Frequency changer

An inverter, also referred to as AC converter and based on the English term as AC / AC converter is a converter that generates an AC voltage different in frequency and amplitude AC voltage. Not to be confused are inverters with transformers that can not change the frequency of the voltage, only the amplitude, but.

Serves the Umrichtung the direct supply of an electrical machine, such as an AC motor in the electric drive technology and the frequency is derived from the condition of the motor, the inverter is specifically referred to as a frequency converter. He then includes additional functions to control the motor, such as a rotational speed measurement, and methods of commutation, to adjust the phase rotation depends on the instantaneous state of the machine.

Converters and inverters are electronic devices without mechanical moving components. In contrast, a transducer is a combination of rotating electrical machines, typically an electric motor and an electric generator, the electro-mechanically provides a frequency conversion.


There are different topologies of converters, as shown in the adjacent diagram as an overview. The main types are the indirect converters, which operate with a DC-operated intermediate circuit and, in principle, represent a combination of rectifier and inverter. Furthermore, there is a direct drive, also referred to as a matrix converter, which do not use a DC- voltage intermediate circuit. The main topologies are divided into different subgroups for specific application areas.

With regard to performance, a distinction between partial converters and full converter. A converter section is only designed to be controlled slip power (up to a maximum of 30 % ), a full converter dominates the full power of the prime mover and the generator. Converter sections are often found in connection with doubly-fed induction generators for wind turbines.

The following are the main principles of inverters circuit is presented.

Indirect inverter

Indirect inverter with DC voltage in the intermediate circuit, english voltage -source inverter ( VSI), consist of a three -phase rectifier ( in the illustrations on the left ), a DC circuit, in which the DC voltage across the capacitor C as energy storage in the DC link is approximately constant, and an output-side inverter ( in the pictures right). Required for the control of the electronic switch control electronics are omitted in the figures for clarity.

Indirect inverter with DC current in the circuit, english Current source inverter ( CSI ) consist of a three-phase rectifier with phase control, an intermediate circuit with a storage inductor L as an energy store, flows through the constant load constant DC current, and an output-side inverter.

The drawn in the simplified diagrams bipolar transistors with insulated gate (IGBT) of an electronic switch may be replaced by other electronic switches such as thyristors, depending on the application. At high voltages of several 100 kV, the switches are connected in series and grouped into so-called Thyristortürmen. Usual this is for higher power ratings up to several 100 MW DC short couplers, which are a form of large inverters between two AC systems with different power supply frequency.

  • Topologies indirect drive

Block diagram of CSI with current link

The advantage of the indirect converter consists of an extensive decoupling of the output from the input via the intermediate circuit and its energy storage. The energy storage, especially at VSI inverters with a capacitor, has according to performance on a relatively high volume. At higher powers, indirect converters are designed as CSI with a coil to store energy. Compared to direct indirect converters inverters have a lower energy density.

Direct drive

With requirements for high energy densities, such as with a compact design of the converter, will find direct converter topologies, also called matrix converter application. The matrix converter can be used in their construction do not split into individual modules such as rectifier or inverter and may cause the voltage and frequency conversion in a single stage, a matrix through.

The matrix consists in three-phase system, as shown in the simplified schematic on the right, from each of three paths per phase, so it is possible to each output phase denoted by the capital letters A, B and C, optionally with one of the input phases a, b or c to connect. The series connection of two IGBT per path is necessary in order to control both positive and negative half waves can. By this drive circuit, not shown for clarity certain time periods of the input voltages are switched to the output, that results in a modified frequency converter of this. The case discontinuous transitions in the range of Changeover expressed in harmonics, which are damped by additional harmonic filter.

Another variant of a direct drive circuit, which finds particular as frequency converters for controlling large AC motors application, is the cyclo - converter. A special feature of the cyclo - converter, the output frequency is always smaller than the input frequency, where the thyristors are used as switching elements, although switched at any desired time, but can be switched off only at a zero crossing.

Mixed topologies

A hybrid between direct and indirect inverter do the indirect matrix converter or sparse matrix converter dar. These have an intermediate circuit, but without energy storage. That the voltage or current is not constant and changes its value cyclically. The advantage is the elimination of the bulky at higher powers failing energy storage. A disadvantage is the greater circuit complexity and the complex control logic. As the cyclo - converter also find the sparse matrix converter primarily in the area of ​​frequency converters for controlling electric motors application.