Commutation
In power electronics, commutation refers to the process in which a current flow passes from one branch to the other.
In addition to the commutation at rectifiers and thyristor circuits there is the commutation even in DC machines , here commutation called. It takes place here mechanically with a commutator ( carbon brushes and collector) or electronically with the help of semiconductor components such as power transistors or thyristors.
In the case of historical choppers or switch mode power supplies and switching regulators , current paths are also switched, but this is usually not referred to as commutation.
Technical details
Natural commutation
Natural commutation occurs when the supply voltage comes from an alternating or three-phase network and the current path is switched itself due to its polarity change. This is the case with line-commutated or load-commutated power converters ( thyristor controller or rectifier circuits ).
If a thyristor is switched on, the load current rises rapidly to the maximum value determined by the consumer. However, a high rate of rise in the current can lead to the destruction of the silicon structure of the thyristor; the thyristor fails. Therefore, care must be taken in circuits for resistive or capacitive loads that the rate of rise of the current is limited: inductors , so-called commutation throttle , in the current branches, a delay effect of the current rise when switching on.
Also with rectifiers, especially with capacitive loads, high current peaks (low current flow angle ) occur, which lead to interference emissions and a high load on the feeding network or transformer . Here, too, inductances can help to increase the current flow angle by maintaining the current flow over a longer period of time (filter throttle).
In transformer- fed circuits, the stray reactances of the transformer often already ensure sufficient limitation of the rate of increase in current or an increase in the angle of current flow. In the case of converters that are connected directly to the mains, on the other hand, commutating reactors often have to be connected in front of the bridge arms.
Commutation chokes of thyristor and other controlled converter circuits or their inductive loads must be protected by snubber elements or protective diodes that can take over the current flow. Otherwise there would be overvoltages if the current flow in an inductance is suddenly interrupted.
Small single-phase and three-phase rectifier bridge circuits , on the other hand, often do not have any commutating reactors, and although the current transfer often leads to interference emissions and high peak currents, these often do not affect the function and reliability. High-frequency interference emissions, which are caused by the reverse recovery time of the diodes, are countered with interference suppression capacitors or line filters connected in parallel with the diodes .
Externally guided power converters
In frequency converters, inverters and four-quadrant and synchronous rectifiers, the current paths are switched independently and outside of the polarity change. Here, switching delays lead as the turn-off time of transistors and thyristors to a variety of Kommutierungsproblemen. On the one hand, a short circuit between two bridge branches must be avoided through dead times; on the other hand, these dead times can lead to overvoltages, loss of efficiency or electromagnetic interference. Here, too, commutation chokes, free-wheeling diodes and snubber elements are used to ensure interference-free current transfer (commutation) between the bridge branches.
The figure clearly shows how the current flow in a two-way converter circuit, driven by the commutation voltage , changes (commutates) between the two branches. During commutation, both current branches are briefly closed (e.g. due to the release time of a thyristor), i.e. This means that there is a short circuit, the current of which is limited by the commutation reactors. When the current has commutated, that is, it has decayed to zero, the switch S1 opens and the load current I flows through the switch S2.
literature
- Olaf Beuth, Klaus Beuth: Electronics 9. Power electronics. Vogel Verlag Und Druck, 2004, ISBN 3-8023-1853-6 , pp. 109–112.