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III. Here are the answers. Make up questions, write them down.
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There are two types of alternators: the revolving-armature type and the revolving-field type.
A voltage will be generated as long as there is relative motion between a conductor and a magnetic field.
All generators are made up of two mechanical parts – a rotor and a stator.
A.c. motors are particularly well-suited for constant-speed applications, since the speed is determined by the frequency of the a.c. applied to the motor terminals.
The induction motor differs from the synchronous motor in that it does not have its rotor connected to any source of power.
IV. Read the statements and define if they are true or false (according to the text content). If the statement is false, correct it.
1. All electrical generators, whether d.c. or a.c., depend for their operation on the action of a coil cutting through a magnetic field, or of a magnetic field cutting through a coil. 2. In a d.c. generator the revolving part is always the winding. 3. The revolving-armature alternator is found in alternators of large power rating and is widely used. 4. It is possible to design a.c. motors to operate from a multi-phase a.c. supply. 5. A.c. motors can hardly duplicate the operation of d.c. motors and are troublesome to operate.
V. Speak on:
a.c. generator, its construction and principle of operation;
a.c. motors and their different types.
Supplementary Texts
Text 1
13. Read and translate the text, give a title to it. Before reading check the meaning of these words in a dictionary:
distinct, add, imply, lead, neutral, star, delta, opposite.
A single-phase alternator has the entire armature conductor connected in series as one winding, across which an output voltage is generated. In a two-pole, single-phase alternator the stator winding is in two distinct coils, both being wound in the same direction round the stator frame. The rotor consists of two poles of opposite polarity. As the rotor turns, its poles induce a.c. voltages in the stator windings. The two coils in the stator winding are connected to each other in such a way that the a.c. voltages induced in them are in phase.
Multi-phase (or 'polyphase') alternators have two or more single-phase windings symmetrically spaced around the stator. The windings are electrically separates from each other.
In a two-pole, two-phase alternator the stator consists of two-phase windings completely separated from one another. Each winding is made up of two parts which are so connected that their voltages add. The rotor is identical to that used in the single-phase alternator.
A three-phase alternator, as the name implies, has three single-phase windings so placed that the voltage induced in any one is phase-displaced (сдвинут по фазе) by 120 degrees from the other two. Instead of having six leads coming out of the three-phase alternator, three leads, one from each phase, are connected together to form what is called a 'star' connection. The point of connection is called a neutral, and the voltage from this point to any one of the line leads will be the phase voltage.
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A three-phase stator can also be connected so that the phases are connected end-to-end; it is then called 'delta connected'. Both the 'star' and 'delta' connections are used for alternators.
Most of the alternators in use today are three-phase-winding machines. This is because three-phase alternators are much more efficient than either two-phase or single-phase alternators.
Exercises
I. Complete the sentences, choosing the words and phrases from the right column:
| 1. In a single-phase alternator the stator winding is in two distinct... | a star connection three-phase |
| 2. Three leads coming out of the three-phase alternator are connected together to form... | alternators coils |
| 3. If the phases are connected end-to-end in a three-phase stator, it's called... | delta connected alternators |
| 4. The 'star' and 'delta' connections are used for... | |
| 5. The most efficient alternators are... |
II. Read the statements and define if they are true or false (according to the text content). If the statement is false, correct it.
1. In a single-phase alternator all the armature conductors are connected in series as one winding. 2. In a two-pole, single-phase alternator the rotor consists of two poles of the same polarity. 3. The windings are electrically separated from each other. 4. Each winding is made up of several parts which are not connected. 5. The voltage induced in any of three single-phase windings is phase-displaced by 180 degrees from the other two. 6. Two-phase and single-phase alternators are more efficient than three-phase alternators.
Text 2
Translate the following text in writing, using the given vocabulary.
| robust | прочный |
| manufacturing cost | стоимость производства |
| due to | благодаря |
| to derive | получать; происходить |
| self - contained | независимый, автономный |
| squirrel - cage | короткозамкнутый |
| brass | латунь, жёлтая медь |
| gap | щель, просвет |
| wound rotor | фазный ротор |
| to set up | создавать, устанавливать |
| to interact | взаимодействовать |
Induction Motors
The induction motor is the most commonly used a.c. motor because of its simplicity, its robust construction and low manufacturing cost. These characteristics of the induction motor are due to the fact the rotor is a self-contained unit which is not connected to the external source of voltage. The induction motor derives its name from the fact that a.c. currents are induced in the rotor circuit by the rotating magnetic field in the stator.
The stator construction of the induction motor and of the synchronous motor is almost identical, but their rotors are completely different. The induction motor rotor is a laminated cylinder with slots in its surface. The windings in these slots are one of the two types. The most common is called a 'squirrel-cage winding', which is made up of heavy copper bars connected together at either end by a metal ring made of copper or brass. No insulation is required between the core and the bars because of very low voltage generated in the rotor bars. The air gap between the rotor and stator is kept very small so as to obtain maximum field strength.
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The other type of winding contains coils placed in the rotor slots. The rotor is then called a 'wound rotor'.
Regardless of the type of rotor used, the basic principle of operation is the same. The rotating magnetic field generated in the stator induces an e.m.f. in the rotor. The current in the rotor circuit caused by this induced e.m.f. sets up the magnetic field. The two fields interact and cause the rotor to turn.
TEST YOURSELF
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