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Do you want to make it yourself, or purchase a commercial unit?
Power inverters are a popular consumer item. Everybody wants reliability and low cost. It practically invites manufacturers (at least a few of them) to offer a substandard unit and claim 'it's rated to provide the power you want, and it's durable for a lifetime'.
I purchased one of these. Rated 1500 W. I think it really was more like 700W. It could not get a refrigerator to run.
Another inverter I got was rated 1000 W. It could run a refrigerator.
This is a popular project. See the list of related threads, further down this page. Or try a search on SPWM (or sine PWM). That should lead you to helpful information.
It has become common to run sine PWM through a transformer, then smooth the output. The transformer does not carry 50Hz, but high frequency PWM. This allows it to be smaller and lighter and less expensive.
Hello Brad, if you are modulating PWM to create 50/60Hz and feeding it thru a mains transformer, then the transformer sees the LF energy (50/60Hz), the only way around this is to use a HF switch-mode transformer to create 380VDc, or so, and then use an H bridge to create the LF sine wave and filter to remove the switching frequency...
A quick sim will convince you that if you want 50/60Hz to come out of a Tx, then that Tx does handle all the 50/60Hz energy...
sorry Brad, for a 1kVA inverter using a 50/60Hz transformer, the transformer rating must be 1kVA no matter how the fundamental is generated...there is no saving...
sorry Brad, for a 1kVA inverter using a 50/60Hz transformer, the transformer rating must be 1kVA no matter how the fundamental is generated...there is no saving...
Yes, watts are watts. I admit I wouldn't know how to design a transformer to carry 1kVA, whether it was at 50 Hz or 50 kHz. I know there would be saturation current to consider, and primary and secondary Henry value, and metal mass, and type of metal, etc.
We can see that SPWM is an alluring concept to so many people who want to make their own step-up inverter.
Evidently SPWM it must offer several advantages. Because there are not many people coming here saying 'I want to build a big, bulky inverter with a 50 Hz transformer, like commercial units made decades ago.'
What you call SPWM can only be applied only in two ways to enact an inverter, 1) to a set of 4 fets running off a 350V bus, to provide the desired Vac out, needs to be filtered to remove the sw freq, and,
2) on a low volt set of fets intended to drive a transformer to get 230Vac (for eg) at 50/60Hz, filtering still needed somewhere, either LV side or HV side.
If the latter then the Tx needs to rated as for a normal mains Tx + the sw losses it will incur.
For 1) above some where a step up stage has been used to create the 350VDC
For 2) the mains Tx does the step up, many commercial inverters and grid tied inverters use a mains Tx this way.
1) to a set of 4 fets running off a 350V bus, to provide the desired Vac out, needs to be filtered to remove the sw freq, and,
2) on a low volt set of fets intended to drive a transformer to get 230Vac (for eg) at 50/60Hz
There have been posters who discussed building these topologies. I have run simulations and I agree they work.
Here is the simulation that demonstrates SPWM through a step-up transformer.
I chose a low carrier frequency so that the waveforms are obvious.
The primary has a small Henry value.
The Ampere waveform going through the primary is of interest. It is a sinewave with spikes superimposed on it. They take on the SPWM pattern.
As it is, however, the spikes make it necessary to have a higher saturation current.
The spikes would be less prominent if the primary inductance were greater. The transformer could get by with a lower saturation current.
I wouldn't present it as an optimum design, but I believe I remember one or more posters who built (or thought they could build) an inverter something like this. The idea is just too appealing, to think it might permit us to make the transformer smaller.
It is tempting to think you can use a smaller transformer, but of course you can't get 50/60Hz out of a small switch-mode type Tx, the average volt seconds over one half cycle dictate the turns and core size, when this is 8.333 or 10mS you are stuck with iron & Cu.
There is one partial exception, say you have a forward converter (or any converter really) and you modulate the output so that it makes positive half sine waves at its o/p, 0 - 325 - 0 - 325...etc then feed this to an H bridge and use the switches to "unfold" this to your load (the mains) so that it matches the mains polarity. This is what Enphase do in their 250W micro-inverters, they use a very high efficiency resonant flyback to generate the "rectified" sine o/p....and 4 x igbt's to unfold it to the mains...
There is one partial exception, say you have a forward converter (or any converter really) and you modulate the output so that it makes positive half sine waves at its o/p, 0 - 325 - 0 - 325...etc then feed this to an H bridge and use the switches to "unfold" this to your load (the mains) so that it matches the mains polarity.
I am experimenting with a simulator, to find an efficient method to apply the concept. It looks like it could be a way to avoid the large bulky transformer/ inductor.
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