120Vdc to 120vAC

[lerameur]

Hello,

I would like to know what would be the best way to convert 120VDC to
120VAC at 5Kwatt power.
I did not find any schematics about this. I do not know if I should
use a microcontroller for the switching or use an analog oscillator to
switch high power transistor or mosfets. any ideas as to which
approach I should use?

regards

Ken

 

[petrus bitbyter]

Hello,

I would like to know what would be the best way to convert 120VDC to
120VAC at 5Kwatt power.
I did not find any schematics about this. I do not know if I should
use a microcontroller for the switching or use an analog oscillator to
switch high power transistor or mosfets. any ideas as to which
approach I should use?

regards

Ken

Some questions. Do you need a common ground for the AC and the DC? What type
of load will the AC get? Do you need a real sinewave or will modified
sinewave be good enough?

petrus bitbyter

 

[default]

Hello,

I would like to know what would be the best way to convert 120VDC to
120VAC at 5Kwatt power.
I did not find any schematics about this. I do not know if I should
use a microcontroller for the switching or use an analog oscillator to
switch high power transistor or mosfets. any ideas as to which
approach I should use?

regards

Ken

That depends on lots of things

120 DC to 120 square wave is doable and efficient. "H" bridge with a
multivibrator driving it. Some PWM "modified" "sine" wave might also
be possible if you load could tolerate it. - downside is you really
need about 170-180 in for a direct conversion without transformers if
you need a modified sine wave.

How good does the output need to be with respect to harmonic content
and what frequency were you going for?

Perfect sine waves are inefficient to generate from DC - A
motor-generator might be called for 5KW - out of the range of most
audio amps.

You'll probably have to design it yourself. There isn't much call for
120 DC to AC. Some research into electric car technology might turn
up something - they use high voltage batteries and drive brushless
motors (usually three or multi phase motors but the ideas could work
for single phase if you need that).

--

 

[captoro]

That depends on lots of things

120 DC to 120 square wave is doable and efficient.  "H" bridge with a
multivibrator driving it.  Some PWM "modified"  "sine" wave might also
be possible if you load could tolerate it. - downside is you really
need about 170-180 in for a direct conversion without transformers if
you need a modified sine wave.

How good does the output need to be with respect to harmonic content
and what frequency were you going for?

Perfect sine waves are inefficient to generate from DC - A
motor-generator might be called for 5KW - out of the range of most
audio amps.

You'll probably have to design it yourself.  There isn't much call for
120 DC to AC.  Some research into electric car technology might turn
up something - they use high voltage batteries and drive brushless
motors (usually three or multi phase motors but the ideas could work
for single phase if you need that).

--

The Idea here is to power a house. I have 11 batteries being charge
by solar and wind power. I thought it would be simpler just to convert
120VDC to 120VAC then 12v to 120cAC. A 5Kw inverter is expensive, but
the way I want to do it, I am saving myself the boost of voltage.


Ken

 

[[email protected]]

The Idea here  is to power a house. I have 11 batteries being charge
by solar and wind power. I thought it would be simpler just to convert
120VDC to 120VAC then 12v to 120cAC. A 5Kw inverter is expensive, but
the way I want to do it, I am saving myself the boost of voltage.

Ken- Hide quoted text -

- Show quoted text -

Then you probably want something closer to a true sine wave. It's not
particularly simpler and it probably will not end up being any
cheaper. Buy the inverter. Check other voltages like 24v. You night
find something there too. If you could get your hands on a used
computer room UPS some of those had big ac inverters in them.

 

[[email protected]]

Then you probably want something closer to a true sine wave.  It's not
particularly simpler and it probably will not end up being any
cheaper.  Buy the inverter.  Check other voltages like 24v.  You night
find something there too.  If you could get your hands on a used
computer room UPS some of those had big ac inverters in them.- Hide quoted text -

- Show quoted text -

It also occurrs to me to point out that a pretty large load. A dozen
or so auto style wet cell batteries are going to do that for maybe a
few hours at best I'd guess. If you really need 5kw then you need
more batteries. And if this is a regular house keep in mind that
standard line service is about 40kw. You are not likely to ever need
that but do not plan on running any big loads like central ac off
batteries. If you're going self reliant power you need to build the
whole house to be far more economical about power usage.

 

[captoro]

It also occurrs to me to point out that a pretty large load.  A dozen
or so auto style wet cell batteries are going to do that for maybe a
few hours at best I'd guess.  If you really need 5kw then you need
more batteries.  And if this is a regular house keep in mind that
standard line service is about 40kw.  You are not likely to ever need
that but do not plan on running any big loads like central ac off
batteries.  If you're going self reliant power you need to build the
whole house to be far more economical about power usage.

So if I aim for a 40kW version, then I will need 8 5KW inverter, this
is why I would like to build the circuit myself. I am good with
electricity but less in electronics, I know I have to put safety
first, but I think this is doable.

k

 

[[email protected]]

So if I aim for a 40kW version, then I will need 8 5KW inverter, this
is why I would like to build the circuit myself. I am good with
electricity but less in electronics, I know I have to put safety
first, but I think this is doable.

k- Hide quoted text -

- Show quoted text -

You would need a room full of batteries and a 40kw inverter is going
to be about the size of a clothes washing machine. I really don't
think you could build it for less than you could buy it.

