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What is a Power Inverter?

A power inverter is an electrical device that converts a direct current (DC) power source into an alternating current (AC)power source. In typical DC/AC power conversion applications, power inverters are used to convert DC power from sources such as batteries, solar panels, or high voltage DC transmission lines, into consumer AC that’s used to run appliances, tools and other residential and commercial electrical products. Other inverter applications include variable-frequency motor drives (VFD), in which the inverter not only converts from DC to AC power, but also provides control of the AC output voltage and frequency.

Types of Inverters

Inverters are typically classified into three categories by the quality of the output waveform they produce; square wave, modified sine wave, and sine wave. Square wave inverters produce the poorest quality outputs and are all but obsolete. Modified sine wave inverters use simpler, less expensive circuits to approximate a sine wave output by generating square waves with dead spots between the half cycles (see the figure below). Sine wave inverters produce the best quality outputs, approaching pure sine waves, with typical total harmonic distortion (THD) better than 3%. Comparatively, modified sine wave inverter THD is typically 24%. Since most AC loads are designed to run on sine wave utility power, the higher THD can cause incorrect operation, overheating, and premature failure.

How Sine Wave Inverters Work

Most modern sine wave inverters employ a voltage source topology and are referred to as voltage source inverters (VSI). The VSI has proven to be more efficient, have higher reliability, and better performance characteristics than other topologies. The VSI operates by using semiconductor switches, such as MOSFETs or IGBTs, to switch a DC voltage source onto the output. Pulse width modulation (PWM) is used to vary the on and off times of the switches, thereby controlling the average voltage of the output waveform. The switched PWM waveform is then filtered to produce a sine wave output. Note that the PWM frequency must be significantly higher than that of the sine wave to be synthesized, in order to filter it effectively, The figure below illustrates the VSI topology for a single phase inverter. The DC source can be provided from batteries, solar panels, or line-fed rectifiers depending on the application.

The control block is used to generate the PWM switch commands. There are many different control techniques that can be used to modulate the switches depending on the application and system requirements. This introduction to power inverters is part one of a series of blog posts about inverters. In future posts we’ll discuss specific applications, including grid-tie, AFE/active rectifier, VMI/stand alone and motor drive inverters, as well as the different control techniques commonly employed.