12V Inveter

[large_ghostman]

What is the technique to making a 12V-230V inverter? I have googled but there seem's to be some dangerous idea's out there. I am interested in taking 12V DC and turning it into 230V AC 50Hz, can someone explain the steps involved? I doubt i will build it because they are pretty cheap , but would be good to know how to do it. Sorry forgot to mention, 750W output.

 

[audioguru]

Electricity in your home has a sinewave, a steady voltage and an accurate frequency.
A cheap inverter has a squarewave, its voltage is too high when the load is light and the voltage is too low when the load is heavy. Its voltage drops as the battery voltage runs down. Many electronic products rely on the higher peak voltage of a sinewave to work but the squarewave voltage is set lower than the peak so that its average voltage creates the same light in an incandescent light bulb or same amount of heat in a heater as with a sinewave. Its frequency is all over the place. It blows a fuse or blows up if overloaded.

A good inverter uses Pulse-Width-Modulation to make a stepped sinewave with its clock at a high frequency where it can be simply filtered out. It has voltage regulation, an accurate frequency and it shuts down when overloaded.

 

[large_ghostman]

So do you think it is worth building one? A decent one, 750W for around 1-2 hours tops,
and we have another bloody power cut!!

 

[Reloadron]

Here is part of the problem, be it a sine wave out, modified sine wave or even a square wave output. You mention 750 Watts, well discounting the inefficiencies 750 Watts @ 12 Volts = 62.5 Amps so you will need a large battery, especially if you hope for a 2 hour run time. As to build? You called it LG in that it is easier and cheaper to buy rather than build. UPS units for home computer systems can deliver 1 KW but when we open them up we find a 24 VDC power source, normally a few 7.5 AH 12 Volt batteries in series.

The true sine wave output units get expensive and are also more difficult to build. Overall cheaper to buy than build over here where I am located. Guys that do use a UPS generally get some large batteries like deep discharge marine application or RV batteries.

My solution years ago was simple. I went with a whole house 18 KW generator unit with automatic start and transfer. Mains power fails and within 10 seconds I am fully back online so my UPS units need not run very long. Fuel is natural gas which we have plenty of. Lose power around here during the winter and things start freezing real quick, including mu butt!

Ron

 

[shortbus=]

Here's a college thesis that explains how a pure sine convertor works, and how to do the math for one. But I agree with Ron, the gen set is a better idea.

 

[tcmtech]

I'm not sure how things are in your part of the world but if a guy hangs a few "I'm looking for," notices around the local car parts and farm supply stores saying you're a teen looking for an old genset to learn how to fix engines and electrical stuff with the odds are you would have too many to know what to do with by the end of the first week.

I know for a fact if you were close to me I would donate at least one or more likely two. One with an engine issue and one with a generator issue and let you figure out which is which!

The point is there are loads a people like me who have odd non functional items like these laying around that are 'too good to scrap' but 'not worth my time to fix that would be happy to see these old pieces of junk go to a good home!

Just something to think about.

 

[audioguru]

That is a good detailed college thesis about their pure sinewave inverter design.
Hee, hee its output voltage regulation was awful because they forgot about controlling it.
Hee,hee again their output filter inductors saturated.
I think one of their sketches of a modified sinewave is mine.

Years ago I was going to design a pure sinewave inverter and post it as a project. I was going to use a class-D audio power amplifier IC by National Semiconductor. I ordered the parts then found that the National IC production was stopped because many of them failed.

 

[large_ghostman]

I have a small gen set, but the problem is, it is a pull start one. I have a deep discharge marine battery, 750W was a maximum and i have seen these cheap. I would guess at most i am looking at 400W, but i wanted headroom, same with 2 hours. In reality it would be very rare to need more than 30 mins.
A whole house gen set would be very expensive, we have wood and oil for heat, but everything else if electric.
Unlikely i am going to attempt to build one, but i wanted to know how it was done . Great paper by the way. My other option would be to build a 12V or 24V auto gen set, we have a great little diesel engine i could use, but i see the engine regulation might be a problem with changing loads. So if i went down that route, then maybe alternators of a lorry and a battery or two. I miss the big Gen set we had!

