mundungus · May 20, 2013

Bump said:
Yeah 24v connected to voltage regulator, connected to 12v battery would do it

No I didn't mean that. An alternator i sbasically a coil and a magnet, one of them spinning to produce an AC voltage. The voltage regulator inside the back of a normal alternator does two things. Firstly you've got a full wave rectifier (bunch of diodes) to turn the AC output to DC. But obviously that on its own gives you an output voltage that goes up with engine speed. So what they do is use an electromagnet instead of a fixed magnet. Then vary the magnetic field based on the output voltage (detected via the field wire) so as to ensure that as the output voltage wants to climb with engine speed, the magnetic field is reduced thereby bringing down the output voltage. Ensuring the usual 14volts output.

I'm not an expert, but if the voltage reg in a 24v unit for whatever circuit design reason caps itself because it doesn't see anything close to 24v on the field wire. It might fail but without catastrophic effect.

mundungus · May 20, 2013

Bump said:
Wire it up to the engine and stick a multimeter on the output?

Or... just rig up a motor to spin it to test it. (old starter motor or something?)

Agree with bump.

That's the only way I can think of doing it. Unless there's a part number somewhere that enables you to identify it.

diver · May 21, 2013

You could connect it up to the vehicle and run it up but you will need a battery on the output. Alternators don't like running without load. If its a 24v alternator it might do something very nasty to a 12v battery but if its a 24v then the exciter wire from your presumably 12v vehicle may not supply the right voltage to excite the alternator anyway.
Safest is to take it to your local auto electrical place and ask them to test it.
Failing that, connect the output to 2 12v batteries in series, connect the exciter terminal to the 24v supply via a 24v bulb of some sort, fire it up and see what voltage you get. If its a 24v then you should show about 27v across the batteries. If its a 12v alternator, then it won't have damaged anything.
Or, it might be knackered anyway and you haven't really proven anything. I'd get it tested.

fanatic · May 21, 2013

regulator part number

SmileySteve · May 21, 2013

Yep 24V syatems are just 2 X 12V batteries wired in series, but they MUST be the same rating ie both 75Ah or 110Ah whatever.

There should be a ratings plate on the Alternator that usually would state voltage and output rating ie 12V DC 45Ah etc.

Bump · May 21, 2013

mundungus said:
No I didn't mean that. An alternator i sbasically a coil and a magnet, one of them spinning to produce an AC voltage. The voltage regulator inside the back of a normal alternator does two things. Firstly you've got a full wave rectifier (bunch of diodes) to turn the AC output to DC. But obviously that on its own gives you an output voltage that goes up with engine speed. So what they do is use an electromagnet instead of a fixed magnet. Then vary the magnetic field based on the output voltage (detected via the field wire) so as to ensure that as the output voltage wants to climb with engine speed, the magnetic field is reduced thereby bringing down the output voltage. Ensuring the usual 14volts output.

I'm not an expert, but if the voltage reg in a 24v unit for whatever circuit design reason caps itself because it doesn't see anything close to 24v on the field wire. It might fail but without catastrophic effect.

Ah ok.

But... you could still use a secondary voltage regulator to charge a 12v battery...

DrDoVeryLittle · May 22, 2013

Just a gut feeling, but it looks to me like a 24V TUM HS Winterised / Waterproof alternator to me. I believe they also have a smaller pulley on to spin faster so you could try measuring that to see if theres a difference. 24V serpentine belts don't fit on 12V standard Defenders due to the pulley size difference.

gemsdad · May 22, 2013

mundungus said:
I think the lorries wire them in banks of two in series. Hopefully a lorry driver will be along soon to confirm or deny

correct a mundo.

mundungus

Active Member

mundungus

Active Member

diver

Active Member

fanatic

RESPECT IS EARNED, BEATINGS ARE FREE ;-)

SmileySteve

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Bump

Well-Known Member

DrDoVeryLittle

Member

gemsdad

NOT BOTHERED.

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