-----Original Message-----
From: On Behalf Of Patrick Delgado
Sent: Monday, July 28, 2003 8:49 PM
To: dakota-truck@dakota-truck.net
Subject: Re: DML: Generator question
<The faster it spins, the more amps it puts out. So max would be around
redline. However, it's probably capable of 40-50 amps at cruise rpm's(~2500)
Patrick Delgado>
Sorry guys, I'll have to weigh in here with a dissent.
First, some history.
The voltage build-up curve on automotive DC generators meant that they did
not put out full voltage at idle. As rpm's increase, the voltage output goes
up.
Quoting from "Automotive Electronics" Graf and Whalen. Published by Sams (My
copy is the 1971 edition)
"A good part of the lower rpm range of the generator is essentially wasted
in building up sufficient field excitation in the pole pieces to produce
slight voltage output."
They also required a mechanical regulator to keep the output voltage from
rising too high. A runaway generator could boil a battery dry in less than a
day. Another drawback is that the output of the generator comes from the
armature, the part that rotates, meaning that it has to be constructed with
heavy gauge wire and have heavy bearings, making the entire assembly very
heavy.
At idle, the load on the generator; lights, radio, wipers, heater fan could
result in low voltage and very dim lights and most importantly, no charge
going to the battery. In fact the battery would be supplying part of the
vehicle load.
This was the situation for the late 30's, 40's, 50's, and early 60's.
In the mid 60's, the increase in current demand; more lights, power seats,
and air conditioners with electromagnetic clutches made adequately sized DC
generators too costly.
The high output alternators in police and other emergency vehicles in the
50's used bulky and expensive selenium rectifiers.
The breakthrough came with the availability of silicon diode rectifiers. The
output now comes from the stator of the alternator with the armature
producing the field. Modern alternators include the rectifiers in the
housing and the output voltage is controlled by the computer.
Now, about the current.
Current doesn't ramp up with rpm. Current only increases as more load is put
on the unit.
At idle, an alternator should be capable of reaching close to its maximum
amp rating. Turn on more accessories, amps go up. Turn them off and the amps
drop, regardless of rpm.
Turn on every light, including the off-road "mini-suns", switch the fan to
the highest speed, add air-conditioning, thumping stereo, wipers, run the
power windows up and down with dirt in the tracks, and eventually the
alternator will be at full output. Beyond this point, the voltage will begin
to fall off and the battery will discharge to supply the load.
One more thing; electric fans don't provide free cooling. The bigger the fan
motor, the more horse power needed from the engine to turn the alternator.
Listen for the rpm drop when jump starting another vehicle. That's
electrical load being transferred to horse power load on the engine.
The best strategy is to turn off most of the non-essential loads under
jump-starting conditions.
This archive was generated by hypermail 2b29 : Fri Feb 06 2004 - 11:46:40 EST