Inverters: Sine Wave and Square Wave, whatever that means!

Household mains is Alternating Current (AC) meaning that the current alternates in direction of flow, first one direction, then the other and so on. Because of the way electricity is generated using mechanical means and magnetic fields, if we were to measure the voltage of mains power and plot it in a graph against time, it would look like a "Sine Wave" (blue line in the graph below, where the black line is zero volts, above the line is positive voltage (measured from the active terminal/wire) and below the line is negative voltage). It is called a Sine Wave because it is the same 'curve' you would get if you plotted the values of Sine on a graph.



An inverter converts the Direct Current (DC) from the battery in your car, caravan and/or camper trailer into Alternating Current. Since the inverter begins with a strictly one-way (direct) current, the easiest way to produce an alternating current is to simply switch the current back and forth. If we plot the voltage of the output of a simple inverter against time, we would get what is called a "Square Wave" (yellow line). As you might imagine, to suddenly switch the full voltage back and forth could effectively 'shock load' some electrical equipment.

Many inverters try to reduce this effect by switching the current "off" for a moment before switching it back the other direction. If we were to plot the voltage of the output of this type of inverter against time, it would result in what we call a "Modified Sine Wave" (red line).

While it is possible to get inverters that produce a Pure Sine Wave, they are more expensive to manufacture, and therefore more costly to buy. Most appliances will operate satisfactorily (at least in the short term) from a Square or Modified Sine Wave inverter. This combined with the cheaper manufacturing costs means they are the most common inverters available.

Microwave ovens can take longer to heat/cook food when operating from a Square or Modified Sine Wave inverter since the peak voltage is not as great. Note that the average voltage is 240V. Because the Square and Modified Sine Wave inverters spend more time at the peak voltage, a lower peak is required to acheive the 240V average. However, this reduced peak reduces the effectiveness of the microwave oven, sometimes by as much as half (meaning it takes twice as long to heat/cook your meal).

Some other effects of using a Square or Modified Sine Wave Invertor are as follows:
• Anything with a motor in it runs slowly and/or noisily (eg. fan)
• Fluoro lights 'buzz'
• The sound system has a 'hum' coming from the speakers
• The TV has lines across the screen
• The variable speed power drill is difficult to control at low speed
• The battery charger doesn't work/fails
• Some equipment (with transformers) can overheat due to the power spending more time at 'peak' voltage rather than peaking only momentarily while oscillating

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