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Re: Controlling the solar scourge



Harry H Conover <conover@max.tiac.net> wrote:
>Clark Dorman <dorman@cochlea.bu.edu> wrote:
>
>>>I like the Air 303. $550 for 375 watts, from Jade Mountain at (800) 442-1972
>>>Now all we need is a cheap, reliable synchronous inverter to feed this tiny
>>>power back to the grid. I'm an electrical engineer. I keep telling people
>>>that a synchronous inverter at this power level should not cost much more
>>>than a lamp dimmer. A simple triac bridge, a backwards lamp dimmer, with a
>>>small microprocessor to determine the optimum load for the windmill, $10
>>>worth of parts. 
>
>I suspect you're missing a few vital pieces.

People have often suspected that :-)

>First, you're going to need an inverter transformer with some efficiency
>(magnetic components don't come cheap).  At 375W power level, this shouldn't
>cost more than $40 or so.

Hmmm. What would I do with that? I'd use a choke in series with the triac
bridge. I have one somewhere up in my attic, made for this purpose, about
10' of 16 ga magnet wire wound on a toroid made of galvanized iron pipe, with
an air gap. The wire is undersized for this 4 kW version and the toroid is
bigger than needed, to keep the wire cooler. A 375 watt version might use
a bent nail for a core. Next?

>Next, you'll need a quantity of Leg switching transistors (6-12 
>should do nicely) so you'll end up with a  waveform somewhat 
>approximating a sine wave.

Why does this remind me of my Kentucky cousin, Ophelia Legg? I'd just a 3 Amp
triac bridge, like a backwards lamp dimmer. Switching frequency 60 Hz. When
the DC output of the windmill is larger than the grid voltage, you turn on
a triac (this would work best with a high DC voltage.) When the grid voltage
becomes larger than the DC supply voltage, the triac turns itself off. I
posted a schematic of this circuit months ago here, with waveforms. 

>After that, a good harmonic filter, at this wattage, probably between $30-50.

That's what the choke's for, to make less than 5% rms voltage distortion
at the grid interface. It also provides current limiting. 

>Combine this with phase/frequency locking

Huh? I think this thing would be called "line commutated."

>and fail-safe "outage" detection (another $20)

I see :-)

>and you'll pretty much cover the OEM bill of materials.

Which is of course what  I  was talking about.

>Now, add in the cost of manufacturing, engineering, advertising, marketing, 
>distributor mark-up and profit, and you'll likely end up with a selling
>price that's about 3-4 times material cost.

The usual ratio is 4-6 times the cost of the parts, to pay for the
salespersons lunches and BMWs.  

>You can save a few bucks by deleting the lamp dimmer components, since
>its components provide no useful function here.

I'm not sure that you understand how this thing would work, Harry.

>What kind of engineer did you say you were?   :-)

I travel with custard pies.

Nick


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