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Voltage Multiplier Question

Discussion in 'Electronic Design' started by [email protected], Aug 25, 2008.

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  1. Guest

    My Usenet reader is playing up so if this is a double post I

    I am experimenting with a wind turbine home built with a brushless,
    permanent magnet,
    three phase Fisher and Paykel washing machine motor.

    To do some tests, I have used a variable resistive load and, by
    varying the load on the wind turbine, I have found that a 6km/h wind
    generate 10W, a 12km/h wind can generate 80W, a 23km/h wind can
    640W and a 46km/h wind can generate over 5KW.

    The problem is when I use batteries instead of a variable load.
    At lower wind speeds, the produced voltage of the motor
    is not higher than the batteries, so the turbine just spins and spins
    does not make any useable power. The turbine only starts charging the
    batteries at 20km/h winds, which is a shame as a lot of power just
    flies by
    without being collected. This is a battery impedance mis-match

    So I think it would be very worth while designing a circuit that
    triples or quadruples the voltage at lower wind speeds, so that I can
    every last bit out of the wind.

    I have looked at voltage multipliers, and since the F&P motor is three
    phase, I wonder what your opinion is of this circuit right at the
    bottom of
    the page:

    I was thinking of using a microprocessor to control mosfets to select
    optimum output voltage for the RPM of the wind turbine. Each mosfet
    would be
    connected to each subsequent voltage doubler point.

    I am not too familiar with voltage multipliers, hence if anyone has
    opinions on their use in this purpose, I would be very interested to
    Voltage ripple is not much of an issue, I am more interested in what
    efficiency of such a circuit would be or any drawbacks that I have not
  2. MooseFET

    MooseFET Guest

    The power available from wind goes as the cube of the wind speed.
    This makes it a lot harder to get useful charging in a light wind.

    [... snip about voltage multipliers ...]

    A voltage multiplier will boost the voltage but I think there are
    better ways to go. A simple flyback booster will boost by a variable
    amount. In the wind generator case, things are fairly easy for the
    flyback design. The MOSFET doesn't have to handle a large amount of

    At low wind speeds, the input voltage to the booster is low. At low
    wind speeds, the torque is also low so the current you want to take is
    low as well. This means that the MOSFET only needs to pass a lowish
    current when it is on.

    At higher wind speeds, the voltage is high. The torque and hence the
    current is also high. In this case, the MOSFETs on time will be short
    making the power in the MOSFET low.

    At very high wind speeds, the boosting isn't needed at all so the
    MOSFET never turns on.

    If you wind your own inductor, you can go a little light on the core.
    It doesn't matter if the core saturates in the high wind case because
    the MOSFET isn't going to turn on.

    Chances are a very simple controller will do what you want. I suggest
    a home brew one that does this:

    If the drain on the MOSFET is at a lower voltage than the battery,
    trigger a one shot.

    The one shoot turns on the MOSFET for a fixed time in most cases.

    A safety circuit consisting of an NPN and a resistor in the source of
    the MOSFET forces this first one shot to reset if the current goes too
    high. This circuit only acts in a fault case.

    A second oneshot enforces a short delay after the first oneshot.
    During this delay, the first oneshot is prevented from firing.

    This second oneshot prevents the first from firing again during the
    time it takes to gat the MOSFET actually turned off. It also helps in
    the very light wind case where the logic given above would turn the
    MOSFET on for too great of a duty cycle.
  3. legg

    legg Guest

    As you have forseen, a multiplier stage cannot be easily deactivated,
    largely because the control switch has to deal with AC current.
    Controlling mosfets in the diode positions will not be effective
    because they only control in one direction.

    You already have rectifiers in the circuit. By locating suitable
    inductors on each phase, before these rectifiers, a single PWM'd boost
    switch located after the rectifiers can be used to transform the
    apparent impedance of the battery load. This is easily disabled at
    higher input voltages, simly by turning the switch off completely.

    Examples in this survey by Jang and Jovanovic:

  4. Joerg

    Joerg Guest

    Go with Ken's suggestion. This can later be expanded into something like
    maximum power point tracking where the conversion ratio is adjuested on
    the fly when the wind changes. This makes sure the blade and motor
    always see a somewhat optimized load.
  5. neon


    Oct 21, 2006
    DO YOU REALY BELIEVE that you can change a battery like a resistor load?
    and on a real system it is not unsual to get comercial power to start spinning the wheels to begin with. and doubleing voltage will get nothing but a less efficient system. you may change energy from a to b but you cannot create b from a from nothing.
    Last edited: Aug 25, 2008
  6. MooseFET

    MooseFET Guest

    There is a way around this problem.

    If the MOSFET is in series with a capacitor and is switched off. the
    substrate diode will conduct for a couple of cycles and charge up the
    capacitor. At that point no further current will flow. This would
    allow you to switch out a tripler without too much trouble. Turning
    the MOSFET back on will be trouble, however because the stored charge
    must be dealt with.

    Another way to attack the problem would be to gear up the speed so
    that the voltage is always higher than desired. You could then use an
    SCR based circuit to reduce the voltage back down. The required
    inductor will be physically large but since the windmill is big and
    heavy, nobody will care.
  7. legg

    legg Guest

    The real problem is the presence of the capacitor in series, and the
    fact that rectification is performed in the circuit. You can't short
    it out - only an open circuit deactivates it - meaning a whole new
    rectification path is needed in it's absence.

    The multiplier has advantages that may be mis-applied in this

    It might be easier to process the energy and to control the effective
    load impedance, if the control function is positioned after the
    rectifier. Easier to do at home, anyways......

  8. MooseFET

    MooseFET Guest

    If the rectifier is a tripler, just opening up the capacitor leaves
    you with a simple rectifier.

    If you make a bridge rectifier, you can make it a doubler by
    connecting one side of the source through a capacitor to ground.

    The MOSFET can be turned on with a photovoltaic isolator. This will
    turn the MOSFET off and on very slowly.
    I'm not suggesting that a multiplier is a good idea. I am just
    pointing out that you missed a point about the bad idea.

    I don't know:

    Go to the junk store and buy a Variac. Replace the knob with a
    pulley. Put a smaller pulley on a small electric motor. Buy some
    weights. The electric motor turns the Variac down and the weights
    pull it up. When the voltage to the motor makes just the right torque
    to balance the weights the Variac setting is just right.

  9. neon


    Oct 21, 2006
    The point is was and still is. POWER if he cannot generate the power to begin with doubling the voltage sticking mos fet is not going to do it. A battery don't give a damn about voltage[ as long as is higherthen itself] it wants current if the current isn't there it will never work.
  10. legg

    legg Guest

    There are many things you can do with a big ball of string, but please
    keep in mind that the OP is working with a 3ph, 'variable' frequency,
    'variable' voltage source. Variable, in this case, being as
    predictable as a mule.

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