An inverter (also Inverter or rotating Richter ) is an electrical device, the DC voltage in the AC voltage converted. In addition to rectifiers , DC voltage converters and converters, inverters form a subgroup of power converters .
Depending on the application, inverters generate either a single 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 loads require alternating voltage or current for operation, but only a direct voltage source is available. Important areas of application include the control of AC motors or in photovoltaics for converting the direct current obtained from solar modules into alternating current, for feeding into the public power supply network or for direct consumption.
There are two types of control of inverters:
- Self-commutated inverters, also stand-alone inverters , use transistors , for example IGBTs . They are used to convert direct voltage into alternating voltage, and the reverse is also possible. Since the valves can be switched on and off with a cycle generated by the inverter itself, no reference from the grid is necessary. Self-guided inverters can thus be used to generate an alternating voltage independently of the power grid and create a so-called island grid (lead - see grid management).
- Third-party or line-commutated inverters usually also use IGBT , but also thyristors or triacs . To function, they require a fixed alternating voltage in the network and draw so-called commutation reactive power . They are used to feed energy from the direct voltage side into the alternating current network; the reverse direction is often also possible. This type has a shutdown of the system in the event of mains disturbances. This avoids overvoltage or voltage in switched-off network sections. This is regulated in the VDE standard 0126 , see device for network monitoring with assigned switching devices .
Application examples for self-guided inverters
- Mountain huts, weather stations without grid connection, mobile devices, inverters in mobile homes and on boats
- Uninterruptible power supplies in hospitals, power plants and data centers
Application examples for externally managed inverters
- to feedable (grid-connected) photovoltaic systems and fuel cells
- for network coupling of wind power plants with variable speed and DC voltage intermediate circuit
- for energy recovery ( braking energy use ) on 2-quadrant frequency converters
Form of the output voltage of self-commutated inverters
Nowadays, three output voltage forms are common for self-commutated inverters:
- Rectangular and trapezoidal shape:
- Square-wave and trapezoidal inverters are often used to generate alternating voltage with an effective value equal to the mains voltage from direct voltage sources (e.g. 12-volt batteries).
- Square-wave inverters generate a square-wave voltage.
- Trapezoidal inverters (they are also known as modified sine or quasi-sine wave inverters in the trade) also generate a square-wave voltage, although there is a pause between the positive and negative rectangle (see adjacent picture).
- Square-wave and trapezoidal inverters can be manufactured more cost-effectively than sinusoidal inverters, since the complex pulse width modulation that is present in the sinusoidal inverter is dispensed with. The square output voltage is problematic for some devices that are operated with it because it deviates significantly from the sinusoidal voltage. Transformers, motors and heating devices can be operated with square-wave voltage, but the steep voltage increases cause interference emissions. Such inverters are unproblematic for devices that behave ohmically (e.g. incandescent lamps, heaters). The problem with trapezoidal inverters are devices that control their power using triacs (vacuum cleaners, some modern coffee machines) - they work in a limited manner or not at all. Some special monitors or refrigerators with an electric thermostat sometimes recognize a difference and indicate a fault if there is no sinusoidal alternating voltage.
- Sinus inverters generate a sinusoidal alternating voltage from a direct voltage. They are suitable for all devices, including those with capacitive behavior (LED lamps, compact fluorescent lamps, switched-mode power supplies). Sinus inverters also generate interference, but these are minor. The interference is caused by the fact that the sinusoid is synthesized by means of a pulse-width modulated square wave voltage, usually in the two-digit kHz range .
With inductive loads (motors, refrigerators, tools), the peak power of the inverter must be sufficiently high due to the starting current. Higher-quality models take into account the need to require a starting current that is around ten times as high for milliseconds. They can withstand a load three times as high as their specified continuous rated power for a short time.
A solar inverter is part of a solar system . On the input side there is usually one or more DC choppers with a maximum power point tracker, which a microprocessor controls and feeds the intermediate circuit. A one to three-phase inverter is located on the output side and is automatically synchronized with the power grid.
Uninterruptible power supplies (UPS)
A UPS contains an inverter which, in the simplest case, is switched to the consumer with a relay instead of the mains in the event of a power failure. The short switchover pause of a few milliseconds is tolerated by most consumers. The inverter consists of an accumulator, which is charged with a charging circuit when the grid is available and kept at the end-of-charge voltage. Older UPSs worked with a line-frequency transistor chopper and a subsequent line-frequency transformer; today's devices use higher-frequency PWM inverters and are therefore lighter.
