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Modified Sine Wave V’s Pure Sine Wave

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So, you have wanted an inverter to run a 240 volt device off road for a while. You will need a pure sine wave inverter but will a cheaper modified sine wave inverter do it?
Before we answer that question, let’s have a quick gander at what AC (short for alternating current) is.
AC is a current that repeatedly changes direction. This applies to the output of both pure- and modified-sine-wave inverters.

sine wave.jpgWhat sets the two apart, is how the current changes direction and how long it stays level. See the image to the left. As you can see, the pure sine wave features a smooth, flowing rhythm. It’s similar to what you’d think of as a “wave”. Consequently, it’s also called a “true” sine wave.

This is similar to the power point at home, and it is what most household appliances are designed to run on. In contrast to this, the modified sine wave features prolonged highs and lows as well as plateaus at zero voltage, giving it a rather squarish look. Not surprising, then, that it’s also called a “square” sine wave. Some appliances are compatible with a modified sine wave; others are not. As a general rule, the more complex the appliance, the likelier it is that it requires a pure sine wave. To be absolutely sure, you should always go by what the manufacturer says. If in doubt a pure sine wave inverter is a safer option for just a few dollars more.

Modified-sine-wave inverters
MSW inverters utilise filters to round the corners of a square wave; hence the word “modified”. As previously mentioned, however, the shape of the wave remains quite square.

Because of the plateauing peak outputs, appliances running on a modified sine wave will have to deal with more power for a longer time, and this equals additional heat. For this reason, many appliances that are designed to run on grid power will overheat if run on a modified sine wave.

Pure-sine-wave inverters

Manufacturing a PSW inverter is a lot more involved than making an MSW inverter, and this translates into a higher price. What you get for the additional cost is peace of mind. Appliances are getting increasingly complex; these days, even seemingly simple devices feature advanced microprocessors, and, often MSW will not agree with these microprocessors. A PSW is the only safe choice.

For example, many devices rely on a PSW to time their operation by counting how often the wave passes through zero voltage. This works well on the smooth grid AC. But when such devices are run off an MSW inverter, their microprocessors are tricked by the MSW’s plateaus at zero voltage, which results in miscalculations of time, leading to poor performance and shorter product lifespan.

In short

  • Modified-sine-wave inverters are relatively simple and cheap products that generally will use battery power more efficiently than pure-sine-wave inverters.
  • Only basic products such as normal lights bulbs and induction or shunt motors can safely be run on a modified sine wave.
  • Pure-sine-wave inverters require many components and therefore come at a higher cost. They produce current that is close to identical to that of grid AC, making them perfect for running sensitive electronics.
  • If in doubt as to whether your appliances can run on a modified sine wave, always check with the manufacturer.

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