Need help on 24V DC input to 240VAC, 500W inverter circuit design

woodyzai

Joined Nov 14, 2009
8
I need to design an inverter circuit for my solar inverter project. My dc input from the panels are 24V DC and I need to get 240VAC, 500W power output, 60Hz operating frequency, modified sine wave or sine wave waveform inverter circuit design. Anyone can give me a guide on this? Thank you very much for your help.

SgtWookie

Joined Jul 17, 2007
22,221
Well, for starters - can your solar system put out about 24A at 24V?
If an inverter were 90% efficient (which would be very, very good for a home-built), you'd need that much current to generate a 500W 240v output.

Your output at 240v would be about 2.083A; it wouldn't power very many appliances.

woodyzai

Joined Nov 14, 2009
8
I will have 3 panels (each max power 120W, open circuit voltage 21.6V, short circuit current 7A). I planned to connect them in parallel to each other. So I think will have about 20A current.

SgtWookie

Joined Jul 17, 2007
22,221
120W x 3 = 360W. You will need an absolute minimum of 555W input power, even with an optimistically 90% efficient inverter. That's about 4.7 panels.

If you want to be more or less certain that you will be able to output 500W @240v for a good portion of a bright sunny day, you will need six 120W panels.

woodyzai

Joined Nov 14, 2009
8
yea I did have 6 panels, 3 panels (big) which have a max power of 120W, open circuit voltage 21.6V, short circuit current 7.75A, 3 panels (small) of max power 120W, open circuit voltage 23.9V, short circuit current 7A.

Before that I planned to connect all of them in parallel, so will have power of 6x120= 720W, lets say due the cloudy weather we just obtained 600W, now I going to design a 500W inverter which have 24VDC input. This inverter is going to connect to a 300W load, excessive power will be stored in the battery charger. So does it make sense?

Since I connect all of them in parallel, I think the current will be very large, so if I connect 3 big panels in series, and 3 small panels in series, then combine them in parallel, how do you think?

whisperingsage

Joined Nov 14, 2009
1
Do you have a battery bank with this system? And is the 240 VAC for a particular application like an electric cookstove? (because it sounds much like one of my projects). I take comfort that the needs for high AC volt appliances are only for short term use. Like power tools and cooking. In this case backing yourself up with a marine (or other deep cycle ) battery bank system will allow this use, even if you don't have 20 panels to back yourself up. All lights and long term use appliances (24/7) are 12 VDC or 24 VDC which doesn't draw down more than my system can charge. Also very useful if you get a lot of wind is the wind generator. The Air X on the market ready to go is a small and very good use of $500-600 investment. We got the older model and it put out 30 amp hrs and we had wind usually all day. It has internal diodes. I haven't opened up the guts of the Air X to make a home version, but the blade design is what whips it through at the lowest wind. In fact, in my windy area, I had to trim down 1 " off the tips to slow it down. Sorry I didn't have any inverter advice, as I am in need of more info myself for homemade inverters, but just on a practical level, you don't have to have just the panel system to depend on. I have worked with this stuff for about 20 years. And I currently only have 6 panels. SgtWookie Joined Jul 17, 2007 22,221 OK, here you go: http://www.islandwaterworld.com/bro...d-sine-500w---24v-230v---sinergex/4,7464.html Just what you're looking for,$120 off the shelf.

You're not going to be able to build something as efficient, reliable, or inexpensive at home.

[eta]
Keep in mind that most items powered with AC are going to have a higher start-up current than run current. It may only be for a fraction of a second, but unless you plan for it, your inverter may simply refuse to power it altogether.

And instead of trying to use the "left over" power to charge the batteries from the inverter; just charge them directly from the solar cells. No point in taking a "double hit" in efficiency losses (one for the DC-AC inversion, and another for the AC-DC buck for the charger)

Last edited:

woodyzai

Joined Nov 14, 2009
8
actually designing an inverter is my final year project. So i need to design it myself instead of buying one. The efficiency can reach about 80% should be ok for it. Do you have any working inverter circuit can show me? thanks

SgtWookie

Joined Jul 17, 2007
22,221
Here is a resource for you: http://www.smps.us/power-inverter.html

Look at the hobbyist schematics for entertainment. The 2kw actually isn't too bad at all, particularly for a 10+ year old design; but it's outdated. If a student showed up with an inverter that contained a huge transformer nowadays, the professor would very likely give them a big, fat goose egg for a grade.

Audioguru

Joined Dec 20, 2007
11,249
Since the student is allowed to simply copy a design by somebody else then maybe he is studying how to solder parts together. For high school?

SgtWookie

Joined Jul 17, 2007
22,221
Audioguru,
The hobbyist portion of the website merely serves as an example of how not to do things.

The other portions of that page are textbooks and notes on various inverter topologies.

There will be a good deal of time, effort and studying involved for our OP to achieve their goal. Nothing has been "handed" to them that they could simply copy and use outright; but that site does have a wealth of useful knowledge that once learned, will be a big help towards their assignment.

woodyzai

Joined Nov 14, 2009
8
erm yea.... we need to build one inverter by referring to someone else in order to have basic idea how to design an inverter before we really designing one for ourselves. We planned to divide the inverter into 3 stages, pull pull converter, inverter then LC filter. But we don't have exact idea how to connect it. Maybe someone can give any guidance and schematics for the circuit? thanks

SgtWookie

Joined Jul 17, 2007
22,221
OK, then start by looking up push-pull converters.
Here's a Wikipedia page for the basics:
http://en.wikipedia.org/wiki/Push–pull_converter

The idea with the push-pull converter is to get your low voltage, high current DC up to a high voltage level.

For the inverter portion, it would be a good idea to review Class D amplifiers. If you can implement the idea of a Class D amplifier in your inverter, it will dissipate much less power in your MOSFETs or IGBTs, whichever you decide to use.

The fellow who posted the 2kw 120v design gave some hints towards the end about using a microcontroller with a sine look-up table. Microcontrollers have become quite powerful nowadays.

The LC filter could be either a simple low-pass filter or a twin T filter tuned to your output frequency. If you wish to experiment with designing your own filters, have a look at Elsie; available in a freeware version here:
http://tonnesoftware.com/elsie.html