An engineering oddity!

[ScottyUK]

I was recently given the dismantled remains of an ancient magneto by one of our electricians, informed that it had belonged to his grandfather, and asked if I could make it work again. Stupidly I agreed to have a go.

I'll try to describe it as I see it - if I use any non-magneto terminology, then please bear with me as it was built at least 40 years before I was born.

The item is housed in a mahogany box with a glass viewing window in the lid (more later). Two electrodes are brought out, and there is a hole for a crank handle to be connected to the mechanism.

Internally, the unit has a pair of bobbin coils of a few hundred turns of cotton-covered wire, each wound on a steel former approx. 45mm dia. x 45mm long . The bobbins are mounted parallel to each other on a steel yoke approx. 60mm long. The yoke is carried on a split shaft on which the whole assembly rotates in insulated (wooden!) bearings.

The two bobbins are connected in series, and the outer ends of the windings are brought out to either side of the split shaft carrying the assembly. Spring contacts take the output of the winding to the external connections. A simple switch shorts the coil twice per revolution.

The bobbins rotate in the field of a large, though not especially powerful, horseshoe magnet, forming a closed magnetic circuit through the bobbins and yoke twice per rotation. Obviously as the yoke rotates the polarity of the magnetic field relative to the bobbin reverses. The magnet has a sliding keeper bar, which drives a dial indicator showing 0-100% through the viewing window. I guess this effectively weakens the magnet as it moves closer to the pole faces.

The bobbin assembly is rotated by a hand crank and geared drive and then pulley system, giving a ratio of about 30:1 step-up. The fastest I think I have managed to get is about 3000rpm, and I got the glorious output of 3.2V at 120Hz or so.

So... my questions:

What is the reasonable output voltage that I might expect from this magneto? I can see that the voltage is generated by the L.di/dt as the switch makes and breaks current.

Am I correct in thinking that the contact timing and duration are critical to how this thing works? How to adjust them, other than trial and error?

What on earth would you use one for? Someone in the office suggested that 'electric shock therapy' used to be fashionable. Guess I missed out! Any other ideas?

 

[electricpete]

I guess there are not many old-timers left to answer your question.

(The scary thing...We are becoming the old-timers.)

 

[buzzp]

You know I did see something that looks like what your describing on a discovery channel show or something. What they were using had a hand crank and it was used to shock people into being "healthy". It might have been the Ripleys Believe it or Not show. I can not remember for sure. Sounds like something fun to work on.

 

[pablo02]

To echo electricpete, we are becoming the old-timers...

My father still has all of his books (and some of my grandfathers) -- I'll look through them to see if anything applies...

 

[jOmega]

Take a look at

http://www.howstuffworks.com/question375.htm

There is a pretty good description with pictures of a magneto .. that I think will answer some of your questions, Scotty.

Best wishes for the new year,

j

&#937

 

[ScottyUK]

Happy New Year to all of you.

Thanks for the responses - much appreciated.

I found a reference on E-Bay in the medical antiques section, which bears out the theory about 'electric shock therapy'.

I'm now wondering if my Fluke 77 is averaging out some very short high voltage pulses to give me the low average reading I'm seeing. I'll take it to work and use a 'scope to see what is really being generated, or maybe get the girlfriend to hold the terminals while I crank it... if she yelps, there's more than 3V present!

A lead from jO's link indicates my assumption about timing is very important. The coils generate a sinusoidal output current which is shorted though the switch. In order to get any significant voltage out, the switch must open when the current is at a peak. The trick is obviously to find this particular point in the rotation.

Thanks again for everyone's interest.

 

[ScottyUK]

An update!

I've re-magnetised the magnet using about a hundred turns of 2.5mm^2 cable wrapped around it, then shorting the two ends across a truck battery for a few seconds until it got hot. Crude, but the magnet is much stronger!

After a bit of fiddling with the contacts, it certainly passes the yelping girlfriend test (she suggested that I had been born out of wedlock!) and the 'scope shows it produces pulses of about 150V at low speed, up to about 300V at higher speed. The pulse duration is pretty short - a few microseconds I think - but I couldn't get a decent trigger for the analogue 'scope to be any more accurate.

