Inverter
An inverter is a circuit for converting direct current electrical power to alternating current. An inverter can have one or two switched-mode power supplies (SMPS).
Most inverters consist of an oscillator driving a transistor, that is used to interrupt the incoming direct current to create a square wave. This is then fed through a transformer to smooth the square wave into a sine wave and to produce the required output voltage.
More efficient inverters use various tricks to try to get a reasonable sine wave at the transformer input rather than relying on the transformer to smooth it. Capacitors can be used to smooth the flow of current into and out of the transistor. Also, it is possible to produce a more sinusoidal wave by having split-rail direct current inputs at two voltages, or positive and negative inputs with a central ground. By connecting the transformer input terminals in sequence between the positive rail and ground, the positive rail and the negative rail, the ground rail and the negative rail, then both to the ground rail, a stepped sinusoid is generated at the transformer input and the current drain on the direct current supply is less choppy. These methods result in an output that is called a "modified-sinewave." Modified-sine invertors may cause some loads, such as motors, to operate less efficiently.
More expensive, true sinewave inverters use Pulse Width Modulation (PWM) with a high frequency carrier to more closely approximate a sine function. Given that utility generated power is subject to external, distorting influences such as inductive loads, a true sine inverter may even offer a better, "cleaner" output than the grid.
See also Switched-mode power supply; SMPS, Power converter.
