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  1. #27
    Join Date
    Jun 2001
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    Moore, Oklahoma, United States
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    4,183
    Quote Originally Posted by syndicated View Post
    Agreed, but therein lies the fundamental difference between internal combustion engines and electric motors. Gas engines need to operate at a minimum rpm to make any useful power, and must fire in fast enough intervals so it doesn't shake itself to pieces think of a lawn mower about to stall.
    To get a variable pumping capacity wouldn't an unloader to the job easier?
    Idle speed is enough to prevent the motor shaking like crazy. At idle the engine horsepower will be minimal. Electric motors don't operate efficiently at less than about 40%, I think the gas engine has the advantage in part load capacity. Underloaders would be more complicated to implement that just varying the throttle position of an engine. Why produce the power then unload it?

  2. #28
    Join Date
    Sep 2004
    Location
    Kansas City, Kansas, United States
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    13,832
    B&S sucks!
    Put in a kohler and they are good units!

    But if you get free gas?

    Get a gas air conditioner!

  3. #29
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
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    14,914
    Quote Originally Posted by 54regcab View Post
    Idle speed is enough to prevent the motor shaking like crazy. At idle the engine horsepower will be minimal. Electric motors don't operate efficiently at less than about 40%, I think the gas engine has the advantage in part load capacity. Underloaders would be more complicated to implement that just varying the throttle position of an engine. Why produce the power then unload it?
    An engine designed to operate at one speed can be optimized for long term reliability, emissions reduction, and fuel consumption, better than one that has to operate at a range of speeds.
    If the engine and compressor were magnetically coupled, unloading could be accomplished via allowing the magnetic coupling to slip at part load.
    The engine RPM wouldn't change when at part load, but the work being done, thus the fuel consumption, would be lower.
    You would want the system to be variable cooling/heating capacity to reduce/eliminate start/stop cycles, which ultimately are what send an engine to its grave.
    If more government is the answer, then it's a really stupid question.

  4. #30
    Join Date
    Aug 2010
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    2,613
    Quote Originally Posted by 54regcab View Post
    Idle speed is enough to prevent the motor shaking like crazy. At idle the engine horsepower will be minimal. Electric motors don't operate efficiently at less than about 40%, I think the gas engine has the advantage in part load capacity. Underloaders would be more complicated to implement that just varying the throttle position of an engine. Why produce the power then unload it?
    What type of motors are you referring to and its inefficient below 40% of what?
    You'd want to modulate the system in such a way that it provides the best COP over a long averaging time (BTU-hr fuel heat content to BTU-hr cooling work)


    Quote Originally Posted by mark beiser View Post
    An engine designed to operate at one speed can be optimized for long term reliability, emissions reduction, and fuel consumption, better than one that has to operate at a range of speeds.
    Traditionally, AC portable generators had to run at 900, 1800 or 3600 RPM as rotor speed dictated frequency.

    The Honda EU series inverter ones don't. The voltage and frequency are regulated by inverter and the actual generator is a high frequency three phase alternator much like the ones we have in our cars. It actually achieves better fuel economy by lowering the engine speed when the load is low.

    If the engine and compressor were magnetically coupled, unloading could be accomplished via allowing the magnetic coupling to slip at part load.
    No, it would be very inefficient. The slip has to be dissipated as heat. This is why multi-tapped PSCs are incredibly inefficient. You can only vary the speed of AC motors two ways.
    1.) Changing the input frequency with a VFD.
    2.) Changing the torque by using taps and varying the degree of magnetic slip.

    Use a watt-meter and compare the power use of those portable fans. The input power at lower settings are disproportionately high, because much of the shaft power reduction is done through magnetic slippage.

    To get good efficiency at multiple speeds on an AC motor without a VFD you'd have to use a mechanical transmission much like a drill press.

    The engine RPM wouldn't change when at part load, but the work being done, thus the fuel consumption, would be lower.
    You would want the system to be variable cooling/heating capacity to reduce/eliminate start/stop cycles, which ultimately are what send an engine to its grave.
    I don't know how Yanmar does it, but the programming of PCM tune is one of the most important part of efficiency.

    An unloader type compressor like Emerson digital coupled with a large flywheel would present itself as a variable torque load on the engine. The modulation speed needs to be fast enough for the flywheel weight so it would impress a fluctuating load on the engine.

    The throttle and compressor modulation should then be controlled in coordition with the PCM mapping to follow optimal efficiency curve just like how the vehicle PCM coordinates air, transmission, fuel and ignition. Gas engines suffer from more pumping loss at low throttle opening.

  5. #31
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,183
    As you stated running a typical A/C motor at below 40% of it's rated output won't reduce input power very much.

  6. #32
    Join Date
    Aug 2010
    Posts
    2,613
    Air handler fan is a different setup. The output is varied by increasing the slippage at motor. If it used a single speed motor and load was changed by changing the load on the motor by mechanical means the efficiency drop won't be as dramatic. So, variable geometry fan blades or a solenoid shifting transmission box.

    Technically feasible but too expensive. Instead, they went with BLDC(ECM) which is the same technology as the fans in your computer and compressor motor in ductless splits. The guts of even the X13 is technically capable of operating like true VFD, but instead they're designed to work with legacy setup so instead they use taps and assign program to each tap.

    As for the compressor, there are two ways... the scroll unloader like the one from Emerson Climate control or using BLDC and inverter motor drive to change the rotor speed.

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