 

[mab2]

Depends what you want to run. There's a lot to be said for the Modified sinewave inverters - a good one will run most things and if you're wanting to run a motor (like a freezer) then they have better surge capability than the pure sine - and of course they're a lot cheaper.

If you do want to make your own then the MSW is a lot easier too. Simplest method is using a push-pull driver like SG3525 to drive FETs to alternately ground the ends of a center-tapped transformer - with the center tap connected to battery +ve (you can use a standard 10-0-10 / 230v mains transformer the other way round).

A better design has a transformer with a 'clamping' coil that is shorted when the main winding is not being driven - this means the transformer is effectively shorted during the 0v parts of the waveform to better mimic normal mains, but requires a more complex driver circuit to switch the clamping circuit and to time the switchings so there's no overlap.

 

[large_ghostman]

Thank you mab2 that has given me something to look into , The loads would all be pretty much resistive, like heating elements, and probably around 60W of small motors (fans). But if it worked out cheap enough, then dad might be tempted to build a bigger one and run it off the spare engine. That would give us a 2 cylinder 15 hp diesel engine, to run a couple of lorry alternators. I need to go and think this through, my gut say's trouble running a battery bank with strings of two in series and several strings in parallel.
The batteries i have in mind are 145 amp hour, deep cycle marine. But i also know where there are two fork lift truck batteries up for grabs (free). No idea what there rating is, but they run a large electric forklift for around 9 hours.
The engine runs all day on around 4 litres of diesel.

 

[large_ghostman]

Ok i know that the question was about inverters, But something else occurred to me. I did a bit of google and ended up on some sites for DIY off grid, it is a little hard to make sense of some of it, but looks like some have made 230V from washing machine motors, and some made wind turbines from 3 phase motors,
so rather than inverter route, could a generator be made from driving some kind of motor without a battery bank? How would you get the 50Hz? Really interesting stuff, i should read up on power electrics really. My head is full of question's on this stuff, I guess i am starting at the wrong end, and should be looking at how energy companies do it, then work back and see how it can be scaled down

 

[tcmtech]

Well if you are looking for normal 230 VAC 50Hz output you might as well just run the logic on your design full circle to the point you come to the conclusion that a engine driving an actual 230 VAC 50 Hz generator is what would work best.

If you only need short term limited power reworking a UPS or two to run off larger external battery works just fine plus does give you the capacity to use a common alternator and whatever engine you want to keep the system going for extended runs.
Granted the larger 500 - 1000 VA units tend to run on 24 VDC inputs but there is no rule that says you can't put your two 12 volt batteries in series to make a 24 volt system.

As the forklift batteries go if you can get them by all means jump all over the offer before it gets away! They are nothing more than massive lead acid batteries made up of a number of 2 volt cells in series.

Many can have their individual jumpers between cells cut or removed to reconfigure them for other voltages besides their typical stock 36 - 60 VDC output.

Just some things to think about.

 

[large_ghostman]

Thats very helpful! I think what i am trying to get too but not making it, is I want to know what is inside a generator! How do they work? Can they be replicated by parts? Surely the most expensive part of say a 9KVA Gent set, is the engine and gear box?
I do put up the hands here and say, I havnt found the basic information yet, so I dont know enough to be really asking this stuff, or to be thinking about doing it, for this reason its a thought exercise, and a want to know thing. How does a gent set work? what is inside say a medium sized one?
Or another way, what hurdles do you have to jump to replicate one
There, I finally framed the question in an accurate way
Man my question skills suck back regions

 

[KeepItSimpleStupid]

If you have a battery involved already, I think your options increase. Much like a car alternator, you can make a high voltage DC bus and then go inverter technology.
The car alternator pulses the rotor field and 3 phase rectifies the output to DC. Residual magnetism does not play a role.

One of the commonalities of a stand-alone, non battery genset is the concept of a partially magnetized rotor. So, one of the reasons a genset doesn't generate power is thhat the field needs to be flashed to create the residual magnetic field.

