II. Соедините два предложения, опустив выделенные слова и употребив соединительные слова, данные в скобках.
1. Circuit breakers protect the circuits from overloads. Circuit breakers also protect the circuits from voltage failure and reversed current (as well as). 2. One type of circuit breaker is open-type circuit breaker. The open-type circuit breaker has characteristics common to all types (which). 3. The contacts pull open by their own weight. The contacts may pull open as a result of spring action (or).
III. Дайте заголовок к каждому абзацу текста.
IV. Письменно изложите содержание текста на английском языке (не более 4-5 предложений).
Text 3
Review 1
A single-phase alternator has an armature which consists of a number of windings placed symmetrically around the stator, and connected in series. The voltages generated in each winding add to produce the total voltage across the two output terminals.
In the three-phase alternator, the windings have voltages generated in them which are 120 degrees out of phase. Three-phase alternators are most often used to generate a.c. power.
The frequency of the a.c. generated by an alternator depends on the speed of rotation, and on the number of pairs of rotor poles.
The change in the output voltage of a generator caused by variations in load is compensated by varying the field current. Voltage regulators are used to provide automatic variation of field current with change in load.
Carbon Pile
A pile of carbon discs assembled under pressure in such a way that the force pressing the discs together can be varied to produce a variation in the electrical resistance of the pile. The variation of the pressure on the discs may be achieved automatically to give automatic regulation of a generator.
Rotating Field
If three windings are placed round a stator frame, and three-phase a.c. is applied to the windings, the magnetic fields generated in each of the three windings will combine into a rotating magnetic field.
E x e r c i s e s
I. Составьте предложения из данных слов и словосочетаний.
1. To generate, are most often used, a.c. power, three-phase alternators. 2. Depends on, the speed of rotation, the frequency, of, number of pairs of rotor poles, and, the a.c., by an alternator, generated. 3. The pressure on the discs, of, the variation, automatically, may be achieved.
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II. Предложите заголовки к 1-4 абзацам текста.
III. Письменно изложите содержание текста на английском языке (не более 5-6 предложений).
Text 4
Review 2
A synchronous motor uses a single or three-phase stator to generate a rotating magnetic field, and an electro-magnetic rotor which is supplied with d.c. The rotor acts like a magnet and is attracted by the rotating stator field. This attraction will exert a torque on the rotor and cause it to rotate with the field. Synchronous motors are not self-starting and must be brought up to near synchronous speed before they can continue rotating by themselves.
The induction motor has the same stator as the synchronous motor. The rotor is different in that it does not require an external source of power. Current is induced in the rotor by the action of the rotating field cutting through the rotor conductors. This rotor current generates a magnetic field which interacts with the stator field, resulting in a torque being exerted on the rotor and causing it to rotate.
The two types of rotors used in induction motors are the squirrel-cage and the wound rotor.
The magnetic field generated in three-phase a.c. motors are 120 degrees out of phase. At any given instance, these fields are combining to produce one resulting field, which acts on the rotor. The rotor turns because the magnetic field shifts its position through a certain angle.
The rotor of an induction motor rotates at less than synchronous speed, in order that the rotating field can cut through the rotor conductors and induce a current flow in them. The percentage difference between the synchronous speed and the rotor speed is known as slip. Slip varies very little with normal load changes, and the induction motor is therefore considered to be a constant-speed motor.
Two-phase Induction Motor
Induction motors are designed for three-phase, two-phase and single-phase operation. The three-phase stator is exactly the same as the three-phase stator of the synchronous motor. The two-phase stator generates a rotating field by having two windings positioned at right angles to each other. If the voltages applied to the two windings are 90 degrees out of phase, a rotating field will be generated.
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E x e r c i s e s
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