轮机英语课件lesson 23.ppt

Lesson 23 Alternating current generators A coil of wire rotating in a magnetic field produces a current. The current can be brought out to two sliprings(滑环) which are insulated from the shaft. Carbon brushes(碳刷) rest on these rings as they rotate and collect the current for use in an external circuit. Current collected in this way will be alternating, that is, changing in direction and rising and falling in value. To increase the current produced, additional sets of poles(电极) may be introduced. The magnetic field is provided by electromagnets(电磁铁) so arranged that adjacent(相邻的) poles have opposite polarity(极性). These ‘field coils(场线圈、励磁线圈)’, as they are called, are connected in series to an external source or the machine output. If separate coils or conductors(导线) are used then several outputs can be obtained. Three outputs are usually arranged with a phase separation of 120°, to produce a three phase supply. The three phase system is more efficient in that for the same mechanical power a greater total electrical output is obtained. Each of the three outputs may be used in single phase supplies or in conjunction for a three phase supply. The separate supplies are connected in either star or delta formation. The star formation is most commonly used and requires four sliprings on the alternator. The three conductors are joined at a common slipring and also have their individual slipring. The central or neutral line(中性线) is common to each phase. The delta arrangement has two phases joined at each of the three sliprings on the alternator. A single phase supply can be taken from any two sliprings. So far, alternator construction has considered the armature(电枢) rotating and the field coils stationary, the same electricity generating effect is produced if the reverse occurs, that is, the field coils rotate and the armature is stationary. This is in fact the arrangement adopted for large, heavy duty alternators. The field current supply in older machines