Converter
In electrical power engineering, a converter is a mechanical combination of rotating electrical machines that converts one type of current into another, for example direct current into alternating current . A conversion between alternating currents with different frequencies is also possible; frequency converters are used for this.
Converters and, in particular, frequency converters have a similar area of application, but in contrast to converters they represent a power electronic device without mechanically moving components.
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Basics
Before the introduction of the alternating current network , transformers, as well as semiconductor rectifiers and converters, were also used for small powers. Today only units for high capacities such as those in traction current converter plants are operated. The Bundeswehr also uses converters to supply aircraft with electrical energy during maintenance work.
construction
A rotating converter consists of an electric motor that drives a generator for the desired type of current. It can be implemented as a converter set, i.e. as two individual machines coupled with their shafts, or as a so-called single - armature converter from just one machine. Single armature converters have no shaft protruding outwards and an armature with a commutator (current converter) and slip rings.
Applications
With an externally excited direct current generator coupled to a mains operated asynchronous motor , a variable direct voltage can be generated, for example by controlling its excitation.
If the output frequency is also to be variable at a constant drive speed, the following options are available:
- a synchronous generator is excited with an alternating voltage of variable frequency
- A further set of converters consisting of a direct current motor and synchronous generator is connected to a variable direct voltage (see above).
Further applications of converters or machine sets are in the interception of load surges , such as occur , for example, in roller drives in steelworks or in the supply of high-voltage laboratories . The centrifugal mass and the mechanical decoupling can avoid repercussions in the electrical supply network from abrupt load fluctuations.
The Leonardsatz , a machine set for the variable-speed drive of large machines, is actually not one of the converters, but due to the similar components and principles used, it is also described here.
Single armature converter
Single armature converters were used for small outputs, for example to generate low voltages from the original direct current network.
Applications in the mobile sector concerned the generation of different voltages from car batteries or corresponding direct current networks, such as 400 Hz three-phase current (36 V) for operating gyro motors in gyro compasses or the artificial horizon from the 28 V direct current electrical system of aircraft. Such small converters had permanent magnet excitation, so their output voltage could only be influenced via the input voltage.
For the Wehrmacht , the converter set "U5a1" supplied 330 V anode voltage at 140 mA via a single armature converter and 5 V heating voltage at 1.2 A via an adjustable series resistor for operating mobile transmission systems from 12 V car batteries; it was manufactured according to central specifications by various manufacturers.
The call and signal machine used in analog exchanges is also a single-armature converter because it converts the mains frequency of 50 Hz into the call current frequency of 25 Hz.
Traction current
The frequency of long-distance traction current in Germany and some other European countries is 16.7 Hz. This low frequency was chosen in order to operate the direct current series motors with alternating current (in contrast to direct current, which were easily transformable) for a long time in electric rail drives can. This made it possible to set the contact line voltage relatively high, which is an advantage for power transmission. The currents, and thus the line losses and the required conductor cross-sections, are therefore significantly lower. This is important for mainline railways with usually quite large distances between feed points.
In the locomotives, the voltage is reduced by a variable transformer to the level permissible for the machine. Since the commutators of the motors were not suitable for operation at a 50 Hz supply frequency, a correspondingly lower frequency was selected. The frequencies used result specifically from the divider ratio 1: 2 or 1: 3 of the 50 Hz mains frequency, so that they could be generated from this with converters with different numbers of pole pairs in the motor and generator on a common shaft. Railway power converter sets have an external excitation so that the voltage output can be controlled. They can be fed back.
Leonardsatz
The Leonard theorem is also called Ward-Leonard converter after its inventor, the American electrical engineer Harry Ward Leonard . It consists of a converter (which converts three-phase current into a variable DC voltage ) and a DC motor connected to it. Until the development of power semiconductors such as thyristors , the Leonard sentence was the only way to implement large variable-speed drives that were fed with three-phase current.
The illustrated converter part of a Leonard set is used to supply and control a pendulum machine of an engine test bench and consists of:
- the drive ( 1 ), an asynchronous motor
- the generator ( 2 ), a separately excited DC machine
- the exciter generator ( 3 ), a small auxiliary direct current generator to generate the excitation voltage for ( 2 )
A Leonardsatz consists of a converter which initially generates a direct current voltage that can be controlled via its excitation ( 5 ) with the aid of an asynchronous motor and a direct current generator mechanically coupled to it (voltage conversion ). The primary drive is usually an asynchronous motor connected directly to the AC mains , but it could also be a DC motor or an internal combustion engine . The variable direct voltage supplied by this converter feeds a direct current motor, the excitation of which is also varied from case to case. The motor can also be housed spatially separated from the converter set, namely with the machine to be driven (rolling mill, elevator, spinning machine, etc.). Load devices on engine test benches use the fact that a Leonard set can be fed back, i.e. In other words, it can feed energy back into the grid when the output of the DC motor is driven - this then works as a generator, the DC machine of the converter set as a motor and the asynchronous machine as an asynchronous generator. See also pendulum machine .
Normally, a Leonard kit also includes an exciter generator (shunt machine ) to provide excitation voltage to the DC machines. This exciter generator is also driven by the primary drive. Its small excitation power can be controlled with a variable resistor ( 5 ).
The armature of the DC generator is connected directly to the armature of the DC motor; this DC link transfers the power, the large current flowing in it does not have to be switched - one of the advantages of the Leonard theorem. The excitation current of the motor is normally not changed in order to have the maximum torque available at all speeds. However, by means of field weakening , its speed can be increased beyond the nominal speed at the expense of the torque.
The Ward-Leonard set enables low-loss, variable-speed drives and energy recovery when braking. In addition, the DC motor can also be loaded intermittently without the load surges being transferred to the network (mass inertia of the machine set). The DC motor or the driven machine is started up by increasing the generator voltage from zero by gradually increasing the excitation current of the DC generator. A high inrush current therefore only occurs when the asynchronous motor is started.
Ilgner converter
The Ilgner converter, named after its inventor Karl Ilgner , is based on the same principle as the Leonard converter. Its specialty is just a large flywheel that is coupled to the three-phase motor. This flywheel stores kinetic energy that can be used in various ways:
- it can be used to bridge drive failures
- Load surges can be absorbed, as they occur, for example, with roller drives in steelworks.
Leonard sentences were used from the beginning of the 1920s and some of them still run today. Good alternatives only emerged with thyristor controllers and frequency converters . However, electrical storage devices are required here to replace the mechanical energy storage device of the Ilgner converter.
The intermittent network load of rolling mills, electric steel furnaces or other electric arc furnaces leads to problematic network voltage fluctuations even today, so they are often built close to power plants whose generators and turbines can compensate for load fluctuations due to their inertia.
literature
- Hans-Günter Boy, Horst Flachmann, Otto Mai, Jürgen Rabens: Electrical machines and control technology - the master's examination . 8th edition. Vogel-Verlag, Würzburg 1990, ISBN 3-8023-0725-9 .
- Peter Bastian, Horst Bumiller, Monika Burgmaier and others: Electrical engineering . 27th edition. Europa-Lehrmittel, Haan-Gruiten 2009, ISBN 978-3-8085-3188-4 .
Individual evidence
- ↑ Converter set U5a1. Radiomuseum.org, accessed September 14, 2017 .
- ↑ Pictures of the converter replacement “U5a1” in Wikimedia Commons
