Alternator.

Alternators are nearly always higher output than the standard dynamo of old, but more to the point give a useable output at low engine revs - even idle. A dynamo if geared to do this would fail at high revs as the commutator couldn't cope.

Good question. There are somethings I will change, under the bonnet is mine, K&N instead of oil bath, vertical generic fuel pump instead of a SU, Webber not a Solex carb. Outside is is original, well as original as you can get for a vehicle with nearly 50 years of bodges, dents and scratches.

Changing the polarity just seams wrong, it is an intrigal part of what makes MY car better (IMHO) ,different, mine than the same type of vehicle parked next to it. My system of keeping a second battery on charge for when I need it works fine (one more pleasure of owning an old vehicle !) but , being basicly lazy, if I could get away with out doing it I would.

Not logical , sensable or mechanicly sound but then neather is running a 50 year old posative earth vehicle for fun.

Not IME.

I played around with car electrics in the 60's and 70's when I was involved in rallying. The best reliable output you could expect from a dynamo was around 17A. The weediest alternators then were putting out 25A, with the average at 35A.

Alternator failures were very common, and people in the trade were not prepared to try to fix them. I found that most failures were physically broken diode packs, and changed lots of them!

Chris

So it's not original given the K& N etc, it's not logical, sensible or mechanically sound, it seems you're doing it just to piss off the 'elders'. Aren't you just cutting off your nose to spite your face?

Steve

I'd have thought all those changes obvious to any enthusiast, whereas a polarity change difficult to detect if done carefully. And would allow such things as electronic ignition which is of far more benefit than a K&N filter. ;-)

If I had to, my approach would be to get an alternator with a separate regulator - early Rover P6 3500 had them, and Jaguars of that period too. IIRC, 45 amp output. The regulator had discrete components and a circuit diagram is available, and it would be quite easy to change the polarity. The rectifier pack is easily removable - but I'm not sure how easy it would be to modify without seeing one.

It was also a damn good alternator - much better than the later replacement battery sensing one with built in regulator.

The standard Lucas dynamo fitted to most small/medium cars was specced at

28 amps. However, if you ran it at peak output for long - as perhaps in a rally car with lots of lights - it would soon burn out. And high revs makes things worse. It was common to change the gearing for racing purposes.

Yep. Hence the "reliable" in my post! I never managed to get one anywhere near to 25A. I drove a Mini Cooper S in road rallies. When I stopped at controls I had to heel and toe to keep the revs high enough to avoid the needle of the ammeter bending against the discharge stop!

That would have been with up to six 100W halogen lamps on the front however...

Chris

So actually, it's been bastardised even more than my Capri has and you're picky about the earthing?

Rory ( snipped-for-privacy@griffon65.freeserve.co.uk) gurgled happily, sounding much like they were saying :

"the normal thing and change it"... to an alternator, y'mean?

There's a good reason that people go through the hassle of a change to -ve with an upgrade to alternator.

Guy King ( snipped-for-privacy@zetnet.co.uk) gurgled happily, sounding much like they were saying :

It's an ancient Landy, ffs. You've got to thrape the living crap out of it to get it to move at more than a snail's pace.

Snip

I have electronic ignition.

Thanks for the info on altenator types I will look out for one on my travels.

Rory

I have just found my old Lucas training centre manual. Looks like you are looking for 10AC or 11AC with 4TR control box and 3AW for the warning light, these are positive earth. I have wiring diagrams should you need them. 10AC is 35A 11AC is 43A nominal output.

HTH

Dave

PC Paul used his keyboard to write :

The voltage from the alternator is nearer to 120v ac.

Are you sure? Even if that's P-P voltage it's still 42V RMS so that leaves the regulator a hell of a lot to drop to get down to 14.4V for charging.

Where does the 120V figure come from? I know electronics but not particularly the innards of car alternators, so you may well be right...

I to found it to be quite a surprise. It came from an item I read upon the alternative uses for car alternators. Remove all of the electronic gubbins from a modern alternator and they make fairly capable electric welders.

Aha, a bit more research, and I see now. The reason alternators work evenly well over a huge rev range compared to dynamos is that they use coils everywhere instead of having permanent magnets. This means that by controlling the (low) current flow in the exciter coils the overall output is regulated - so the regulator isn't dumping excess power, it's reducing the exciter coil field strength so less power is generated in the first place. Clever.

An unregulated alternator will build up to 120Vish at high RPM. But when it's being used for 14.4V car battery charging it doesn't get anywhere near that high internally, it regulates it down first, making the alternator easier to turn and hence making the overall system more efficient than with a dynamo.

A day when you learn something is a day not wasted... so I'm happy now :-)

The message from Adrian contains these words:

Ah, yes, my brother's got one of those. I'd forgotten just how sluggish they could be. I learned to drive in one when I was 12 - the local doctor wanted me to drive so he could sit on the front and shoot rabbis [1] on his bird reserve.

[1] Ooops!

In this respect, alternators are no different from the generators they replaced. Both use regulated field coils instead of permanent magnets as a means of controlling output.

In an alternator the field coils spin, and the AC generated in the stator is rectified by diodes. In the earlier generator the field coils are fixed and the armature rotates inside, with rectification to DC achieved _mechanically_ by use of a commumator and brushes.

It's the use of an armature and communator which limits both rotational speed and maximum current handling (it needs to pass full output current through the brushes). Such a generator uses a regulator which is both a voltage and current limiting device, whereas an alternator uses just a voltage regulator.

If the generator used permanent magnets, this might be the case.

John

I really do know how to spell "commutator" when I stop and think about it :)

John