>My Usenet reader is playing up so if this is a double post I
>apologize:
>
>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
>manually
>varying the load on the wind turbine, I have found that a 6km/h wind
>can
>generate 10W, a 12km/h wind can generate 80W, a 23km/h wind can
>generate
>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
>but
>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
>problem.
>
>So I think it would be very worth while designing a circuit that
>doubles
>triples or quadruples the voltage at lower wind speeds, so that I can
>get
>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:
>
>
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>
>
>I was thinking of using a microprocessor to control mosfets to select
>the
>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
>any
>opinions on their use in this purpose, I would be very interested to
>know.
>Voltage ripple is not much of an issue, I am more interested in what
>the
>efficiency of such a circuit would be or any drawbacks that I have not
>forseen.
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:
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RL