Modified Sine Wave

Modified Sine Wave is a term that you'll see tossed around when people are speaking or writing about inverters.

What it means is that the inverter produces the 'right' amount of power, at the right 'frequency' but with the 'wrong' waveform. Wrong in this case means as compared with what the grid normally would produce. A waveform as sampled at your normal grid powered household outlet is a very pure waveform called a 'sinewave'. This sinewave goes up and down in a very predictable way, has an exact period during which it does its rise and fall and has a minimum of 'harmonics'. A 'Modified Sine Wave' usually produces something that tries to approximate the sinewave but would more properly be called a square wave (at best).

The difference between the two is not really appearant until you try to drive an inductive load (such as an induction motor). The motor will try to follow the 'modified sinewave', but it really can not and any discrepancy between the input wave and the actual motor movement will show up as heat in the motor windings (and in the inverter!). This is a little bit comparable to driving your car with the clutch pedal slipping all the time. In the best case this will lead to inefficiency, in the worst case either your motor, inverter or both will burn up.

The practical upshot of all this is that if you know in advance that you will be driving inductive loads then you're much better off with a 'pure sine wave' inverter, but beware they are *MUCH* more expensive.