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Wood gas electronic governor controll

Discussion in 'Renewable Energy' started by Patricktj, Aug 10, 2013.

  1. Patricktj

    Patricktj New Member

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    Hi all
    I need some help please.
    I'm a mechanic, so my electronics is very limited.
    I have a Chevy 4.1 6 cyl running on wood gas, I need to build a governor to control engine speed to keep the alternator at 50 Hz.
    The input will be the same as a rev tachometer.
    It will be coming from the (-) pole of the coil,
    The engine will be running at 2000 rpm so there will be 3 cylinders firing per rev.
    So 2000 x 3 = 6000 pulses per min
    = 100 pulses per sec
    = 100 Hz

    The power will be 12 v dc
    I need to control the throttle butterfly with a servo of some kind.
    I need to make it adjustable.

    Here is a youtube vid of the engine running.
    https://www.youtube.com/watch?v=J0A3XZGO9oo


    Thanks
    Patrick
     
  2. alec_t

    alec_t Well-Known Member Most Helpful Member

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    Welcome to the forum.
    You could use a first monostable circuit triggered by the coil pulses, smoothe the mono output to provide a voltage linearly related to the pulse frequency (hence rpm), scale/offset that voltage with an op-amp, use the scaled/offset voltage to control the width (in the range 1ms-2ms) of pulses at ~50Hz repetition rate from a second mono, and use the output from the second mono to control a standard heavy-duty modeller's servo.
     
  3. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    Hi Patrick, can you get hold of an automotive cruise-control servo?

    Will the engine make enough vacuum to operate the servo? (It might not, if the engine is running WOT)?
     
  4. dave

    Dave New Member

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  5. tcmtech

    tcmtech Well-Known Member Most Helpful Member

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    If it was me I would just use a common mechanical belt driven governor. They have been used on millions of gensets and are easy to set up.

    Odds are you could pull a used one off a piece of old farm machinery at a salvage yard for under $50.
     
  6. shortbus=

    shortbus= Well-Known Member

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    Cruise controls haven't used vacuum motors since the late 1970's or early 1980's. They are electronic servos.
     
  7. Patricktj

    Patricktj New Member

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    Hi Alec_t all you said just went way over my head. Could you draw a circuit diagram that I could get some one to build for me ?
    Thanks

    Mikemi the lag time on a cruise control system is to great and the engine would hunt terribly, it was one of my first thoughts but when I asked about it that's what the experts said.

    Tcmtech I like the mechanical governor system, it's just hard to come by in South Africa. I haven't seen one.

    Thanks for your help guys
    Will keep you posted If I fined a mechanical one.
    Otherwise Alec if you could do a circuit diagram that I could have built then at lease I have a direction to go in.

    Thanks patrick
     
  8. alec_t

    alec_t Well-Known Member Most Helpful Member

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    How accurately do you want to control the speed? 5% , 1% ......tolerance? How rapidly does the engine respond to throttle adjustment?
    The monostable system I suggested would need > 1 sec to smoothe the output pulses, so might not be fast enough if you want tight control of speed and a rapid response. A digital solution may be preferable (e.g. using a programmed micro).
     
  9. Patricktj

    Patricktj New Member

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    Hi Alec
    It has to control the speed to 1%, to hold the alternator at 50 Hz and do it very quickly.
    What can you suggest ?
    Thanks
    Patrick
     
  10. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi Patrick,

    Does the engine have a flywheel on the output shaft.? If so what mass is it.?

    A 1% speed control will be difficult to maintain, especially if your alternator/generator load is being switched.

    Why do you need 1% on 50Hz, many domestic devices will accept some small drift in frequency.

    E
     
  11. Mr RB

    Mr RB Well-Known Member

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    You have little chance of holding 1% with a crude speed controller and something as unpredictable as wood gas (which has a randomish amount of fuel per cubic foot, depending on the wood temperature, and which piece of wood just outgassed/burned).

    There's no way of doing it "quickly". Your best bet is to improve the time constants of the system, ie stabilise and slow the output time constant with a decent sized flywheel on the motor output shaft, and stabilise and slow the input time constant by homogenising the wood gas in an accumulator so it is evenly mixed and reliable in fuel content per cubic foot, at least in the short term. Once you have smoothed those two factors you might have some chance of holding 1% speed, provided your generator load remains fairly constant.
     
  12. alec_t

    alec_t Well-Known Member Most Helpful Member

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    I can't suggest anything to control it 'very quickly' :(. I think it would take several seconds to stabilise the speed of a 4.1L 6 cyl engine in response to a sudden load change on the alternator.
     
  13. Mr RB

    Mr RB Well-Known Member

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    A gasoline engine is not quite that slow. :)

    I've done the electronics for a couple of closed-loop engine dynamometers (for racing use) and engines respond reasonably quickly once the throttle vane has been moved. Maybe 50-100mS for the new increased fuel/air charge to hit the cylinders, maybe another X mS (depends on RPM) for the charge to hit all cylinders (to produce max shaft torque) then there is only a short time for the inertia of the crankshaft, generator rotor etc to bump back up to speed. If the throttle vane is moved very quickly it's speed can jump up in less than 0.5 second, even with the dyno load applied. But that's with good fuel and a performance engine.

