----- Original Message -----
From: "J. Forster" <jfor@onemain.com>
To: "Military Vehicles Mailing List" <mil-veh@mil-veh.org>
Sent: Wednesday, October 31, 2001 5:32 AM
Subject: Re: [MV] Vehicle/Trailer Mounted Generators
>
>
> Joe Foley wrote:
>
> > Also, with one of these generators you have to watch the load as it
changes
> > the engine speed which changes
> > the output frequency, then you have to adjust the governor to match.
This may
> > be a problem for radio
> > equipment or computer stuff.
>
> Most equipment, including radio and computer gear, does not care much
about
> line frequency (50 to 70 HZ is usually OK), but can be damaged by wide
voltage
> swings. I assume, but do not know, if these puppies have voltage
regulators. Are
> you sure the voltage regs are working, BTW?
>
You do have to be careful about several things.
Expect a shift in frequency, even a good engine governor will necessarily
have some hysterisis and probably a shift in output frequency by a few hertz
from no load to full load.
Domestic generators, even of reasonable size, more than likely have a simple
self-excitation method, this produces acceptable voltage stabilisation
although not what you'd expect from a commercial supply, worse, the output
is usually a quite poor sine wave that becomes worse with load and any
reactance like transformers or power factor corrected items. (fluorescent
lights).
Things with switch mode power supplies, like computers, don't worry as they
will take anything AC from typically 90V - 300V and 40Hz - 600Hz. Things
designed for true sine-wave mains input will possibly not work, or have
severely reduced output, including battery chargers.
I expect all of us with AC voltmeters have mean reading devices since the
true RMS meters are delicate things for lab use only, only on a pure sine
wave does mean = RMS so you can fully expect your meter to lie measuring a
typical generator.
The lower end generators are made this way for cost and low maintenance as
this novel self-excitation method doesn't need slip rings and a stable DC
supply for excitation, slightly flickering lights are a symptom of this
method but they're fine for resistive loads and commutated power tools as
intended.
The mil generators may well have proper excitation methods and are
continuously rated at full loads, the necessary maintenance to slip rings is
not a factor to the mil and nor is high manufacturing costs to good
standards. The Onans we see here are very large devices for just 6kW but
will likely run for ever, both Onan and Powerlite have a UK agent who is
immensely helpful to the private owner.
Apart from the larger, high-end domestic generators, the only smaller
devices with controlled and pure sine wave outputs are the smaller Honda's
(something with i in its title) with electronic AC generation using UPS
technology where the engine/generator just supplies rough "watts" to the
electronics package, there is not only no governing per se but the engine is
actually revved up with a stepper motor controlled throttle above about 1/3
power to make more "watts" as demanded by the electronics. These are very
quiet and very frugal at lower powers as the engine is just running at a
fast idle. They will stagger though with sudden huge loads since there has
to be a short delay whilst the engine revs to meet the demand, auto-switched
AC motors are a perennial problem.
Richard
Southampton - England
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