An Engineer in Wonderland – A car alternator surprise

I was asked to have a look at a car that was not charging properly yesterday.

alicealterator2.jpgI was asked to have a look at a car that was not charging properly yesterday.

It is a kit car based on a Renault 6.

When I measured the battery, it was charging at about 13.8V – with and without the lights on. This is a bit on the low side for a car, but acceptable.

As the owner said it had not been charging at all, I thought I would pull the alternator brushes, just in case they were worn.

So we took the thing off, a SEV Marchal unit, and dismantled the back end where the brushes normally are.

No brushes. 

There didn’t seem to be much room at the front end for brushes, but they had to be there.

So with much effort we got the pulley off and dismantled the front part.

Still no brushes.

I had an I-must-be-going-mad moment, then noticed that there was something stationary inside the rotor.

A brushless car alternator?

I had never heard of such a thing.

But it was.

The three-phase output stator is around the outside – as with all the alternators I have ever come across.

And the rotor is within this, but the claw poles only extend a little way in from each end, allowing the field coil to be supported from the external stator half way down the machine.

What a neat design. With nothing to wear except the bearings.

I am sure there must be disadvantages, or all car alternators would be made this way.

Suspicion is now on the external electromechanical regulator.

Anyone know of an external electronic regulator I could bolt on instead?

The field coil is grounded at one end, and therefore needs a positive feed.


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  1. I have discovered that early CB500 and 750 Honda motorcycles also had fixed field and output coils, with an unwound iron rotor between them.

  2. Interesting! I’ve never seen one of those either.
    Some early Lucas alternators had an external electronic regulator, TR4. If I remember right it could be connected to give +ve or -ve earth, and maybe either on the feed side of the field or the earth side, as on modern built-in regulators (all the ones I’ve seen, anyhow). I have data on this at home, I’ll check tonight and if it’s OK post some info tomorrowish.

  3. I too would guess its the same technology as the motorbike arrangement. These started appearing on bikes in the late 80’s, with control, efficiency and reliability advantages. Not sure about weight but may have been a factor before before newer rare earth permanent magnets came into the picture.
    The Kawasaki unit is known for its reliability and performance and is the one usually described in articles about bike electrics.

  4. Thanks for your comments.
    I don’t think it is a variable inductance generator as I am fairly sure these have no flux source in the centre, and need sophisticated control electronics.
    Although I am happy to be corrected here.
    As for the motorcycle type.
    In my experience, these are mostly permanent magnet rotor types.
    The regulators for these, as far as I know, are unique to this generator type and use thyristors or well-finned shunt regulators to short the stator outputs then the battery voltage gets too high.
    Permanent magnet generators are proposed as a way of increasing efficiency and cutting weight in cars.
    I don’t know if they have got there as all the cars I have anything to do with are a bit long in the tooth.
    The unusual kit-car alternator is still a field current controlled three phase generator, I think, but with the magnetic field source suspended in a rotating claw-pole armature.
    It therefore has an extra set of air gaps in the magnetic circuit and must therefore waste some field current.
    For those new to car alternators, the standard type has the field coil rotating as part of the claw-pole armature, fed through slip-rings and brushes.

  5. My 1991 Kawasaki bike has a 3 phase generator arangment, its really low profile and sits on the end of the big end oppsite the flywheel / clutch. Its a really well desiged arangment. For a regulator it may be worth looking at a device from a Bike.

  6. This sounds like a switched reluctance generator. Contact Switched Reluctance Drives, Harrogate North Yorkshire, it might be one of theirs.

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