How Does A Pure Sine Wave Inverter Work ?
Inverters in Renewable Energy
Inverters are a very important part of the transition to renewable energy. They are necessary because solar panels give a direct current (DC) power output, which basically means the current flows one way. However, nearly all of our homes and businesses use alternating current (AC) power, where the current flows in both directions at a given frequency. Inverters sit between the solar array and the house or business, converting the DC output from the solar panels into useable AC output.
Development of inverter technology has been a key part of the explosion in renewable energy. Early inverters were expensive, inefficient (throwing power away and heating up) and problematic. Even now, problems with inverters are the most common type of problem experienced by owners of solar arrays. Modern inverters are more efficient, cheaper, smaller, smarter and much more reliable than their earlier counterparts.
DC Power vs AC Power
DC power is pretty self-explanatory. The current runs one way only. In the case of solar cells, the current will vary fairly slowly through the day as the suns’ intensity changes, but the current will always flow the one way. If we plot current vs time, we get the DC graph shown below.
AC power is different. The current not only flows both ways, but it’s intensity changes rapidly. When current is plotted against time, the curve forms a ‘wave’. There are all sorts of different types of waves for AC power. However the type of wave that we use in our homes and businesses is called a ‘sine wave’. The AC curve in the figure below is a sine wave.
The inverter’s job is to take the DC power and convert it to an AC power curve.
Converting DC Power to AC Power
Early inverters used mechanical switches to create simple versions of AC power, and there are some (cheap) inverters using mechanical switches still available today. The simplest version just switches on and off, producing the ‘chopped’ waveform shown below. For higher frequency, the switch turns on and off more rapidly.
The next step up is instead of turning the current off, the switch is more complex and actually reverses the current. This converts to the DC current to an alternating ‘square wave’ current. Again, the frequency can be adjusted by changing how fast the switch operates.
Some types of equipment without sensitive electronics can run on this type of power. However homes and businesses need their AC power to be more like a sine wave.
Sine Wave Inverters
Changing DC current to sine wave AC current requires more complex electronics. The figure below is a circuit diagram for a ‘do-it-yourself’ sine wave inverter.
Sine wave inverters work in three stages: the oscillator stage, the booster or amplifier stage, and finally the transformer stage.
The oscillator stage does what the title says it does: changes the DC current to an oscillating AC current. The oscillating current can be set to a particular frequency: for the United States the frequency is 60 Hz. This means there are 60 full waves per second. The DC current is converted to this type of AC current using integrated circuits. However at this stage the oscillations, or wave heights are quite small, too small to power anything useful. The wave heights need to be increased, hence the next stage.
The booster stage simply takes the signal from the oscillator stage and amplifies it. This creates waveforms with much higher wave heights, high enough for useful power. However there is one thing left to get right before the power can go to a home or business: the voltage.
The final transformer stage gets the voltage right. A typical residential array may have DC voltages up to about 600V. Commercial arrays can have even higher voltages, for example 1000V or even higher. In the United States, AC power is delivered at 120 V. Stability of this voltage is very important for stability of the grid and equipment that runs off the grid. Hence voltage control is a very important part of an inverter.