    The main problem here is that wood gas is not a great fuel and the fuel content etc changes with every pop and crackle of the wood. Because the fuel has limited and unstable power the whole closed loop system would need to be detuned, slowed and damped, much slower than a well tuned engine on good fuel. So in this case "several seconds" might not be far off. :)
     
  14. alec_t

    alec_t Well-Known Member Most Helpful Member

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    Is your alternator the standard Chevy one? If so, won't its output be rectified 3-phase AC, i.e. DC? In which case why is 50Hz important?
     
  15. Patricktj

    Patricktj New Member

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    Hi Erick and Mr RB
    I have the original engine flywheel and belt drive pulleys on the alternator and engine.
    So hopefully there is a lot of inertia stored in both of those to level out the surges.
    As you say wood gas chances CV easily, if it is mixed to fare along in the pipe system and you have a back fire well lets say the result is ground trembling !
    Excuse my ignorance but what is the usual frequency fluctuation on domestic power? I have never measured it.
    I do have fluctuation loads as saws start up and shut down and also my compressor, my other option is to fit soft starts to all my electric motors, but that is more money!
    So obviously 1% is not obtainable, so what is the best I can afford?

    Thanks
    Patrick
     
  16. Patricktj

    Patricktj New Member

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    Hi Alec
    The Chevy will be driving a 22kva 380 volt 3 phase alternator, which will be powering my farm, and saw mill.
    My goal is to saw wood with wood.
    Thanks
    Patrick
     
  17. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi Patrick.

    Most motors will tolerate +/-5 thru +/-10% variation in 50Hz frequency.

    At the lower limit the motors will run slower and slightly hotter if the frequency remains at -5% below 50Hz continuously.

    A strange question, :rolleyes:, at what rotational speed is the fastest rotating accessible part running at.??

    I would say the chevy is running at 3000rpm.?? if there is no gearing between that and the alternator.?

    E
    EDIT:
    hi P,
    Revisited your Vid and post I now see 2000rpm..:eek:
     
    Last edited: Aug 16, 2013
  18. Mr RB

    Mr RB Well-Known Member

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    That's an awesome project! Using your scrap wood to provide up to 22kW of useful electricity. :)

    You said you were using the standard pulleys and fanbelt to drive the alternator? :eek: The original car alternator and belt were only capable of 1-2 hp at most. Your 22kW 3-phase alternator is capable of 29hp (22kW=29hp), and assuming it is 75% efficient that 29hp output requires 38hp shaft power.

    The original car fanbelt and pulleys will never transfer more than a couple hp at "generator" speeds like 2000 or 3000 RPM at the car engine.
     
  19. Patricktj

    Patricktj New Member

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    Hi Eric and Mr RB
    The Chevy will be running 2160 rpm. The alternator is a 22kva tractor driven alternator so needs to run at 540 rpm I have to gear down the motor.
    I have a 125 mm 5 belt pulley mounted to the fly wheel of the Chevy and a 500mm 5 belt pulley on the input shaft of the alternator.
    Eric I can get a probe to pick up off the ring gear of the flywheel. Or the W terminal of the 12v engine alternator, or the (-) negative of the engine coil where your rev counter usually picks up from.

    When I switch over from mains power to wood power, I have to switch over everything, so it will power my house with computers, tv, fridges, washing machines etc so I have had problems in the past when when the alternator goes to far of frequency it fries the motherboard in the fridge!

    Thanks guys
    Hopefully I will have wood power by next week, the pulleys arrived yesterday with out any keys! So now waiting for keys!
     
  20. tcmtech

    tcmtech Well-Known Member Most Helpful Member

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    So you are going from 2160 RPM by belts down to 540 then up to either 1500 or 3000 RPM by gera sets and you want a 1% throttle response? :confused:

    Typical 50 - 60 Hz gensets with mechanical governors tend to run around +- 3 - 5 Hz from the base frequency, 3 - 5 HZ high at no load and 3 - 5 hz low at full load, without ever causing any issues with whatever they are powering so with that I find the 1% requirement unrealistic. Especially with the multistage power transfer system.

    If it was me I would be looking into taking the 540 gear box off the generator and doing a direct drive from the engine itself if its a 1500 RPM type. If its a 3000 RPM unit then I would be looking at running it with a double B size belt drive from the flywheel end.

    Now relating to what I understand of wood gas it has a slow combustion rate which makes it very unsuitable for higher RPM engine operation which means the slower the engine can turn and still carry the load the better.
     
  21. alec_t

    alec_t Well-Known Member Most Helpful Member

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    Here's a possible basis for a simple controller:
    RPMcontrol.gif
    With the component values shown, incoming pulses at a target 100Hz from the ignition/alty result in output pulses of 1.5ms duration and a 50Hz rate for driving a standard servo which would be linked to the throttle. An error of up to ± 5% in the incoming pulse rate results in a proportional change in output pulse width of up to ± 0.5ms, i.e the pulse width is in the 1ms-2ms range which is the usual full-scale range for the servo. The output pulse rate changes proportionally too, but this is non-critical.
     

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