 

[Noway2]

You would need a room full of batteries and a 40kw inverter is going
to be about the size of a clothes washing machine.  I really don't
think you could build it for less than you could buy it.- Hide quoted text -

- Show quoted text -

A 120Vdc bus is a standard for "industrial" UPS's, called a "low bus"
design. There are companies that make them, such as SCI and Gutor.

To get a clean sinewave will require a lot of complex control in the
inverter algorithm AND require an expensive set of filter inductors
and capacitors. This is by no means a trivial task. Shooting from
the hip, I would guess than buying a unit for this application would
cost at least $20K USD. On top of that, you would need some serious
battery capability. A string of 'car' batteries will not give you
much up time.

 

[Nobody]

The Idea here is to power a house. I have 11 batteries being charge
by solar and wind power. I thought it would be simpler just to convert
120VDC to 120VAC then 12v to 120cAC. A 5Kw inverter is expensive, but
the way I want to do it, I am saving myself the boost of voltage.

Only if you're planning on using square wave output. 120V AC means 120V
RMS, which requires 170V peak for a sine wave or 147V peak for a "modified
sine" wave.

 

[Bob Engelhardt]

Parallel your batteries & run 4 1500w 12v inverters off them ($100 ea):
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=96498

 

[Tom Biasi]

The Idea here is to power a house. I have 11 batteries being charge
by solar and wind power.

Ken

Can your charger keep up with your demand?

Tom

 

[Nobody]

What are you most comfortable with? You could do this either with a
low-level sine wave oscillator and an analog amplifier, or you could do
it with a processor that has on-board PWM generation. I'd go with the
microprocessor because that's what I'm comfortable with, but I know some
excellent designers who would blanch at the thought, and would have a
wien-bridge oscillator or some such over at the top left of their schematic.

Maybe you missed this part from one of the OP's earlier replies:

So if I aim for a 40kW version, then I will need 8 5KW inverter, this
is why I would like to build the circuit myself.

The thermal issues of an analogue design would be bad enough at 5kW. At
40kW, he's going to need 5kW just to power the cooling fans.

And:

The Idea here is to power a house. I have 11 batteries being charge
by solar and wind power.

If the objective is energy efficiency, a switching design is the only way
to go. But at that kind of power level, this isn't a beginner's project.
Unless you have a good understanding of EMI issues, 40kW of DIY SMPS is
going to screw up TV reception a hundred miles away.

 

[ehsjr]

The Idea here is to power a house. I have 11 batteries being charge
by solar and wind power. I thought it would be simpler just to convert
120VDC to 120VAC then 12v to 120cAC. A 5Kw inverter is expensive, but
the way I want to do it, I am saving myself the boost of voltage.


Ken

There are some serious issues here.
You'll be drawing in the neighborhood of 50 amps from each of the
batteries to get ~5kw. That means you want some heavy conductors
to put the batteries in series - think automobile battery cables
for size. They will need to be switched out of series if they
are charged individually from your sola5r and wind sources. If
they are switched out of circuit for charging and back into
series when powering the load, you'll need switching circuits
that can carry that relatively high current with low loss.
None of that is a "show stopper" but it *must* be addressed
in the design. And that needs to be addressed _before_ worrying
about how to build the inverter - i.e. there's no point in
having the inverter you want if you cannot provide it with the
needed input power.

Safety is a huge issue. If you do not know how to accomplish
what you want, then you *cannot* know how to address the safety
issues.

Cost. The inverter/batteries/solar&wind charger setup will
cost you WAY more than a 5 kw gasoline powered generator.
You need to figure the run costs to see if your plan has
merit. If it is just for backup when the grid is down, the
gas generator is the way to go. If it is a house that is
far from the grid and far from a supply of gas, solar/wind
may be viable (or possibly the only real cost effective
solution). You need to work out a detailed energy budget
and compare.
Space. You need to consider how much space is available to dedicate
to whatever equipment you use to provide the 120VAC, and you need to
consider the reguirements of that equipment - airflow, ambient
temperature, service clearance etc.

Location. Part of the engineering is determining whether there
is enough solar/wind energy available at your location to make
the effort worthwhile.

As a general rule, solar/wind is not a viable alternative to
grid connection to power a house.

You indicated that you already have batteries being charged by
solar/wind. That *might* give you a leg up on some of the issues
mentioned above - but it might not.

Ed

 

[captoro]

Maybe some experimenting is in order to determine if you need a sine
wave.

I'm running some 120 V compact fluorescent lamps off a little kludged
together 12VDC/120VAC pair of cross connected transistors driving a 12
radio shack transformer.  It only puts out 95 volts AC but the lamps
work and a switching power supply I use for 12V projects works on the
95 V square wave input.

Providing the switching supply has fast enough input rectifiers, I
can't see a problem with the idea.  

With 120 VDC you might just get by with DC into a switcher - but each
load would have to be looked at separately.  A switcher usually has a
FWB input to charge some caps to 150 VDC then circuitry and a
transformer to lower and isolate the output.  More complex stuff that
incorporates power factor correction may just smoke . . .

You could buy a commercial unit for the sensitive stuff and build your
own device for lighting circuits and none sensitive loads.
--

I might put this project on hold or possibly go a different route.
thanks for all the feedback.

K