Inverter (electrical)

For the digital logic gate, see Inverter (logic gate).

An inverter is a circuit for converting direct current (DC) to alternating current (AC). Inverters are used in a wide range of applications, from small switched power supplies for a computer to large industrial applications to transport bulk power.

Inverter circuit description

Basic inverter designs

In one simple inverter circuit, DC power is connected to a transformer through the center 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 electromechanical version of the switching device includes two stationary contacts and a spring supported moving contact. The spring holds the movable contact against one of the stationary contacts and an electromagnet pulls the moveable contact to the opposite stationary contact. The current in the electromagnet is interrupted by the action of the switch so that the switch continually switches rapidly back and forth. This type of electromechanical inverter switch, called a buzzer, was once used in automobile radios. A similar mechanism has been used in door bells, buzzers and tattoo guns.

These electromechanical inverters explain the source of of the term "inverter". Early AC to DC converters combined a synchronous AC motor with a commutator so that the commutator reversed its connections to the AC line exactly twice per cycle. This results in AC-in, DC-out. If you invert the connections to a converter you put DC in and get AC out. Hence an inverter is an inverted converter. ^*

As they became available, transistors and various other types of semiconductor switches have been incorporated into inverter circuit designs.

More advanced inverter designs

In more advanced inverter designs various techniques are used to improve the quality of the sine wave at the transformer input, rather than relying on the transformer to smooth it. Capacitors and inductors (but not freewheel diode as it is AC) can be used to filter the waveform at the primary of the transformer. 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 inverters 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, and are thus often used in these areas. Nevertheless, there do exist inverters with greater pulse ratings.

Simple inverters generate harmonics which affect the quality of power obtained using them. But PWM inverters eliminate this by means of a sine wave cancellation using the properties of Fourier Series.

Inverter applications

The following are examples of inverter applications.

DC power source utilization

An inverter allows the 12 volt DC power available in an automobile to supply AC power to operate equipment that is normally supplied from a mains power source.

Inverters are also used to provide a source of AC power from solar cell and fuel cell power supplies.

Uninterruptible power supplies

One type of uninterruptible power supply uses batteries to store power and an inverter to supply AC power from the batteries when mains power is not available. When mains power is restored, a rectifier is used to supply DC power to recharge the batteries.

Induction heating

Inverters are used to convert low frequency mains AC power to a higher frequency for use in induction heating. To do this, AC power is first rectified to provide DC power. The inverter then changes the DC power to high frequency AC power.

High-voltage direct current (HVDC) power transmission

With HVDC power transmission, AC power is rectified and high voltage DC power is transmitted to another location. At the receiving location, an inverter in a static inverter plant converts the power back to AC.

Variable frequency drives

A variable frequency drive controls the operating speed of an AC motor by controlling the frequency and voltage of the power supplied to the motor. An inverter provides the controlled power. In most cases, the variable frequency drive includes a rectifier so that DC power for the inverter can be provided from mains AC power. Since an inverter is the key component, variable frequency drives are sometimes called inverter drives or just inverters.

Textbooks

External links

VESA Announces Formation of LCD Inverter Special Interest Group
Solar Panel Inverters explained in great detail.

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