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A converter , also known as AC inverter and on the basis of the English term as AC / AC converter called, is a power converter , which consists of a AC one in the frequency and amplitude generates different AC voltage. Converters are not to be confused with transformers , which can only change the amplitude but not the frequency of the voltage.

If the conversion is used for the direct supply of an electrical machine such as a three-phase motor in the context of electrical drive technology and the frequency is derived from the condition of the motor, the converter is specifically referred to as a frequency converter . It then includes additional functions for motor control, such as speed measurement and commutation processes to adapt the rotating field depending on the current state of the machine.

Converters and frequency converters are electronic devices without mechanically moving components. In contrast, a converter is a combination of rotating electrical machines, usually an electrical motor and an electrical generator , which electromechanically enables frequency conversion .


AC / AC converter
Converter with DC link
Hybrid Matrix Converter
Direct converter (matrix converter)
AC / DC-DC / AC converter with voltage DC-link (U-BBC)
AC / DC-DC / AC Converter with Current DC-link (I-BBC)
Hybrid Direct Matrix Converter (HDMC)
Hybrid Indirect Matrix Converter (HIMC)
Direct Matrix Converter
Indirect Matrix Converter
Conventional Matrix Converter (CMS)
Full Bridge Matrix Converter (Open Motor Windings)
AC / DC-DC / AC Converter without DC-link Capacitor
Indirect Matrix Converter (IMC)
Sparse Matrix Converter (SMC) (VSMC) (USMC)
Three-level matrix converter

There are different topologies of inverters, as shown in the diagram opposite as an overview. The most important types are the indirect converters, which work with an intermediate circuit operated with DC voltage and which in principle represent a combination of rectifier and inverter . There are also direct converters, also known as matrix converters, that do not require a DC link. The main topologies are divided into various subgroups for specific areas of application.

In terms of performance, a distinction is made between partial and full converters . A partial converter is only designed for the slip power to be controlled (up to a maximum of 30%); a full converter controls the full power of the drive machine or generator. Partial converters are often found in connection with double-fed asynchronous generators on wind turbines .

The most important circuit principles for converters are shown below.

Indirect converters

Indirect converters with DC voltage in the intermediate circuit , English Voltage Source Inverter (VSI), consist of a three-phase rectifier (on the left in the figures), a DC voltage circuit in which the DC voltage on the capacitor C as an energy store in the intermediate circuit is almost constant, and an output-side Inverter (in the pictures on the right). The control electronics required to control the electronic switches are omitted from the figures for the sake of clarity.

Indirect converters with direct current in the intermediate circuit, English Current Source Inverter (CSI), consist of a three-phase rectifier with phase control , an intermediate circuit with a storage choke L as an energy storage device , through which constant direct current flows under constant load, and an output-side inverter.

The bipolar transistors with insulated gate electrode (IGBT) as electronic switches shown in the simplified circuit diagrams can also be replaced by other electronic switches such as thyristors , depending on the application . At high voltages of a few 100 kV, the switches are connected in series and combined into so-called thyristor towers . This is common for higher outputs of up to a few 100 MW with direct current short couplings , which are a form of large converters between two alternating voltage networks with different network frequencies .

The advantage of the indirect converter is that the output is largely decoupled from the input via the intermediate circuit and its energy store. The energy store, especially in VSI converters with a capacitor, has a comparatively high volume depending on the power. For higher powers, indirect converters are designed as CSI with a coil as energy storage. Compared to direct converters, indirect converters have a lower energy density.

Direct converters

Basic circuit of a direct converter

Direct converter topologies, also referred to as matrix converters, are used when high energy densities are required, such as with a compact converter . The structure of the matrix converters cannot be broken down into individual modules such as rectifiers or inverters and converts the voltage and frequency in one stage, a matrix.

