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[aprssig] backup pwr systems

Bob Bruninga bruninga at usna.edu
Tue Dec 12 04:33:21 UTC 2006


>> My thought is just a sensing circuit that only fires 
>> the inverter MOSFET's on the rising side of the 
>> 60 cycle waveform.  And then fires them for 50ms 
>> or so on each cycle. If there is no waveform, then 
>> the inverter transistors don't fire.  Should 
>> be very simple. 
>
> You would have the power company beating down your
> door that way.  Switching the MOSFETs on only a 
> half cycle would introduce even more harmonics 
> into the output power and get fed back into the grid.  

No, that's what every light dimmer does,  And if it is running a 400 watt lamp load like a dining room light then it is the same harmonics.  A simple RFI filter can smooth it up so even a HAM can run on the same power.

THink about it.  The load of the "grid" is something like maybe 0.01 ohm or much less.  There is little that a single mosfet (or a single light dimmer) can do that would cause any discernable gliches on the waveform.  All the mosfet would do would be to contribute a flea power of current, and it would never leave my house, since with only 400W going in, and my houose drawing 1 kW, then nothing leaves my system.  I just reduce my current is all.

> Best to just stick with a standard UPS for each 
> item (or group of items) ... or go with a 
> grid-tie inverter system.

Yes, that was the orginal thread.  I'm looking for a synchronous inverter (call it a grid tie system) in the 400 watt class that should be able to be built for under $100 in electronics.  The cheapest grid-tie systems are in the thousands of dollars.  Hence by post.

>  Having something that plugs into the wall as you 
> describe is a sure-fire way to electrocute a line 
> worker when the power goes out (because you don't 
> have an anti-islanding relay installed as would 
> normally come with a grid-tie inverter). 

No, you didnt read my post correctly.  I said a synchronous inverter which means it cannot generate power on its own with out the presence of the 60 Hz grid in the first place.
It is inherently safe by design.

Bob






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