Converting DC to AC: Open Discussion - All methods

[Grumpy]

Everyone on the "alternative energy path" eventually comes to the crossroad where the signs read: DC-AC Motor-Generator, and DC-AC Inverter.  With 12vdc or 24vdc, the inverter looks pretty nice, but with something like 144vdc, inverters become hard to find for direction invertion.

So, I'm thinking that an option like using an electric motor designed for automobile convertions might be a good way to go if you have higher voltage DC available.  Some of these are 144vdc and relatively inexpensive. Couple this to a generator in place of the gas or diesel engine and you're good. 

Is there an easier electronic way to convert HV DC to AC that is readily available?

[Grumpy]

AC generator output vs motor/engine size

6000W peak, 5000W continuous    11 HP
5000W peak, 4000W continuous      8 HP
3000W peak, 2500W continuous      5 HP

The Ac Generator heads are readily available to couple to whatever you have to drive it with.

12HP DC motor for EV is about $1200, but I have seen them much cheaper.

(Could probably get off the grid for $2500, assuming you have some means to power it.)

[humbugger]

I would check out some of the grid-tie solar/wind inverters available.  I'll try to find a link for you, sure I've seen some designed for HVDC input up to 300V which have wide-ranging inputs.  They put out a nice clean sine wave at 120VAC 60Hz or 230VAC if you want it that can sync itself to the grid, just in case you're producing more energy than you need!  

Yeah...here you go:

http://www.pvpowered.com/downloads/Residential-Inverters-Disconnects.pdf

That's just one of many and probably too expensive, but there are lots of others out there.  Look up "String Inverters" on Google.  They are designed to accept the HVDC inputs you are wanting to use.  I'm sure I've seen small inexpensive ones, too.  They can be paralleled as well, so you can start small and grow your system.

I'd stay far clear of motor/generator approaches unless you really need the shaft input to your generator (for use with gas engine or whatever).  No motor/gen system will be as efficient, quiet, maintenance-free as a good solid state inverter.  And good luck ever trying to tie the AC output into the grid with a motor/gen setup.  

JM2C

Humbugger

From Wki:

String Inverter - A relatively new term and misleading term in the world of alternative energy, it applies primarily to solar electric generating equipment. Grid Tie or Non gris Tie or Grid Interactive is a better term. Generically, an inverter is a device which changes electricity from DC (direct current) to AC (alternating current). Prior to the popularization of system which interact with the utility grid, most photovoltaic (PV) electicity-generating systems were based on one or more solar panels which produced 12, 24 or 48 volts nominal DC power. In situations where more power (more panels) were used, the panels would be connected in parallel, to increase the available current flow while keeping the voltage output at 12, 24 or 48 volts, depending on the nominal panel output voltage. In Grid Tie system the panels are wired in series which increases the voltage and keeps the current low so that wiring is simpler and wire size can be smaller. When panels are wired in series it is a STRING of panels. Hence the term String Inverter. However the you can have string of panels on systems where the the controller changes the voltage to battery voltages and the inverter is not a "String Inverter" or Inverters which are Grid Tie and work off Battery voltage. String refers to the way the solar panels are wired and has nothing to do with the inverter.

[Grumpy]

Thanks Much.  This sounds like what I'm looking for.

[humbugger]

You are quite welcome, sir.

Actually, the Wiki descrption is not quite accurate and a bit confusing.  Grid tie inverters can still be limited to 12/24/48 VDC inputs, and many are.  The way it evolved is this:

When it became possible and legal and profitable to tie a solar or wind power to the grid, many folks were excited because the battery requirement was effectively eliminated.  In other words, if you have the national power grid available and you can get paid for pumping into it when you are not using all the power you produce, it becomes your effective "battery".  You can take out and put in whenever convenient.  It has unlimited "storage" capacity in effect.

Once this whole idea started to catch on, the 12/24/48 volt constraint was removed for inverter makers.  It is generally easier and more efficient to design power conversion devices at the higher voltages because losses are lower as current goes down.  A 5KW inverter that uses 12VDC nput will draw 417 Amperes!  The same inverter designed for 300VDC input power will draw a mere 17A.

Since losses in circuits and copper wiring (anything with any resistance at all) follow the I-squared X R rule, you can see that I-squared gets enormous in low voltage systems.  Just think of the difference in wire sizeneeded to keep the losses low  between 417 Amps and 17 Amps!  Same idea applies to the MOSFETs, transformer primaries, etc. in the inverters.

So, grid-tie inverters are actually defined as inverters that can synchronize the output AC to the grid and that have been approved for doing that.  A side benefit of this syncronizing ability is that they can be tied together at the output (and input) in parallel, so they can be modularized easily in affordable increments.  A grid-tie inverter may still be designed only for 12/24/48 volt DC input.  But the higher input voltage types of grid-tie inverters have become popular, partly because no one really wants to use 400 pounds of giant fat wire to get 12V at 417A down from the solar panels on the roof!

So-called "string inverters" are pretty much all higher voltage DC input and are a sub-group of grid-tie inverters. If you don't have grid access or don't want to sell-back to the grid for some reason, you pretty much need the batteries.  Or if your grid is unreliable, as in third-world countries.  There may be string inverters that don't have the grid-tie feature, even.