Sunday, August 16, 2009


An inverter is an electrical device that converts direct current (DC) to alternating current (AC);

the resulting AC can be at any required voltage and frequency with the use of appropriate transformers, switching, and control circuits.

Static inverters have no moving parts and are used in a wide range of applications, from small switching power supplies in computers, to large electric utility high-voltage direct current applications that transport bulk power. Inverters are commonly used to supply AC power from DC sources such as solar panels or batteries.

The electrical inverter is a high-power electronic oscillator. It is so named because early mechanical AC to DC converters were made to work in reverse, and thus were "inverted", to convert DC to AC.

The inverter performs the opposite function of a rectifier.

In one simple inverter circuit, DC power is connected to a transformer through the centre tap of the primary winding.

A switch is rapidly switched back and forth to allow current to flow back to the DC source following two alternate paths through one end of the primary winding and then the other.

The alternation of the direction of current in the primary winding of the transformer produces alternating current (AC) in the secondary circuit.

The switch in the simple inverter described above produces a square

voltage waveform as opposed to the sinusoidal waveform that is the usual waveform of an AC power supply.

Using Fourier analysis, periodic waveforms are represented as the sum of an infinite series of sine waves.

\mbox{THD} =  {\sqrt{V_2^2 + V_3^2 + V_4^2 + \cdots + V_n^2} \over V_1}

The sine wave that has the same frequency as the original waveform is called the fundamental component. T

he other sine waves, called harmonics that are included in the series have frequencies that are integral multiples of the fundamental frequency.

The quality of the inverter output waveform can be expressed by using the Fourier analysis data to calculate the total harmonic distortion (THD).

The total harmonic distortion is the square root of the sum of the squares of the harmonic voltages divided by the fundamental voltage:

\mbox{THD} =  {\sqrt{V_2^2 + V_3^2 + V_4^2 + \cdots + V_n^2} \over V_1}

The quality of output waveform that is needed from an inverter depends on the characteristics of the connected load. Some loads need a nearly perfect sine wave voltage supply in order to work properly. Other loads may work quite well with a square wave voltage.