In a message dated 98-10-07 02:45:33 EDT, egil@kvaleberg.no writes:
> On 7 Oct 1998, DANMAS@aol.com wrote:
>
> > It is normal for the smaller, underpowered, alternator on a TR6 to
> experience
> > a small voltage drop with all loads on.
>
> Small?
>
> If I remember correctly, we are talking of more than 1 volt below proper
> charging voltage, which is not what I would call small. For winter use,
> such a mediocre voltage means constant reliance on home charging for
> trouble-free starting at all temperatures and long battery life (a good
> battery should last 10 years, insufficient charging will half that).
Egil,
Obviously, you know a lot more about alternators than I do, so I won't argue
with you too much. I guess "small" is a relative term, but it has been my
observation that one volt is about the normal drop for a fully loaded Lucas
alternator (or any other low power unit, for that matter). I think your
description of the problems of winter use is just about what folks in the cold
north are seeing. A lot depends on the driving conditions, or the number of
times the engine is started. If it is started once in the morning, and ran
most of the day, even one volt over the battery voltage will still recharge
the battery during the day's travel, as long as excessive cranking is not
required to start the car. If it is hard to start, or started often and
driving only for short distances, yes, the battery will see a lot of home
charging. This is true of most any car/alternator system.
Perhaps a good battery will last 10 years, but then I've never had a good
battery. The most I've ever gotten, on any car, was 7 years. Normally, I get
around 5.
> The problem is inherent with the design of the built-in regulator. I would
> imagine the early, seperate regulator units to be better, but I've never
> tried them. The Lucas DC generator with relay-based regulator does
> definitely maintain the charging voltage over load much better then do the
> Lucas alternator! With the exception of when the engine is running at
> idle, of course.
> I should hasten to add that is not only Lucas alternators that have this
> problem, most brands of the 70s and 80s are just as bad (I'm not really
> familiar with 90s).
You may be right about the internal regulators vs the relay type, I have no
data to compare the two. I would think by now though, with all the advances in
solid state engineering, the newer units would be better than the relay type.
It may also be true that the generators maintained voltage better over their
load range, but it is also unfortunate that the load capability of the
generators was very limited. Adequate for most purposes with the limited
electrical loads of the time, but severly inadequate for modern cars.
> Not being able to leave things alone, I've replaced the poor built in
> regulator with a seperate, adjustable PI regulator measuring actual system
> charging voltage (PI in this case meaning Proportional/Integrating).
Very interesting! I assume this is a circuit of your own design? I've always
wondered why the designers chose to use a regulator circuit that emulated the
"on-off" action of the relay type, rather than designing an analog system,
such as your PI type. It seems to me that the technology was there, and the
integrated circuit manufacturing capability was sufficiently advanced for
this.
> With such a regulator, the Lucas 18 ACR alternator is able to keep
> correct charging voltage even with all loads applied (PI pump, heated rear
> window, headlights and extra driving lamps, fan and wipers).
According to the manuals, the 18 ACR was rated at 45 amps. Do you have an idea
how much yours wound up producing?
> Now enter the following tale of car electricals engineering: It turns out
> that the rectifier diodes have been dimensioned with the limitations of
> the built-in regulator in mind! With the increased output commanded by the
> new regulator, the diodes get so hot that over time the solder starts to
> melt! After having replaced the diodes once, before I understood what the
> problem really was, I resorted to installing proper Schottky rectifier
> diodes instead. They have the additional benefit of increasing useful
> alternator output by an additional 50W due to significantly lower losses.
I'm not at all surprised at your first two statements. Did the Schottky diodes
fit without a lot of modification? 50 watts is not a lot, but every little bit
helps. They would be of benefit even without the new regulator.
> Of course, fitting a big GM unit is probably more inexpensive and simple,
> but I'd just wanted to say that you it is in fact possible to have
> satisfactory charging with a Lucas unit (much modified, admittedly).
Inexpensive and simple is definately an attraction! Near universal
availability (at least in the US) is another. On the other hand, there are a
lot of folks that would prefer to keep their cars as stock as possible. I
could see a very good market for modified Lucas units as you mentioned. Do you
have any plans to market either completed units, or kits for owners to
install? Is the external regulator you describe small enough to be hidden out
of site for these folks? I can see where you could get rich off this!
> A much simpler (and quite customary) approach to increasing poor
> alternator output is to mount a silicon diode in series with the
> regulator, increasing the charging voltage by 0.7 volts. Just make sure it
> doesn't bring the charging voltage sans load too high, though.
There's no doubt that a diode will increase the voltage output, but what will
it do to the maximum current capacity? I don't see how that could help the
overall charging capability of the alternator, when the applied load is in
excess of the capacity. If the applied load is not excessive, than the
standard alternator will be adequate.
> Ensuring first that the brushes are OK goes without saying - they do not
> last particularily long on the Lucas unit, but are cheap and easy to
> replace.
Certainly true.
Thanks for your input and information.
Dan Masters,
Alcoa, TN
'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
http://www.sky.net/~boballen/mg/Masters/index.html
'74 MGBGT---3000mile/year driver, original condition - slated for a V8 soon
'68 MGBGT---organ donor for the '74
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