| 单词 | commutation | ||||||||||||
| 释义 | commutationenUKcom·mu·ta·tionC0519800 (kŏm′yə-tā′shən)commutation(ˌkɒmjʊˈteɪʃən)com•mu•ta•tion(ˌkɒm yəˈteɪ ʃən)n.
commutationnounCommutationenUKcommutation[‚käm·yə′tā·shən]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. Commutationthe 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. CommutationThe 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.  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  commutationenUKCommutationModification, exchange, or substitution. Commutation is the replacement of a greater amount by something lesser. To commute periodic payments means to substitute a single payment for a number of payments, or to come to a "lump sum" settlement. In Criminal Law, commutation is the substitution of a lesser punishment for a greater one. Contrasted with clemency, which is an act of grace eliminating a sentence or punishment, commutation is the modification or reduction of a punishment. The change from consecutive prison sentences to concurrent sentences is a commutation of punishment. commutationn. the act of reducing a sentence resulting from a criminal conviction by the executive clemency of the Governor of the State, or President of the United States in the case of federal crimes. This is not the same as a pardon which wipes out the conviction or the actual or potential charge (as when President Gerald R. Ford pardoned ex-President Richard M. Nixon even without charges having been officially made--a rare instance). A pardon implies either that the conviction was wrong, that there has been complete rehabilitation of the party, or that he/she has lived an exemplary life for many years and deserves to have his/her name cleared in old age. Commutation implies the penalty was excessive or there has been rehabilitation, reform, or other circumstances such as good conduct or community service. Commutation is sometimes used when there is evidence that the defendant was not guilty but it would prove embarrassing to admit an outright error by the courts. (See: executive clemency, pardon) commutationthe reduction in severity of a penalty imposed by law.COMMUTATION, punishments. The change of a punishment to which a person has been condemned into a less severe one. This can be granted only by the executive authority in which the pardoning power resides. commutationenUK
Synonyms for commutation
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commutation - the travel of a commuter