Power inverter

An inverter (also inverter ) is an electrical device, the DC voltage to AC voltage, ie direct current into alternating umrichtet. Form inverters, rectifiers and inverters in addition, a subgroup of the current judges.

• 3.1 photovoltaic
• 3.2 Uninterruptible Power Supplies (UPS)
• 3.3 Frequency and power recovery
• 3.4 inverters in motor vehicles
• 3.5 lighting

• 4.1 Mechanical
• 4.2 With electron tubes
• 4.3 With semiconductors

General

AC inverters may be designed depending on the circuit for the generation of single-phase or three-phase (three-phase ). Models with semiconductors made ​​of silicon carbide achieve efficiencies up to about 98 percent.

Apply inverter where an electrical consumer alternating voltage required for operation, but only a direct voltage source, such as a car battery, is available, or to direct current into an alternating or three-phase feed network.

Species

There are electromechanical vibrator, motor generator and electronic inverter with tubes or semiconductors ( principle of oscillator or astable multivibrator / multivibrator )

Control

There are two control types of inverters:

• Self-commutated inverters, also stand-alone inverter, use transistors, for example, IGBTs. They are used to convert DC voltage into AC voltage, as a side case, the reverse is also possible. Since the valves can be with a clock generated by the inverter itself turned on and off, no power is required by the reference. Self-commutated inverters can be used independently from the power supply and build a so-called off-grid so that for generating an alternating voltage ( lead - see network management ).

• Foreign - or line-commutated inverter using IGBT also mostly, but also thyristors or triacs. You need to function a fixed AC voltage in the network and obtain so-called commutation. They are used to feed power from the DC side to the AC mains, the reverse direction is often as possible. This type has a shutdown of the plant during grid faults. So overvoltage or voltage in the switched-off power sections is avoided. This is regulated in the VDE standard 0126, see device for grid monitoring with assigned switching elements.

• Mountain huts, weather stations without network connectivity, mobile devices, inverters in RVs, supply of rural areas in developing countries
• Uninterruptible power supplies in hospitals, power plants and data centers

• To infeeding ( grid-connected ) photovoltaic systems and fuel cells
• For grid connection of wind turbines with variable speed and variable DC voltage intermediate circuit
• For energy recovery (braking energy use) in 2 -quadrant drives

Of the output voltage self-commutated inverter

Today, three output voltage forms of self-commutated inverters are common:

• Rectangular and trapezoidal shape:

• Sinusoidal shape

For motors (refrigerators, tools), the peak power of the inverter must be high enough due to the starting current. The need to require an approximately ten times as high start-up power for milliseconds, carry higher quality models in accounting. They tolerate briefly three times as high as their load duration specified rating.

Applications

Photovoltaic

A solar inverter is part of a solar system. On the input side there is usually one or more DC-DC converter with maximum power point tracker, the controls a microprocessor and the DC input. On the output side there is a one-to three-phase inverter and is automatically synchronized with the power grid.

Uninterruptible Power Supplies (UPS)

A UPS contains an inverter which is switched on power failure in the simplest case with a relay instead of the network to consumers. The short changeover delay of a few milliseconds can be tolerated by most consumers. The inverter operates from a rechargeable battery which is charged with mains supply with a charging circuit and held on the final charge voltage. Older UPS worked with a line-frequency transistor - chopper and a subsequent line-frequency transformer, current devices use higher-frequency PWM inverter and are therefore easier.

Frequency and power recovery

Another use of the inverter as a component of a frequency converter. Here is an AC voltage other frequency is generated from an AC voltage after rectification ( DC ). Thus, for example, can be regulated in an asynchronous motor speed. The energy during deceleration of the engine, he then acts as a generator, is converted to simple frequency converters in a braking resistor to heat. In order to feed this energy back into the grid instead, a line-commutated inverter can be connected to the DC link. The result is a 4-quadrant converter. Such inverters can be realized without rectifier and DC link (matrix - converter).

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 usually designed for connection to the cigarette lighter or for fixed connection ( RVs, buses, trucks). They are available for 12 volt ( car) and 24 volts (trucks, buses).

The first vehicle manufacturer to offer an inverter to the mains voltage 230 V in a production car, was the Volkswagen AG. Meanwhile, to have a power outlet for Euro connector as an option for various car models, inverter, inverter with Schuko sockets are also available.

In the operation of inverters of higher power via the cigarette lighter on a 12 - volt electrical system is to be noted that at the low voltage of 12 V, a very high current must be maintained ( heat, contact load ). The cigarette lighter is normally protected with 15 A and should be permanently loaded with no more than 10 A in order to keep the heating up of contacts within limits. It can be operated on the cigarette lighter so only consumers with up to about 100 to 150 watts of continuous power input. In addition, the strong stress on the onboard battery and its low cycle life is observed. A depth of discharge below 30% should be avoided, thus can be useful from a conventional 50 Ah battery max. Refer to 35 Ah.

With engine running, it should be noted that the alternator though can supply a charging current in the range of 50 A, a considerable portion is but picked up by lighting and other loads, which can already now take place a discharge of the battery under load current of 20A.

Lighting

Applies the inverter, in the form of a resonant converter, with power ranging from a few 10 W as electronic ballast in fluorescent lamps.

Another major area of ​​application of this inverter is the power supply of fluorescent tubes ( CCFL), which are often used as a backlight for TFT flat screens.

Construction of the past and today

Mechanically

Inverters can be realized electromechanically as a chopper or engine generator, or electronically with tubes or semiconductors. In the formerly used choppers ( Contact inverter), a mechanical contact periodically reverses the polarity to the applied DC voltage with a Wagnerian hammer. The contact wear occurring decreased the turbo inverter. With him periodically switching contacts are replaced by a mercury jet which operated in a closed chamber by a motor rotates in a circle.

With electron tubes

Realized with vacuum tubes inverters are suitable only for small loads, are mechanically sensitive and were hardly built. Inverter higher power were realized with controllable mercury valves ( thyratrons ). Later was used for this purpose thyristors ( to or from GTO thyristor ).

With semiconductors

All of these worked in the clock frequency of the frequency of the AC voltage to be generated and could not produce a sinusoidal output voltage. These inverters are in the switching frequency is therefore limited to a few hundred hertz, mostly they worked with 50 Hz power transistors ( bipolar, MOSFET, IGBT) can chop the DC voltage with high efficiency and accomplish without wear, they worked among others in UPS in square-wave operation at 50 Hz and fed as well as earlier the chopper a 50 Hz transformer. Such a circuit would, for example, a four-quadrant controller.

However, transistors also allow switching frequencies up to several 10 kHz and then work in the chopper operation. This is also known as subharmonic method: With the transistors used as switching elements (usually IGBTs) is by pulse width modulation ( PWM) in the chopper operation a sinusoidal AC voltage of short pulses of high frequency (some up to about 20 kHz) is simulated ( sine wave inverter ). The transistors poland, as well as earlier the chopper, the DC voltage periodically around, but with higher frequency. The average of the high frequency pulse width modulated switching frequency, the output AC voltage. So We get together the AC output voltage of small pulses of different widths and so approaches to the net usual sinusoidal voltage curve. To smooth the PWM chokes serve, but these are much smaller than those that were required for smoothing the output AC voltage of previous inverter. For motors can be entirely dispensed with a choke. The basic circuits are found in switching power supplies. The difference is the modulated reference voltage for controlling the output voltage.