Regards to all.

 

[jbartos]

Suggestion: IEEE Std 100-2000 "Dictionary" defines Magneto as Magnetoelectric Generator (Electric Installation on Shipboards). It is an electric generator, in which the magnetic flux is provided by one or more permanent magnets.

 

[jbartos]

Suggestion: The modern magnetos are more advanced. Visit

http://www.morrismagneto.com/

for: World's fastest motorcycle ignition
Be carefull around Harleys!

 

[UKpete]

Scotty, are you in the IEE? I was browsing their on-line library and noticed the following title:
Magneto & Dynamo-Electric Machines, Higgs P (1884).
Yes, 1884!
There are some gems available in the free postal service, if you aren't aware of it:

http://www2.iee.org/Catalogue/simple-search.html

(type in author, title or search terms)

BR
Peter

 

[enginesrus]

If there are mechanical breaker points that switch the primary circuit. They would just start to open as the magnet moves between the poles. And you ask what kinda voltage to expect. Most that I know of produce in the 20 to 50 kv range.

 

[ScottyUK]

Hi Peter,

I'm in Teesside in the north-east of England, although since I'm a Geordie, 'the south' is everything below Gateshead on the map...

I wasn't aware of that service - I'll investigate it. Sounds really useful if they have documents going back over 100 years.

xr7755 -

Those unit must have a transformer in them to produce that sort of voltage. The simple unit I'm playing with produces the voltage directly from the breaking of primary current. I agree with your timing of the switch, although I think the exact position depends on the X/R ratio of the coils.

 

[enginesrus]

I'm an engine guy. I just know on engine magneto's
that opening postion for breaker points is called E-Gap.
E meaning efficiency. If it is opened out of time, the efficiency is compromised. Basicly you don't want any magnetic flux from the magnets in the core of the coils, when the field is colapsing.

 

[dede61]

Hi all,

Happy New year.

ScottyUK, maybe this link sheds some lights.

http://www.tepco.co.jp/rd/shiryokan-e/exhibition/edison-e.html

According youre explemation it sounds like an Edison generator, or a deviation of that.

I seen one in Germany last summer, that one was localy protected as a historical heritage, it was in good shape and in revers working state (for demonstration purposes only that is) but its great to see these old mashines.

regards,

Danny

 

[panelman]

Scotty

My contribution to the family gardening is to make sure the mower starts easily for my gf, a few years ago she couldn't start it and i popped my trusty old fluke 77 fault finding on the magneto

i was only spining it by hand but the output killed the meter, magnets and coils can produce some big voltages

 

[ScottyUK]

Danny,

Thanks for the link. It looks absolutely nothing like my machine, but it did get me back on Google. I've found something that is almost identical, with the exception that the sliding keeper bar and assoiated dial indicator are not fitted in the one shown.

http://www.agassiztrading.com/museum/electro-shock.htm

ps. I may have to revise the earlier statement that it was made "at least 40 years before I was born" to at least 100 years before I was born!

 

[dede61]

Hi Scotty, (scotty like Scottland ?)

I agree they look different, I haven't seen an electro-shock before. I also agree it must be at least like 100 years older then we where borm.
I have one book about almost 200 years before we where born (1782) subject "electricity" containing many oddities and what where lab tests (sorry for the Englisch speaking it was written in Dutch). These things are collectors items.

regards,

Danny

 

[jbartos]

Suggestion: There are some devices now that cure the nervousness, the magneto in the museum was meant for in the above ScottyUK link. See

http://www.ndelectronics.co.za/sir-acu.htm

for: Positive, Negative and Alternating Current treatment choices

 

[ScottyUK]

Hi Danny,

Scotland has only one 't'. My surname is Scott; I've been Scotty amongst friends and colleagues since I was a kid, I'm Jon to my girlfriend, and Jonathan to my mother on Sundays!