alt.energy.homepower

On Fri, 14 Sep 2007 04:15:45 -0700, rip_t@ wrote:

>ive been building a minature genorator based on a small rc car engine
>and have almost finished, but before i start it up poroperly i have a
>couple of questions that worry me
>
>the engine is rather basic and has no govenor. but the load on the
>genorator will always be a switch mode power supply (either 15 in
>energiser nimh battery chargers which ive opend up and am pretty sure
>a smps or a laptop 12v car power supply.
>
>so how can i match the engine power to the load needed. becasue when
>charging the load will be 120w yet at full throttel the engine can put
>out 200-300w. my fear is that the speed will rise and so will voltage
>until it become too great for the smps to handel. is their any risk
>and how can i match the load, either for fear of damagign equipment or
>to save on fuel?

Man, I hope your construction techniques are better than your spelling and grammar!

This little generator

/Generator/

is of about the same architecture as you describe - a fixed throttle engine (see the
throttle setting nut on top of the carburetor) attached to a 3 phase permanent magnet
alternator driving a 60 hz inverter.

Speed control is effected by an ignition cutout rev limiter built into the electronic
control box. This thing cuts ignition sparks in a random fashion to hold the speed
constant.

The general procedure is this. Start the engine and rev it up using the manual
throttle nut to a point where it will accept load and the green light flickers. Apply
the load. Reduce the engine speed until the green light comes on solid, then
increase it until the light flickers. The flicker shows the operation of the rev
limiter.

If your engine is glow plug based then your easiest option is to replace the glow
plug with a spark plug and implement a similar scheme. Barring that, about your only
other reliable and efficient option is to connect up an R/C servo to the throttle and
drive it with some electronics that look at engine speed or alternator output. A
BASIC Stamp or PIC processor will do the job nicely. You could probably do it in the
analog domain with a dual 556 timer chip but the digital method would be so much
simpler and reliable.

If fuel efficiency isn't a major issue then you could design a variable energy dump
into your controller. Energy not used by the load is shunted to a dummy load
resistor, maintaining a constant load on the engine. This control scheme is
frequently used on micro-hydro electric plants where the "fuel" is free. Perhaps the
fuel penalty won't be too severe if you advance the throttle only enough to handle
the beginning of the charge cycle and then let the energy dump handle things as the
battery charges.

My preferred approach would be the servo/microprocessor route.

John
--
John De Armond
See my website for my current email address

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