt, then gives a forwards kick when the chain takes up the slack. A very interesting phenomenon, and I suppose could also be realised with a geared drive, if the mesh was left a little on the slack side.

I have already decided I will be going with a smaller chain drive system, these people do a 6mm wide series for very reasonable prices.

http://www.technobotsonline.com/index.php?dispatch=categories.view&category_id=193


Bogs

[John Stevenson]

Sounds a bit like the impulse magneto's where the drive pauses, winds a spring up and then lets go.

John S.

[madjackghengis]

Hi John, I just had to go through this build log again, what with a new posting, and was reminded of the rare earth magnets you were going to be putting in to test for low speed operation, I'm hoping you will be posting a similar video of the outcome of that, the Webster with your maggie is a very impressive display, in a large part because it is merely idling, which where mags are weakest, and show their faults, and yours runs perfectly at that speed.  I'm looking forward to what comes out of the neo magnet replacement, and hope to see some testing.  I've got a spark tester for cars, essentially a point with a clamp on it, with a plastic holder holding a screw with a point, and a clear cover, so you can adjust the distance the spark has to jump, and it's a good "seat of the pants" test of how much energy the spark actually carries, better than just a "fat" spark at the plug, or a "weak and skinny" one.  Just cleaning a working magneto, getting the brass contacts for the high voltage clean, and ensuring clean points and good gap can easily double the spark output and it is a repeatable test with this little cheap and easy bit of tool.  In any case, it was good to refresh my mind on your magggie build and the success you show is available for such engines.   Cheers, Jack

[Bogstandard]

You might be quite correct on that John, but if it does allow a larger or stronger spark to be produced, then advantage has to be taken of it. So a bit of a slack drive it is.


Jack,

I received my unit back today, and as promised, he has sent me a specially made sintered neo magnet, that looks to be either chrome or nickel plated to make it a little stronger, as they are renowned for being a little susceptible to shock loads. But because of the the size it has been made to means that a slight mod to the main frame and a couple of new parts will need to be made before it can be tested again, so that might take a bit of time before I can get around to it.


John

[narrowgauger]

Hi Bogs

noted that you were using some small roller chain.  Looking at the dimensions of the chain you are using it seems to me that this is still rather oversized and out of scale with the model.

as an alternative roller chain may I suggest the following miniature stainless steel chain from Precision Scale Model Engineering ( psme@psmescale.com ):

product code: MRC-1475 0.1475 (3.75mm) pitch roller chain
Price: approximately $US 3.75 per inch with a minimum of 6 inches per order.

there is also a good range of sprockets available from the same supplier.

I have been dealing with the PSME owner, Lawrence DeMilo for many years and find him a great supplier.

Hope this assists
have fun
Bernard

[Bogstandard]

Bernard,

Many thanks for taking the trouble to reply and attempting to assist me.

But in reality, 6mm chain and sprockets will be perfect for the job to come. The engine it will be fitted onto actually has two rather large 9" flywheels, unlike the very small engine that was shown in the video.

Thanks again


Bogs