Science Fair Project Encyclopedia
Inverter (electrical)
An inverter is a circuit for converting direct current to alternating current. They are used in a wide range of applications, from small power supplies for a computer to large industrial applications to transport bulk power. An inverter can have one or two switched-mode power supplies (SMPS).
Simple 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 slightly sine wave-shaped waveform and to produce the required output voltage. Advanced inverters have started using more advanced forms of transistors or similar devices such as thyristors.
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 and inductors 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 power inverters use Pulse Width Modulation (PWM) with a high frequency carrier to more closely approximate a sine function. The quality of an inverter is described by its pulse-rating: a 3-pulse is a very simple arrangement, utilising only 3 transistors, whereas a more complex 12-pulse system will give an almost exact sine wave. In remote areas where a utility generated power is subject to significant external, distorting influences such as inductive loads or semiconductor-rectifier loads, a 12-pulse inverter may even offer a better, "cleaner" output than the utility-supplied power grid.
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