The matrix in the three-phase system, as shown in the simplified circuit diagram on the right, consists of three paths per phase, which makes it possible to connect each output phase, designated with the capital letters A , B and C, to one of the input phases a , b or c . The series connection of two IGBTs per path is necessary in order to be able to switch both positive and negative half oscillations. Due to the control circuit, which is not shown here for the sake of clarity, in this converter certain time segments of the input voltages are switched to the output in such a way that a changed frequency results. The discontinuous transitions in the area of ​​the switchover times are expressed in harmonics that are dampened by additional harmonic filters .

Another circuit variant of a direct converter, which is used in particular as a frequency converter for controlling larger three-phase motors, is the cyclo converter . As a special feature of the cyclo-converter, the output frequency is always lower than the input frequency if thyristors are used as switching elements, which can be switched on at any point in time, but only switched off at a zero crossing .

Mixed topologies

Sparse matrix converter

The indirect matrix converters or sparse matrix converters represent a hybrid form between direct and indirect converters . These have an intermediate circuit, but without energy storage. I.e. the voltage or current is not constant over time and changes its value cyclically. The advantage is that there is no need for the voluminous energy storage devices that fail at higher outputs. Disadvantages are the greater complexity in terms of circuitry and the more complex control logic. Like the cyclo converters, the sparse matrix converters are primarily used in the field of frequency converters for controlling electric motors.


"Static" converters based on semiconductors are increasingly being replaced by "rotating" converters.

  • In the field of welding, rotating converters, recognizable by the cylindrical housing and the noise of the motor, were common in the 1960s. Around 1980, welding "rectifiers" with transformers appeared, the iron core of which was typically mechanically adjusted using a crank protruding from the top in order to limit the welding current. Modern welding "inverters" are considerably lighter and more efficient: A device for 150 A welding current weighs around 4 kg and can be carried with a shoulder strap. Large devices became connectable around 2010, so they communicate in their control system in order to add their welding currents, for example for rough welding on tram rails.
  • In April 2014, the world's largest converter system for traction current was built in Datteln with an output of 413 megawatts . It prepares around 25% of DB's traction current in Germany with high efficiency. The system with 4 blocks in IGCT (Integrated Gate Commutated Thyristor) technology was developed at ABB Switzerland in Turgi (AG).

See also


  • Joachim Specovius: Basic course in power electronics . 4th edition. Vieweg + Teubner, 2010, ISBN 978-3-8348-1307-7 .

Individual evidence

  1. Jump up ↑ JW Kolar, T. Friedli, F. Krismer, SD Round: The Essence of Three-Phase AC / AC Converter Systems . Proceedings of the 13th Power Electronics and Motion Control Conference (EPE-PEM), September 1-3, 2008, Poznań 2008, p. 27-42 .
  2. ^ Ralf Lohde, Christian Wessels, Friedrich W. Fuchs: Power electronics generator systems in wind turbines and their operating behavior. (PDF; 620 kB) (No longer available online.) Christian-Albrechts-Universität Kiel, archived from the original on April 10, 2014 ; Retrieved July 9, 2013 .
  3. ^ I. Takahashi, Y. Itoh: Electrolytic Capacitor-Less PWM Inverter . Proceedings of the IPEC'90, Tokyo, 2-6. April 1990, p. 131-138 .
  4. ^ MH Bierhoff, FW Fuchs: Pulse Width Modulation for Current Source Converters - A Detailed Concept . Proceedings of the 32nd IEEE IECON, November 7-10, 2006, Paris 2006.
  5. WI Popow: The forcibly commutated direct converter with sinusoidal output voltage . In: Elektrie . tape 28 , no. 4 , 1974, p. 194-196 .
  6. Mahesh Swamy, Kume, Tsuneo: Present State and Futuristic Vision of Motor Drive Technology . In: (Ed.): Power Transmission Engineering . December 16, 2010.
  7. The world's most powerful converter system for traction current efficiently converts traction current for Deutsche Bahn, April 29, 2014, accessed October 26, 2018.