Date:         Tue, 05 Jul 94 18:59:29 CDT
Sender:       Vanagon Mailing List <vanagon@vanagon.com>
From:         Joel Walker <JWALKER@ua1vm.ua.edu>
Subject:      Re: alternators

On Tue, 5 Jul 94 17:29:24 CDT Alistair Bell said:
>Does there exist a table of the relationship between electrical
>load and voltage produced for a given rated alternator?
>Another example of idle curiosity.

aarrrrrrrrrrrrrgggggggggggggggggghhhhhhhhhhhhhhhh. i'll bet you guys used
to just bug the living hell outa your professors! :)

bosch gives tables of alternator current (produced) vs rotation speed (rpm),
but not vehicle electrical demand (that is what you wanted, right?) vs
voltage.

tidbits from bosch:
Figure 6 shows that in most cases the DC generator only delivers power above
the idling speed of the engine, while the alternator delivers approximately
1/3 of its rated power output even at idle.

<maybe this is what you are looking for>
Why is a voltage regulator necessary?  In our discussion of basic principles,
we have, so far, failed to deal with an important aspect of alternators in
motor vehicles: the voltage regulator.
The regulator has the task of keeping the generator voltage constant over the
entire engine-speed range irrespective of the load on the alternator or its
rotational speed.
Automatic regulation of the voltage is no easy task because of the variations
in the speed of the engine and the fluctuations in the connected loads are
considerable.  Depspite these constantly changing operating conditions, it
must be ensured that, at high engine speeds and with low electrical loading
due to loads, the voltage is limited to a set value.  This protects the loads
against over-voltage and prevents the battery from being over-charged. In
addition, the electro-chemical properties of the battery must be taken into
account when it is bening charged. Therefore, the charging voltage must be
higher in cold weather than in warm weather.  The voltage regulator allocated
to each alternator ensures that this requirement is complied with.  A
distinction is made between regulators for mounting directly on the alternator
and those for mounting on the vehicle body.
..................
Vehicle electrical system
The voltage level in the vehicle electrical system, and thus in the battery
charge, are also affected by the wiring between alternator, battery, and
electrical loads.  If all loads are connected on the battery side, the total
current in the charging cable is Ig = Ib + Iv. The charging voltage is lower
due to the high voltage drop.  If all loads are connected on the alternator
side, the voltage drop is less and the charging voltage is higher.  This
could be disadvantageous to loads (such as electronic circuitry) which are
sensitive to voltage peaks or high voltage ripples.  Those electrical devices
which feature high current draw and relative insensitivity to overvoltage
should therefore be connected at the alternator, and voltage-sensitive loads
with low current draw sould be connected at the battery.  Voltage drops can
be minimized by suitable conductor cross sections and good connections whose
resistance remains low even after a long period.
...................................
<just a little aside:>
Fuel consumption
A small portion of the fuel used by the vehicle is used to drive the
alternator and transport the combined weight of the starter, battery, and
alternator (approximaely 5% in a medium-sized car).  Average fuel consumption
per 100 km: For a weight of 10 kg, approx 0.1 liter; for 100 Watts of drive
power: approx. 0.1 liter.  Alternators with a high part-load efficiency
therefore contribute to fuel economy even though they are slightly heavier.
------------------------------------------------------------------------
<me again>
so ... i think the answer (sort of) to your  question (sort of) is something
like this:
the alternator isn't thought of as outputting 'voltage', but only current.
the voltage is stablized/cut-off by the regulator, regardless of the speed
of the engine/alternator.  the charts showed some rather startling differences
in alternator types, though, as relates to current output vs engine rpm.
a Bosch K1 alternator, for example, starts producting current at 1000 rpm,
produces 10 amps at 1200 rpm, by 2000 rpm, it's up to 20 amps, by 3000 rpm
it's putting out about 28 amps and begining to level off, 4000 rpm produces
only 32 amps, and 5000 rpm only 34 amps.

a Bosch T1 alternator, on the other hand, has a much steeper curve. it begins
to produce current at about 700 rpm, at 800 rpm it is producing 10 amps, at
1000 rpm it's producing 30 amps, 2000 rpm is 50 amps, and then it flattens
abruptly at 3000 rpm and 60 amps ... producing no greater current than 60 amps
for 4000 and 5000 rpm.

so which one do we have? beats the heck outa me. another table lists the T1
as 'buses', but i'm sure this is the big mutha greyhound-types. :) cars and
trucks are listed as a Type G1 (which is described as similar to K1). it
does say that a code is stamped on all alternators:
K1() 14V 35A 20
where K - housing outside diameter. G=100...109mm, K = 120...139mm,
                                    T=170...199mm, U = >200mm
      1 - claw-pole alternator (2 = single-pole alternator,
                                3 = generator with permeable rotor,
                                4 = claw-pole alternator with exciter)
      () - direction of rotation. (->) or R  = clockwise
           *as seen looking onto  (<-) or L  = counterclockwise
           the driven side of     (<>) or RL = either direction
           the alternator*
     14V - alternator voltage in volts (should NOT be confused with the
           nominal voltage <6v, 12v, etc.> which is used to identify starters
           and accessories which are operated at this voltage)
     35A - max current in amps
     20  - speed (rev/min) in hundreds at which 2/3 of max current is reached
           (i.e., 20 = 2000 rpm).

yadda yadda yadda. so what? well,i guess it means
(1) if you turn everything on (and have a lot of stuff that runs off 12v)
    while the car is idling, you'll run the battery down. hmmm. that pretty
    much cuts out the idea of idling all night with the a/c running!
(2) even if you are blasting down the highway at 4000 rpm, it's possible to
    have enough lights :) and wipers and radios and stereos and a/c and
    all sorts of other stuff that would suck up so much alternator current
    that your battery would never get any charge ... and would slowly die
    over a night run to Vegas. not likely that you (or i) would have ALL of
    that stuff running at the same time, but possible.

i guess what all this boils down to is that if you stick a lot of electrical
'consumer' gizmos on your bus, then you'd better get one more: a voltmeter.
or an ammeter. or both. so you can see when you are overloading the system.

yeah, i know: it's more than i wanted to know, too. :)

joel