A7 alternators again....

[Stuart Giles]

Following on from this 'vintage' thread from last year: https://www.austinsevenfriends.co.uk/for...p?tid=6540  On and off (mostly off in between other 'must do' workshop stuff) I have been working on a replacement for a friend's 12v Dynamator which has not been inspiring confidence after I needed to repair it for him last year.

My basic idea was to use a small permanent magnet alternator as fitted to lorry refrigeration units and some garden tractors. They produce an output to charge a 12V battery at up to 14A; low by modern alternator standards, but still a more than adequate 168W output Vs. the 55ish Watts of the A7 unit.

Here's what these alternators look like 'out of the box' the gold coloured drum contains some powerful permanent magnets which rotate around the stator coils in order to provide a single phase  AC output which is connected to an external regulator.

 

The main components:



Machining the stator casting to locate the A7 distributor drive housing.



Assembled on the re-machined Lucas dynamo shaft along with the permanent magnet rotor.



Finished: 

[Ivor Hawkins]

Now that looks neat, excuse my electro dyslexia but would that work with a six volt system?

[Bob Culver]

I gather "modern" motor bikes have pm alternators. Am not clear on how regulated. But apparently can cook themselves on low load.
Edit. On brushing up it seems the regulator dumps output. Dont know what load is maintained on the gears but a constant load well above original max may tax the gears. I have seen arrangements for extra lube. Because of the sliding action, some books recommend low load running in of cross gears

[Charles P]

Nice work.
I have sketched out an approach that puts the revolving magnet inside a slightly wider "vestigial" dynamo body, disguising the gubbins in a VSCC compliant way. It's a matter of eeking out every small fraction of an inch.
One year I may actually do it.

Charles

[andrew34ruby]

I guess it could be regulated to 6v but think it would still produce the same maximum current so that would be half the power. How is it regulated?

The 168 watt output is hardly enough anyway. A pair of tungsten headlights and there's nothing left to power everything else. It's still more than my original dynamo regulated to 12 volts, 120 watts, but if you are going to all that trouble and work it would be nice to think it could power everything.

[Stuart Giles]

The 168 watt output is hardly enough anyway. A pair of tungsten headlights and there's nothing left to power everything else.

They must be impressive headlights you're using? The Quartz Halogen headlamps on my special are 60W main beam 55W dip. So 120W for the pair. Add side & tail lights 20W and for a tourer or saloon a wiper motor @ perhaps 30W. That's pretty much the 168W output of the little alty. You may continue to be fortunate with your 60W A7 dynamo pushing out 120W as a 12V dynamo, but I have pretty much lost count of the ones I have seen where they have flung the solder off the armature commutator joints. The spindle I used in this conversion was one of those.

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Now that looks neat, excuse my electro dyslexia but would that work with a six volt system?

well........... You could run it at 6V but it would give the regulator a really hard time. The main component in the regulator for a permanent magnet alty is a thing called a Zener Diode. Basically these have a set Zener Voltage 14.5 ish volts for a 12v system, or 7.8 ish volts for a 6 volt system. When the incoming voltage exceeds whatever the Zener voltage happens to be, the Zener diode sends it to earth, so immediately the voltage will drop below the Zener voltage and charge gets forwarded to the battery. Then when the voltage rises, once again it's earthed by the Zener diode. You can see why all the permanent magnet alty regulators have fins on them, have instructions that they should be in free air and/or should be bolted to a metal surface etc. It's because they are sinking power. Giving the alty setup above a quick test last night, the alty was pushing out almost 15V AC at not especially high revs; a regulator for a 6V system would function with this alty, but it would be working really hard and I think may well not live for very long.

["Slack Alice" Simon]

Put a 2:1 transformer in somewhere?

[andrew34ruby]

The 168 watt output is hardly enough anyway. A pair of tungsten headlights and there's nothing left to power everything else.

They must be impressive headlights you're using? The Quartz Halogen headlamps on my special are 60W main beam 55W dip. So 120W for the pair.  

No Stuart my headlights are not impressive, it's just that my brain is even less impressive. We all know that  2 x 60  = 180, don't we?

I'm down with covid, feeling much better, but the brain is still struggling.

[Stuart Giles]

The 168 watt output is hardly enough anyway. A pair of tungsten headlights and there's nothing left to power everything else.

They must be impressive headlights you're using? The Quartz Halogen headlamps on my special are 60W main beam 55W dip. So 120W for the pair.  

I'm down with covid, feeling much better, but the brain is still struggling.

Always better to be down in the garage or down in the workshop than down with Covid. Hope you get well soon

[Ivor Hawkins]

---

Now that looks neat, excuse my electro dyslexia but would that work with a six volt system?

well........... You could run it at 6V but it would give the regulator a really hard time. The main component in the regulator for a permanent magnet alty is a thing called a Zener Diode. Basically these have a set Zener Voltage 14.5 ish volts for a 12v system, or 7.8 ish volts for a 6 volt system. When the incoming voltage exceeds whatever the Zener voltage happens to be, the Zener diode sends it to earth,  so immediately the voltage will drop below the Zener voltage and charge gets forwarded to the battery. Then when the voltage rises, once again it's earthed by the Zener diode. You can see why all the permanent magnet alty regulators have fins on them, have instructions that they should be in free air and/or should be bolted to a metal surface etc. It's because they are sinking power. Giving the alty setup above a quick test last night, the alty was pushing out almost 15V AC at not especially high revs; a regulator for a 6V system would function with this alty, but it would be working really hard and I think may well not live for very long.

Thanks Stuart, I seem to remember a Zenor diode on the front of my old BSA motor cycle, so it must be established technology which works...rather than bugger up my regulator I’ll stick with the standard dynamo for the time being, but I like the conversion...