Frequency converter and regenerative power supply
The inverter is also used as a component of a frequency converter . Here, an alternating voltage with a different frequency is generated from an alternating voltage after rectification ( intermediate circuit ). This can be used to control the speed of an asynchronous motor , for example . The energy when braking the motor, which then works as a generator, is converted into heat in simple frequency converters in a braking resistor. In order to be able to feed this energy back into the grid instead, a grid-commutated inverter can be connected to the intermediate circuit. A 4-quadrant converter is created. Such converters can also be implemented without a rectifier and intermediate circuit (matrix converter).
A 4-quadrant converter is also required on variable-speed hydro or wind power plants.
Inverters in motor vehicles
Inverters for use in motor vehicles are usually designed for connection to the cigarette lighter or for permanent connection ( mobile homes , buses, trucks). They are available for 12 volts (cars) and 24 volts (trucks, buses).
The first vehicle manufacturer to offer an inverter with a mains voltage of 230 V in a series car was Volkswagen AG . In the meantime, inverters with a socket for a Euro plug are available as special equipment for various car models , inverters with household sockets are also available.
When operating inverters with higher power via the cigarette lighter on a 12-volt on-board network, it should be noted that at the low voltage of 12 V, a very high current must be carried (heat generation, contact load). The cigarette lighter is usually fused with 15 A and should not be permanently loaded with more than 10 A in order to keep the contact heating within limits. This means that only consumers with up to 100 to 150 watts of continuous power consumption can be operated on the cigarette lighter. In addition, the heavy load on the on-board battery and its short cycle life must be taken into account. A depth of discharge of less than 30% should be avoided; this means that a normal 50 Ah battery can be used for max. Take 35 Ah .
When the engine is running, it must be noted that the alternator can deliver a charging current in the range of 50 A, but a considerable part is absorbed by the lighting and other consumers, which means that the battery can also discharge at a load current of 20 A.
Inverters that are intended for use in motor vehicles must have an E label. This symbol indicates that the component has passed the required tests and approvals and can therefore be installed in motor vehicles.
Construction then and now
Inverters can be implemented electromechanically as a chopper or motor generator or electronically with tubes or semiconductors . In the earlier common chopper (contact inverter), a mechanical contact periodically reverses the polarity of the DC voltage supplied with a Wagner hammer . The turbo inverter reduced the contact wear that occurred . The periodically switching contacts are replaced by a mercury beam that rotates in a closed chamber operated by a motor.
With electron tubes
Inverters implemented with vacuum tubes are only suitable for smaller outputs, are mechanically sensitive and have rarely been built. Inverters of higher power were realized with controllable mercury valves ( thyratrons ). Thyristors (with quenching thyristor or GTO ) were later used for this purpose .
All these frequency converters worked with the frequency of the alternating voltage to be generated and could not generate a sinusoidal output voltage. The switching frequency of these inverters is therefore limited to a few hundred Hertz ; they mostly work with 50 Hz. Power transistors ( bipolar transistors , MOSFET , IGBT ) can chop up the DC voltage with high efficiency and without wear, they work, etc. a. in UPS in square wave operation with 50 Hz and, like the chopper in the past, fed a 50 Hz transformer. Such a circuit would be e.g. B. a four-quadrant actuator .
However, transistors also allow switching frequencies of up to a few 10 kHz and then work in chopper mode. This is also known as the undershoot method: with the transistors used as switching elements (mostly IGBT), a sinusoidal alternating voltage from short high-frequency pulses (some up to over 20 kHz) is simulated by pulse width modulation (PWM) in chopper mode (sinusoidal inverter). The transistors, like the chopper in the past, periodically reverse the DC voltage, but at a higher frequency. The mean value of the high-frequency, pulse-width-modulated switching frequency is the output AC voltage. The output alternating voltage is thus composed of small pulses of different widths and thus approximates the normal sinusoidal voltage curve. Chokes are used to smooth the PWM, but they are much smaller than those that were required to smooth the output AC voltage of earlier inverters. There is no need for a throttle for motors. The basic circuits can be found in switched-mode power supplies . The difference lies in the modulated reference voltage used to control the output voltage.
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