2017-11-30
543
DIRECT-CURRENT (DC) GENERATORS
1. Both motors and generators consist of two basic units, the field, which is the electromagnet with its coils, and the armature that supports the current-carrying conductors
2. If an armature revolves between two stationary field poles, the current in the armature moves in one direction during half of each revolution arid in the other direction during the other halt. To produce a steady flow of direct current from such a device, it is necessary to provide a means of reversing the current flow outside the generator once during each revolution. In older machines this reversal is accomplished by means of a commutator. DC generators are usually operated at fairly low voltages to avoid the sparking between brushes and commutator that occurs at high voltage The highest potential commonly developed by such generators is 1500 V. In some newer machines this reversal is accomplished using power electronic devices, for example, dunk rectifiers
3. Modern DC generators use drum armatures that usually consul of a large number of windings set in longitudinal slits in the armature core and connected to appropriate segments of a multiple commutator. In an armature having only one loop of wire, the current produced will rise and fall depending on the part of the magnetic field through which the loop is moving commutator of many segments used with a drum armature always connects the external circuit to one loop of wire moving through the high-intensity area of the field and as a result the current delivered by the armature windings is virtually constant. Fields of modern generators are usually equipped with four or more electromagnetic poles to increase the size and strength of the magnetic field. Sometimes smaller interpoles are added to compensate for distortions in the magnetic flux of the field caused by the magnetic effect of the armature.
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4. DC generators are commonly classified according to the method used to provide field current for energizing the field magnets. A series-wound generator has its field in series with the armature, and a shunt-wound generator has the field connected in parallel with the armature. Compound-wound generators have part of their fields in series and part in parallel. Both shunt-wound and compound-wound generators have the advantage of delivering comparatively constant voltage under electrical loads The series-wound generator is used principally to supply a constant current at variable voltage. A magneto is a small DC generator with a permanent-magnet field.
5. Прочтите 2-й абзац текста и письменно ответьте на следующий вопрос:
What is necessary to produce a steady flow of current?