That residual field is what gets the control circuits humming.
RPM controls frequency.
The field controls amplitude.
The residual magnetism is required to start the process.

It's useful to look at automatic transfer switches to see what's important.
Oil level/pressure
Frequency
Amplitude

are the top requirements.

Then add:

Water temp.
Oil pressure

A few things have to met before an actual transfer occurs and weekly testing is a given. It may find no fuel or mice that have made dinner out of a circuit board or wires.

 

[large_ghostman]

If you have a battery involved already, I think your options increase. Much like a car alternator, you can make a high voltage DC bus and then go inverter technology.
The car alternator pulses the rotor field and 3 phase rectifies the output to DC. Residual magnetism does not play a role.

One of the commonalities of a stand-alone, non battery genset is the concept of a partially magnetized rotor. So, one of the reasons a genset doesn't generate power is thhat the field needs to be flashed to create the residual magnetic field.


That residual field is what gets the control circuits humming.
RPM controls frequency.
The field controls amplitude.
The residual magnetism is required to start the process.

It's useful to look at automatic transfer switches to see what's important.
Oil level/pressure
Frequency
Amplitude

are the top requirements.

Then add:

Water temp.
Oil pressure

A few things have to met before an actual transfer occurs and weekly testing is a given. It may find no fuel or mice that have made dinner out of a circuit board or wires.

The engine is a old kabota compact tractor one, with most the bits , also we have all kinds of tractor spares, so automating a engine shut down for say, oil pressure, low fuel etc would be really easy. Mice is easy , the rest needs a good google session to get the head around it!

 

[cowboybob]

I know I'll catch some flack for this, but...

As to any potential frequency concerns (50hz your side, 60hz mine):

If your loads are resistive, frequency is, essentially, irrelevant. For any inductive loads (motors) you'll get slightly higher RPMs (50 to 60 ups it times 1.2). This matters little for most devices. For most other home inductive devices, such as transformers (single phase) there are no significant issues.

Obviously devices "timed" by frequency (e.g., motor driven clocks) DO care about line frequency.

That said, don't rule out old US (60hz) Military genset auction items that may be available and, I suspect, dirt cheap.

I base this on the use of AEG (European) kitchen appliances (freezer, fridge, oven [with built-in fan], dishwasher, all rated at 230VAC, 50hz)) in my home for the past 32 years. The dishwasher finally died last year . So sad...

 

[mab2]

the inverter / battery / engine & alternator route has the advantage that you can have short term power from the battery and longer term from the engine, and also that the inverter should give you a regular 230v/50Hz. But if you still want 750w then you would need to maintain a stonking big battery whether or not you're actually using it.

If the power is 750w of heaters and small motors then the direct engine/generator route has some merit. You can use a standard induction motor as a generator - I use one on my hydro-electric project - google induction motor as generator. However whilst it is a cheap form of generator, it is not entirely straightforward as the output voltage is rather dependant on load, and it has very limited surge capability so can only start relatively small motors.

 

[large_ghostman]

Nothing I would run would matter regarding frequency , also most stuff i have including alot of test equipment has a switch to run at 120V 60hz

 

[large_ghostman]

the inverter / battery / engine & alternator route has the advantage that you can have short term power from the battery and longer term from the engine, and also that the inverter should give you a regular 230v/50Hz. But if you still want 750w then you would need to maintain a stonking big battery whether or not you're actually using it.

If the power is 750w of heaters and small motors then the direct engine/generator route has some merit. You can use a standard induction motor as a generator - I use one on my hydro-electric project - google induction motor as generator. However whilst it is a cheap form of generator, it is not entirely straightforward as the output voltage is rather dependant on load, and it has very limited surge capability so can only start relatively small motors.

Good news! biggest single motor is 240 mA, so no issue there , maybe i could do a hybrid system? i will google the induction motor. We have a stream running through our land, and we are on a hill. Dad has already mentioned very small hydro a neighbor has a small one that puts out 2KWh

 

[KeepItSimpleStupid]

50 Hz stuff on 60 Hz is usually no problem. 50 Hz stuff on 60 Hz power can be. 50 Hz stuff weighs a little more.