Simon,
The cause is the difference in the way generators and alternators operate and
how their output is regulated.
A generator (or dynamo) produces voltage in a more or less linear flow
dependent on the speed of the armature.
For that reason it produces little energy at low speeds and increases rapidly
until the regulator circuit opens to limit
The output. In other words the voltage, and therefore the rate of charge for
the battery in amps, can vary all over the
Place depending on engine speed and electrical load. Because of that variance
an ammeter is useful for determining the
Condition of charge in the battery.
An alternator (or AC generator), on the other hand, because of it's 3 phase
design produces much more energy at a
Much lower rotor speed. The power curve is very steep and the voltage is
controlled by the use of a Zener diode which cuts
Power to the field to control output voltage at a constant point slightly
above the nominal battery voltage (13-13.8 volts.
Since the alternator voltage is applied constantly any power usage is
recovered almost instantaneously. An ammeter will register the change, but it
will be gone probably before you notice it. For that reason an ammeter (to
detect battery condition is pretty much useless. Most manufacturers, when
switching to alternators, also installed volt meters and called them battery
condition gauges.
Bill Lawrence
BN1 #554
> From: simon.lachlan@homecall.co.uk
> To: healeys@autox.team.net
> Date: Wed, 27 Feb 2013 16:22:40 +0000
> Subject: [Healeys] Ammeter query in a generator to alternator conversion
project.
>
> Another one for the guriest of the gurus....
>
> The car is a MkII BT7. (Negative earth, of course)
>
> So, let's do this stage by stage.
>
> 1) When the car ran with a generator, I installed an ammeter. I
> disconnected the brown wire that ran from A1 on the control box to the
> solenoid at both ends. I ran a new wire from the solenoid, through an
> ammeter and back to A1. This worked well.
>
> 2) I bought a Lucas ACR17 alternator on eBay and installed it. You
will
> all doubtless know these Lucas alternators. There are three terminals. The
> two large terminals are connected inside so that one can connect more than
> one heavy duty cable for whatever purpose. For reference I used this:-
> http://mgaguru.com/mgtech/electric/ac101.htm
>
> I have to laugh when I see his advice to just "physically mount the
> alternator". (Almost as funny as the reference in the BMC manual to "gently
> easing the gearbox rearwards". Me and Superman.)
>
> 3) I bought an old control box on eBay and gutted it. I soldered a
> large wire across terminals A & AI inside this control box. I then used the
> control box as a junction box per the site's fourth diagram. (At this
stage,
> remember that the brown wire is still cut at A1 and the solenoid).
>
> 4) Per the MGAgurus suggestion (safety first) I did install an
> additional heavy cable from the alternator to the solenoid.
>
> 5) I switched on and everything worked fine - I think - except the
> ammeter needle which twitched about feebly. The dash charging light behaved
> as it should.
>
> 6) So, I suspect that the ammeter is wrongly hooked up.
>
> I suggest that I need to link both big wires from the ammeter together at
> the solenoid, run a cable from the solenoid through the ammeter and back to
> the joined terminals A & A1.
>
> I think that the brown cable remains disconnected, but that I could connect
> it to the first feed from the alternator and use it to run the connection
> down to the other feed at the solenoid.
>
> Am I right?
>
> I have no particular area of expertise in cars, but electrics would be at
> the bottom of the list. And I don't want to wreck everything.
>
> I would be most grateful for some help,
>
> Thank you.
>
> Simon
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