Difference Between Pure & Modified Sine Wave Inverters
Pure Sine Wave
Normal AC mains, given to us from Eskom is of a pure sine wave nature. It has very little harmonic distortion, subject to outside interference, such as lightning and large AC loads etc. All AC equipment in the stores today has been designed to run off a pure sine wave form, including computers, motors, lights etc.
The internal components of a pure sine wave inverter/UPS inverts the incoming DC voltage into a sine wave form, but because of the sophistication of these components, this makes the inverter/UPS more expensive.
Modified Sine Wave
Modified sine wave has been created to save cost. Modified sine wave inverters and UPS systems just switch their DC supply between positive and negative poles to give you a simulated sine wave form.
Modified sine wave inverters and UPS systems are very cheap to produce but could be very costly if installed onto the incorrect equipment, for example, motors, fluorescent globes with standard ballasts, anything with a high inrush, speed control motors or anything that has sensitive information stored on it, or is of a sensitive nature itself. These should not be installed onto a modified sine wave inverter/UPS.
Please note that some UPS systems most notably the 600VA, under-the-desk type UPS systems are modified sine wave, but because a PC only runs off the inverter for a short period, when there is a power failure, they are less likely to be damaged as a result from the modified wave form.