Commutation


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commutation

[‚käm·yə′tā·shən]
(communications)
The sampling of various quantities in a repetitive manner for transmission over a single channel in telemetering.
(electronics)
The transfer of current from one channel to another in a gas tube.
(electromagnetism)
The process of current reversal in the armature windings of a direct-current rotating machine to provide direct current at the brushes.

Commutation

 

(of electric circuits), various ways of switching the contacts of wires, cables, machines, transformers, apparatus, and instruments; used in installations that generate, distribute, and consume electric power. Changes in the direction of current flow (polarity changes) are also called commutation. Commutation is usually accompanied by transient processes that arise because of rapid redistribution of currents and voltages through the branches of an electric network.


Commutation

 

the replacement of the serfs’ labor obligations and payments in kind by a money rent; the process took place as a result of and in proportion to the penetration of commodity-money relations into the feudal village. The term “commutation” is usually used in the context of agrarian relations in Western European countries.

Commutation

The process of transferring current from one connection to another within an electric circuit. Depending on the application, commutation is achieved either by mechanical switching or by electronic switching.

Commutation was conceived over a century ago through the invention of the direct-current (dc) motor. When direct current is supplied to a winding on a rotor that is subjected to a stationary magnetic field, it experiences a rotational force and resulting output torque. As the stator north and south poles are reversed relative to the rotating winding, the rotor current is reversed by a commutator in order to maintain the unidirectional torque required for continuous motor action. See Direct-current motor, Windings in electric machinery

The principle is illustrated in Fig. 1. In its simplest form, a single rotor winding is connected between two segments of a cylindrical copper commutator which is mounted axially on the rotor. Connection to the external dc supply is through sliding carbon contacts (brushes). The segments have small insulated gaps at A and B. As A and B pass the brushes, the current in the rotor winding reverses. In the short interval where the brushes short-circuit the segments, the rotor current decays before building up in the reverse direction. The angular position of the brushes is selected to reverse the current at the appropriate rotor position.

Basic commutator for a dc motorenlarge picture
Basic commutator for a dc motor

The same principle of commutation applies to the ac commutator motor and universal ac/dc motor, which are common in variable-speed kitchen appliances and electric hand tools. See Alternating-current motor, Universal motor

The equivalent of mechanical commutation occurs in solid-state converter circuits such as those used for rectifying ac to dc or inverting dc to ac. Figure 2 shows a three-phase converter widely used in industry. For simplicity, the ac supply network is represented by equivalent phase voltages in series with the effective supply inductance. (Often this inductance is mainly the inductance per phase of a converter transformer that interfaces the converter and the three-phase supply.) Usually, supply resistance is relatively low and plays a negligible role in the converter action. As shown, thyristors 1 and 2 are conducting the dc current from phase a to phase c. A smooth dc current does not produce a voltage across the inductance L in each phase. In the cyclic conduction sequence, the dc current is commutated from phase a and thyristor 1 to phase b and thyristor 3. To achieve this, thyristor 3 is gated in a region of the ac waveform when its forward voltage is positive. Turning it on applies a reverse voltage to thyristor 1 (phase b being more positive than phase a), which ceases conduction to complete the commutation of the dc current. This is repeated in sequence for the other thyristors in each ac cycle. See Converter, Semiconductor rectifier

Three-phase converterenlarge picture
Three-phase converter
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After that, he granted 13 pardons in December 2013, along with 8 commutations, added a single commutation in April 2014, and then another 12 pardons in December 2014 to go along with 11 commutations--8 of which went to drug offenders, and 3 of which were for members of the "Cuban Five who were released from U.
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attorney for the district in which he was tried both now supported his commutation request.
Each country has its own conditions and circumstances and it is neither sensible nor legal to commute punishment against repeat offenders who had previously benefited from commutation of sentence, he said during his sermon.
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Nevertheless to reach a compromise between switching frequency, efficiency and output power, the optimization of parameters, which influence transistor's commutation mode, is necessary.
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President Bush's commutation should see the men released on or about March 20.