miniature generator

 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?

incase it makes a ny diffrence im using a brushless plane motor so
effectivly a 3 phase motor which i rectife. how can these things be
so  powerful in such a small space for £20 i got a 1kw motor that is
about as big as a walnut. howcome i dont see these things and the also
teeny pwm speed controllers being used for other projects ect

any help would be greatly appreciated

 
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?

incase it makes a ny diffrence im using a brushless plane motor so
effectivly a 3 phase motor which i rectife. how can these things be
so  powerful in such a small space for £20 i got a 1kw motor that is
about as big as a walnut. howcome i dont see these things and the also
teeny pwm speed controllers being used for other projects ect

any help would be greatly appreciated


  I would think that an RC car engine would have some
simple preportional servo accuated throtle controls.  You
should be able to modify them to respond to your generator's
output instead of a radio receiver.  You might even have it
able to respond based on the battery level, for the charger.

  Just a first impression, from your general description.

  Luck;
      Ken

 
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?

incase it makes a ny diffrence im using a brushless plane motor so
effectivly a 3 phase motor which i rectife. how can these things be
so  powerful in such a small space for £20 i got a 1kw motor that is
about as big as a walnut. howcome i dont see these things and the also
teeny pwm speed controllers being used for other projects ect

any help would be greatly appreciated

Sounds like you want a governor like on a arc welder.
A combination of droop with a idle switch.
You can see both mechical control like on the old welders and electronic
control like on the new welders.
no load it goes to idle....strike an arc it goes to constant velicity with a
droop control like on tractor or a standard generator.
You looked at a helicopter RC engine control  since at least on the full
size and drone turbine helicopters engines, you are running constant
velicity with a droop.There is also an idle position.I would think the RC
engine would be straight electronic like on the arc welder more so than
 the complex system used on turbines(They are a combination of electronic
/hydro-pnuematic.Because of all the stuff beyond speed control that
you are needing......acceleration ,pressure,etc. Like an old Woodard control
used on both steam and gas turbines.)










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 Small engines are usually less efficient than bigger for some reason
or another... otherwise I guess we'd have lots of small engines in
ships instead of one ot two big ones. Anyway, I think you should get a
4 stroke model airplane engine, which should have better specific fuel
consumption than a 2 cycle of the same displacement, and set it up on
a bench with a fuel flow meter and an alternator with a rheostat on
the field coil, and find the 'sweet spot' in the rpm and alternator
load setting that minimizes fuel consumption. Once you know this exact
volts and amps in the alternator, you can get a BLDC kit and put on a
custom winding to match that exact setting. If the load varied more or
less than this tuned setting, the effciency would drop on either
side... careful measurement would show if the power used to excite the
field coils in the alternator (a dozen watts?) could be tolerated in
order to gain the advantage of matching a varying electrical (and thus
mechanical) load. Since I'm a programmer, I think this would be a
great job for a $5 microcontroller, but you might be able to find a
nice young man from Shanghai that would adjust the knob whenever the
generator was running for  whatever the prevailing wage is over there.
Couple bucks a day?

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


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

This little generator

http://www.neon-john.com/Generator/generator.jpg

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
http://www.neon-john.com
http://www.johndearmond.com  <-- best little blog on the net!
Tellico Plains, Occupied TN
What do you call 4 Blondes in an Abrams?  Air Tank.