Re: Voltage Multiplier Question

I am experimenting with a wind turbine home built with a brushless,

>permanent magnet, >three phase Fisher and Paykel washing machine motor.

Good choice. It might be useful to see how such a motor is controlled and used in the washing machinery. See:

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.

Yep. Power out is proportional to the cube of the wind speed (and the square of the turbine diameter). Are these measured or theoretical? If measured, methinks 5kw is a bit high for a washing machine motor.

Watts = 14.3 PAV^3 where: P = air density (2.3 x 10-3) A = area swept by turbine blades (sq mtrs) V = wind velocity (km/hr)

What did you use for a turbine blade (propeller)? Getting reasonable efficiency over a wide range of wind speeds usually requires a variable pitch propeller.

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.

What voltage battery pile are you using? I'll assume you have installed 6 diodes to form a 3 phase, full wave rectifier instead of 3 diodes to form a half wave.

The 6 diodes will produce about 30% LESS voltage than the 3 diode array, but deliver twice the power. In this case, power is what you want, so live with the lower voltage.

Once you have some semblance of DC coming from the diodes, your problem is basically a DC to DC converter. There are numerous complicated charge controllers available on the alternative energy market that will do that. The problem is that you will need to size your controller for the expected output. One of the reasons small wind generators favor permanent magnet generators is that they work well at low wind speeds. Washing machine motors work well over a wide range of rpm's (a good thing) but crap out at both low and high speeds. Treadmill motors are PM DC motors with about 1kw maximum output. There's no ideal motor, that works over the entire range of expected RPM's and loads, although complicated hybrid designs seem to be improving. (I haven't kept up to date).

6km/hr (about 3.7mph) is lower than even the smallest commercial wind generator suggest as useful. Most small DC PM generators start producing power at about 5 mph. I don't know where your 3 phase washing machine motor starts, but I suspect 6 km/hr is too low.
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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Those are half wave voltage multipliers. You need to use both halves of the cycle. If you insist, see full wave voltage multipliers as in:

I'm not sure how to drive this with a 3 phase Y.

The problem with both of these is sizing the cazapitors. At higher power loads, the current through the caps is going to be rather high. Each will need to store half the output power of the generator and deliver it to the load at every half cycle. It's possible, but methinks this creates more problems than benefits.

At low RPM's, you're also driving the voltage multiplier at a fairly low frequency. That depends on your propeller and gearing, but it could easily be a few Hz. The required capacitance is going to be rather large for that to work.

However, that doesn't stop anyone from using the idea:

Note that this is for a small bicycle hub generator, not a much larger washing machine motor.

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.

Well, if you wanna use smarts, you could use a hybrid rectifier design. Use a full wave voltage doubler at low RPM's to get it to produce useful voltages at low wind speeds. When the wind speed gets above a specific threshold, switch the rectifier configuration to a more conventional full wave non-voltage doubler design. Adding smarts offers lots of other possibilities, such as pitch control on the prop, smart feathering in high winds, non-violent yaw (direction) control, turbulence and resonance mitigation, performance monitoring, lifetime enhancement (mechanical wear detection), and maybe even a bird strike alarm.

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.

Half wave voltage doubles only use half the cycle and therefore deliver only half the power. That's one reason why you see voltage multipliers mostly in low corrent applications.

Your biggest problems will probably be dealing with tower construction and too much wind. It's been perhaps 25 years, but I lost two wind turbines that way.

The basics:

(Dig through the links in the article).

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Jeff Liebermann     jeffl@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
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