[Nut-upsuser] Slightly OT: Generators & UPS

[Charles Lepple]

On 9/1/06, Julian Stacey <jhs at berklix.org> wrote:
> "Basheer Noorgat" wrote:
> > Hi,
> >
> > Can anyone shed some light on this:
> >
> > My PC's are on a separate power circuit. Each PC has its own UPS.
> >
> > I have an old Honda generator. When the power goes out, I fire up the
> > generator. However, the UPS's still run on battery. If I connect the
> > PC directly to the socket and bypass the UPS, the PC works.
> >
> > I was told by a UPS supplier, that the power output from the
> > generator was "dirty" hence the UPS's preferred to remain on battery
> > power.
> >
> > Recently, the power went out and fired up the generator.
> > Surprisingly, all UPS worked and accepted the generator power.
> >
> > This time the only difference was that we also powered off some
> > halogen spotlights and some energy saving compact flourescent
> > lighting from the generator.
> >
> > Can anyone shed some light on what happened?
> >
> > When I get a chance, I will try and repeat what happened and see the
> > outcome...
>
> I guess the Halogen lights are a straight resisitive load ?  (ie
> no complex electronics in the back, although light bulbs somehow
> seem to get inductive )

Incandescent light bulb fillaments are coiled, which would probably
account for the inductance you're talking about. However, it is fairly
minimal (given that the "inductor" has an air core, and most
intentional inductors use ferrite cores).

> Thr traditional simple way mechanical way to iron out noise from a
> generator is to feed it into a motor witha big flywheel, & couple
> a generator to the flywheel.

Funny, because the original poster mentioned that the (suspected)
dirty power was coming from a generator... I have some perpetual
motion machines in my shed; maybe I should dust them off and try to
sell them <grin>

For generators and UPSes, I would advise staying in the middle of the
load range. Overloading is obviously bad, but low load conditions can
be problematic as well - the voltage control loop may not be stable.

If you have ever driven a car with a manual transmission, you probably
have already seen this. As you slow down with the car in gear and the
clutch engaged, the engine works well until the RPMs approach idle.
Below idle, the engine runs rough, if at all. At this point, you would
usually put in the clutch. If you have a lot of weight in your car,
the inertia can keep the engine turning, but there is a speed below
which you will not be able to speed up again without downshifting
and/or slipping the clutch.

It's not a perfect analogy, but in both cases, you need more load to
smooth the system operation.

As a point of reference, if I plug a 60W incandescent light bulb into
an entry-level 500W Tripp-Lite UPS on battery power, it flickers
visibly until I add more load (light bulbs, or a computer). How much
of this is attributable to low inductance versus low load is up for
grabs, but with the push for unity power factor on PC power supplies,
I would lean towards "low load".

-- 
- Charles Lepple