Understanding & Using Solar DC-AC Inverters ? Part 1

November 12th, 2010

DC-AC inverters are electronic devices used to produce .mains voltage. AC power from low voltage DC energy (from a battery or solar panel). This makes them very suitable for when you need to use AC power tools or appliances but the usual AC mains power is not available. Examples include operating appliances in caravans and mobile homes, and also running audio, video and computing equipment in remote areas.

Most inverters do their job by performing two main functions: first they convert the incoming DC into AC, and then they step up the resulting AC to mains voltage level using a transformer. And the goal of the designer is to have the inverter perform these functions as efficiently as possible . so that as much as possible of the energy drawn from the battery or solar panel is converted into main voltage AC, and as little as possible is wasted as heat.

Modern inverters use a basic circuit scheme like that shown in Fig.1. As you can see the DC from the battery is converted into AC very simply, by using a pair of power MOSFETs (Q1 and Q2) acting as very efficient electronic switches. The positive 13.8V DC from the battery is connected to the centre-tap of the transformer primary, while each MOSFET is connected between one end of the primary and earth (battery negative). So by switching on Q1, the battery current can be made to flow through the .top. half of the primary and to earth via Q1.

Conversely by switching on Q2 instead, the current is made to flow the opposite way through the .lower. half of the primary and to earth.

Therefore by switching the two MOSFETs on alternately, the current is made to flow first in one half of the primary and then in the other, producing an alternating magnetic flux in the transformer.s core. As a result a corresponding AC voltage is induced in the transformer.s secondary winding, and as the secondary has about 24 times the number of turns in the primary, the induced AC voltage is much higher: around 650V peak to peak.

By the way if you.re wondering why MOSFETs are used as the electronic switches, to convert the DC into AC, it.s because they make the most efficient high-current switches. When they.re .off. they are are virtually an open circuit, yet when they.re .on. they are very close to a short circuit (only a few milliohms). So very little power is wasted as heat.

(In DC-AC inverters designed to deliver high power, there are actually quite a few MOSFETs connected to each side of the transformer primary, to share the heavy current. However because they.re essentially connected in parallel, you can still think of them as behaving very much like the single transistors shown in Fig.1.

They just behave like very high-power MOSFETs, able to switch many tens of amps…)

Note that because the switching MOSFETs are simply being turned on and off, this type of inverter does not produce AC of the same .pure sinewave. type as the AC power mains. The output waveform is essentially alternating rectangular pulses, as you can see from Fig.2. However the width of the pulses and the spacing between them is chosen so that the ratio between the RMS value of the output waveform and its peak-to-peak value is actually quite similar to that of a pure sinewave. The resulting waveform is usually called a .modified sinewave., and as the RMS voltage is close to 230V many AC tools and appliances are able to operate from such a waveform without problems.

It.s true, though, that this kind of waveform is not close enough to a sinewave for some appliances. That.s mainly because the rectangular pulses contain not just the fundamental mains frequency, but quite a lot of its harmonics as well.

So if the inverter is operating at the Australia/New Zealand mains frequency of 50Hz, the output will also contain components at 100Hz, 150Hz, 200Hz, 250Hz and so on.

These harmonics can disturb the operation of some appliances. It.s because of this shortcoming that manufacturers have come up with a more complex type of inverter, which does deliver a pure sinewave output. More about these later.

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