TASK 1: Fill in the missing words from the word bank (use, make, consist, place, call, know, make, be)
UNIT 8 ACgenerators Advantageпреимущество Applianceаппарат, прибор Applicationприменение Choiceвыбор Commonраспространенный Completeполный Complexсложный Continuouslyпостоянно, непрерывно Convenienceудобство Disadvantageнедостаток Efficientэффективный Frequencyчастота Housingкорпус Identicalодинаковый, идентичный insteadofвместо чего-либо Lightлегкий, незначительный Pairпара Portableпортативный Revolutionоборот Rightправильный rpm (revolutions per minute)оборотывминуту Setкомплект, набор Singleодиночный, единственный slipringsтокособирательные (токосъемные) кольца, контактные кольца step-downtransformerпонижающий трансформатор Thereforeпоэтому tobeequippedwithбыть оснащенным чем-либо toconnectсоединять toeachotherк друг другу tomeanзначить, означать topreventпредотвращать totakecareзаботиться Twiceдважды Valueвеличина, значение Whileв то время как
READ AND TRANSLATE THE TEST
Ac generators or alternatorsoperate on the same fundamental principles of electromagnetic induction as DC generators. Practical AC generators are usually equipped with a small auxiliary generator called an exciter. The exciter supplies direct current for the electromagnets used to create the magnetic field in the AC generator. The armature of the AC generator consists of copper wire wound in hundreds of coils around slots cut in an iron core. The electromagnets consist of copper bar wound around iron cores.
In most AC generators the armature is the stator and the field structure is the rotor.
The slip-rings are used to carry the direct current from the exciter-generator to the electromagnets in the field structure. Outside wires connected to the armature coils take the alternating current induced in the armature directly from it. This kind of AC generator is also called a synchronous generator because it generates a voltage that has a frequency proportional to, or synchronized with the speed of the rotor.
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In a single-phase AC generators the armature has as many sets of coils as the field structure has poles. But most AC generators have three sets of armature coils for each pole. These generators produce three currents of electricity at one time and are called three-phase generators. They generate more power for the amount of materials used than single-phase generators and therefore are more efficient.
A generator that produces a single, continuously alternating voltage is a single-phase alternator. The stator windings are connected in series. Single-phase alternators are found in many applications. They are most often used when the loads are light. Power, that is used in homes, shops and ships to operate portable tools and small appliances is single-phase powers.
Two-phase alternators are designed to produce two completely separate voltages. The windings are physically at right (90) angles to each other. The rotor is identical to the one used in single-phase alternators. The advantage is output voltages while having one set of bearings, one rotor, one housing, and so on to do the work of two. There is the disadvantage of having twice as many stator coils, which require a large and more complex stator.
In the three-phase alternators the three single-phase windings have voltages generated in them which are 120 degrees out of phase.
The three phase alternator is the most common of all alternators in use today because they are much more efficient than two-phase or three-phase alternators.
Three phase alternators connections may beDELTA or STAR (WYE) depending on the application. The AC power on the ships is usually taken from the ship's generators through DELTA connections for the convenience of the step-down transformers.
Because of the many choices that three-phase systems provide, take care to see that any change of connections doesn't provide the load with the wrong voltage or the wrong phase.
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In an alternator voltage is induced in the armature windings when magnetic fields of alternating polarity are passed across these windings. Theamountofvoltageinduceddependsonthreethings:
1. the number of conductors in series per winding
2. the speed (alternator rpm) at which the magnetic field cut the winding
3. the strength of the magnetic field
The number of windings, of course, is fixed when the alternator is designed. Also, if the output frequency is required to be a constant value, then the speed of the rotating field must be constant. This prevents the use of the alternator rpm as a means of controlling the voltage output.
So, the only practical method to control the voltage is to control the strength of the rotating magnetic field. The strength of this electromagnetic field may be varied by changing the amount of current flowing through the field coil.
TASK 1: Fill in the missing words from the word bank (use, make, consist, place, call, know, make, be)
An alternator ________ an electrical machine which converts mechanical energy into alternating electric energy. They _________ as synchronous generators. Main parts of the alternator, obviously, _________
of stator and rotor. In most of the alternators, field exciters are rotating and the armature coil is stationary. The stator ________ for holding armature winding. The stator core ________ up of lamination of steel alloys or magnetic iron, to minimize the eddy current losses. Rotor: There _______ two types of rotor used in an AC generator / alternator:
(i) Salient and (ii) Cylindrical type. Salient pole type _______ in low and medium speed alternators. Cylindrical type rotors _________ in high speed alternators, especially in turbo alternators. This type of rotor consists of a smooth and solid steel cylinder having slots along its outer periphery. Field windings _______ in these slots. Connecting an alternator in grid ________ assynchronizationofalternator.
TASK 2: Write the necessary conditions to run generators for parallel work:
1.
2.
3.
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