Alternating-Current (AC) Generators (Alternators)
Alternating-Current (AC) Generators (Alternators)
As you know, the simple generator without a commutator will produce an electric current that alternates in direction as the armature revolves. Such alternating current is advantageous for electric power transmission. Most large electric generators are of the AC type. An AC generator differs from a DC generator in only two particulars: the ends of its armature winding are brought out to solid unsegmented slip rings on the generator shaft instead of to commutators, and the field coils are energized by an external DC source rather than by the generator itself. Low-speed AC generators are built with as many as 100 poles, both to improve their efficiency and to attain more easily the frequency desired. Alternators driven by high-speed turbines, however, are often two-pole machines. The frequency of the current delivered by an AC generator is equal to half the product of the number of poles and the number of revolutions per second of the armature.
It is often desirable to generate as high a voltage as possible, and rotating armatures are not practical in such applications because of the possibility of sparking between brushes and slip rings and the danger of mechanical failures that might cause short circuits. Alternators are therefore constructed with a stationary armature within which revolves a rotor composed of a number of field magnets. The principle of operation is exactly the same as that of the AC generator described, except that the magnetic field (rather than the conductors of the armature) is in motion.
The current generated by the alternators described above rises to a peak, sinks to zero, drops to a negative peak, and rises again to zero a number of times each second, depending on the frequency for which the machine is designed. Such current is known as single-phase alternating current. If, however, the armature is composed of two windings, mounted at right angles to each other, and provided with separate external connections, two current waves will be produced, each of which will be at its maximum when the other is at zero. Such current is called two-phase alternating current. If three armature windings are set at 120° to each other, current will be produced in the form of a triple wave, known as three-phase alternating current. A larger number of phases may be obtained by increasing the number of windings in the armature, but in modern electrical-engineering practice three-phase alternating current is most commonly used, and the three-phase alternator is the dynamoelectric machine typically employed for the generation of electric power. Voltages as high as 13 200 are common in alternators.
Words and expressions
electric power transmission - передача электроэнергии
external DC source - внешний источник постоянного
- тока
unsegmented - без пластин
slip ring - скользящее кольцо
high-speed turbines - высокоскоростные турбины
mechanical failures - механические поломки
composed of a number - состоящий из множества
number of windings - количество обмоток
dynamoelectric machine - динамическая электрический
Exercise
Переведите на русский язык следующие предложения:
- A simple generator without a commutator produces AC current.
- The current alternates as the armature rotates through the magnetic field.
- AC generation is best for power distribution.
- There are 2 differences between AC and DC generators.
- AC generators have 2 slip rings instead of a commutator.
- AC field coils are energized by an external DC source.
- Low-speed AC generators are built with as many as 100 poles.
- The AC frequency depends on the number of poles and the speed of rotation.
- 100poles rotated at 1 revolution per second = 50 cycles.
- 2 poles rotated at 50 revolutions per second = 50 cycles.
- Revolutions is normally quoted per minute so 1 per second =50 RPM
- 50 per second = 2500 RPM.
- To reduce transforming higher voltages are generated which require special generators.
- The armature is not rotated to reduce sparks. The magnetic field is rotated.
- Alternators therefore have a rotating magnet inside coils of conductor.
- AC current alternates as follows:
- Conductor at 90 degrees to pole = no current;
- Conductor at 0 degrees to pole =maximum current;
- Conductor at –90 degrees to pole = 0 current;
- Conductor at 180 degrees to pole =Maximum negative current;
17. This is known as single-phase current.
18. An armature with 2 coils at 90 degrees produces 2 phase current.
19. An Armature with 3 coils at 120 degrees produces 3 phase current.
20. 3 Phase generation is the most common form of generation.
21. Voltages as high as 13,200 are common in alternators