An inverter is an electrical device that converts direct current into alternating voltage. In addition to rectifiers, DC converters and converters, inverters form a subgroup of power converters. Depending on the application, inverters generate either a single-phase or multi-phase output current or a single- or multi-phase output voltage. The efficiency of semiconductor-based devices can reach up to 99 percent.
Inverters are used where electrical consumers require alternating voltage or current for operation, but only one DC voltage source is available. Important areas of application include the control of alternating current motors or in photovoltaics for converting the direct current generated by solar modules into alternating current, for feeding it into the public power supply grid or for direct consumption.
Photovoltaics
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A solar inverter is part of a solar system. On the input side, there are usually one or more DC controllers with a maximum PowerPoint tracker, which is controlled by a microprocessor and feeds the DC link. On the output side, there is a single- to three-phase inverter that automatically synchronizes with the power grid.
Uninterruptible Power Supplies (UPS)
A UPS contains an inverter that, in the simplest case, is connected to the consumers with a relay instead of the mains in the event of a power failure. The short switching pause of a few milliseconds is tolerated by most consumers. The inverter is powered by an accumulator (usually a special lead-acid accumulator), which is charged with a charging circuit when the grid is present and kept at the end-of-charge voltage.
Some UPSs work with a mains-frequency transistor chopper and a downstream mains-frequency transformer, while other devices use higher-frequency PWM inverters and are therefore lighter.
Frequency converters and grid recovery
The inverter is also used as a component of a frequency converter. Here, an alternating voltage of a different frequency is generated from an alternating voltage after rectification (DC link). This can be used, for example, to control the speed of an asynchronous motor. The energy when the motor brakes, which then work as a generator, is converted into heat in a braking resistor in simple frequency converters. In order to be able to feed this energy back into the grid instead, a grid-driven inverter can be connected to the DC link. The result is a 4-quadrant converter. Such converters can also be implemented without rectifiers and DC links (matrix converters).
On variable-speed hydro or wind turbines, a 4-quadrant converter is also required.
Inverters in motor vehicles
Inverters for use in motor vehicles are designed to be connected to the on-board voltage socket (cigarette lighter socket) or for fixed connection (motorhomes, buses, trucks). They are available for 12 volts (cars) and 24 volts (trucks, buses). For some car models, built-in inverters with a socket for Euro plugs or an earthed socket are available as an option.
When inverters are operated via the 12-volt cigarette lighter socket, which is usually fused with 15 A, the power is limited to about 100 to 150 watts. The discharge leads to a higher internal resistance of the on-board battery, which is why the starting of the engine can be affected. Starter batteries also have short cycle lifetimes.
When operating with the engine running, the alternator can supply considerable electricity, but this is economically and ecologically unfavorable.
Lighting
The inverter, here in the form of a resonance converter, is used as an electronic ballast in fluorescent lamps with powers in the range of a few 10 W. Another major application of these inverters is the power supply of fluorescent tubes (CCFL), which are often used as backlights for TFT flat panel displays.
