III. Заполните пропуски, выбрав нужный вариант.
1. A synchronous motor in its pure form has no starting ... . 2. The ... falls into step with the rotating field. 3. Single-phase a.c. applied to a ... ... produces a pulsating magnetic field. 4. Small single-phase synchronous motors may use ... ... . 5. A synchronous motor cannot be started by applying ... ... to the stator.
(rotor, permanent-magnet rotors, torque, three-phase a.c., stator winding)
IV. Найдите многокомпонентные определительные словосочетания в приведенных ниже предложениях. Переведите их на русский язык.
1. A high-speed rotating field appears when the instant a.c. is applied to the stator. 2. When the rotor has been brought near to synchronous speed by the starting device, it is energized by connecting it to a d.c. voltage source. 3. Single-phase a.c. applied to a stator winding produces a pulsating magnetic field. 4. Small single-phase synchronous motors may use permanent-magnet rotors.
V. Определите, в чем состоит различие в употреблении окончания -s в выделенных словах.
Образец: fields – мн. число от имени существительного a field; starts – 3-e лицо ед. числа от глагола to start.
1. The rotating field rushes past the rotor poles. 2. The rotor falls into step with the rotating field. 3. Permanent-magnet rotors do not have field windings. 4. When a rotor pole approaches a stator winding, the direction of the stator field must be such as to attract the pole. 5. A synchronous motor has some disadvantages.
VI. Вставьте вместо пропусков it’s (it is) или its (притяжательное местоимение).
1. ... usual to start the work of a synchronous motor with a help of a small induction motor. 2. A synchronous motor in ... pure form has no starting torque. 3. ... obvious that one of the disadvantages of a synchronous motor is that it cannot be started from a standstill by applying three-phase a.c. to the stator. 4. The direction of the stator field must be such as to attract the pole and to give it a torque in the direction of ... motion.
VII. Выразите согласие или несогласие с данными утверждениями, используя следующие фразы: You are quite right; I agree with you; I am afraid you are not right; On the contrary (парная работа).
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1. A synchronous motor in its pure form has no starting torque. 2. Single-phase a.c. applied to a stator winding cannot produce a pulsating magnetic field. 3. The field current in a stator winding must pass through a cycle in the interval between rotor poles of opposite polarity. 4. Small single-phase synchronous motors may use all kinds of rotors. 5. Permanent-magnet rotor does not have field windings nor requires d.c. to be applied to them.
Text 5
Induction Motors
The induction motor is the most commonly used a.c. motor because of its simplicity, its robust construction and its 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 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 are almost identical, but their rotors are completely different. The induction motor is a laminated cylinder with slots in its surface. The windings in these slots are one of 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 the 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.
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 a magnetic field. The two fields interact, and cause the rotor to turn.
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When a.c. is applied to the stator windings, a rotating magnetic field is generated. This rotating field cuts the bars of the rotor and induces a current in them. As you know from you study of meter movement and elementary motors, this induced current will generate a magnetic field around the conductors of the rotor, which will try to line up with the stator field. However, since the stator field is rotating continuously, the rotor cannot line up with it, but must always follow along behind it.
As you know from Lenz’s Law any induced current tries to oppose the changing field which induces it. In the case of an induction motor the change is the motion of the stator field; so the force exerted on the rotor by the reaction between the rotor and the stator fields will set about trying to cancel out the continuous motion of the stator field. That is to say, the rotor will move in the same direction as the stator field, and will try to line up with it. In practice, it gets as close to the moving stator fields as its weight and its load will allow.
Words and Word Combination
| robust a | прочный |
| low a | низкий, невысокий |
| manufacturing cost | стоимость производства |
| due to | благодаря |
| self-contained a | независимый, автономный |
| external a | внешний, наружный |
| derive v | получать, происходить |
| circuit n | цепь; контур; сеть |
| squirrel-cage | короткозамкнутый |
| copper bar | медный пруток (полоса, заготовка) |
| brass n | латунь, желтая медь |
| gap n | щель, просвет |
| obtain v | получать; достигать |
| wound rotor | фазный ротор |
| cause v | вызывать; заставлять |
| interact v | взаимодействовать; влиять друг на друга |
| try v | пытаться, стараться |
| oppose v | противодействовать, препятствовать |
| set about | начинать; распространять |
| cancel v | аннулировать, сводить на нет |
| allow v | позволять, разрешать, допускать |
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E x e r c i s e s
I. Прочитайте слова и определите, к каким частям речи они относятся. Назовите их русские эквиваленты :
simplicity, manufacturing, external source, to derive, circuit, cylinder, surface, insulation, core, air, strength, wound, to cause, to generate, meter, movement, elementary, continuously, always, to cancel, weight, to allow, identical.
II. Найдите в тексте эквиваленты следующих слов и словосочетаний :
прочная конструкция, невысокая стоимость производства, получить название (от), наружный источник напряжения, одинаковый, короткозамкнутая обмотка, воздушный просвет, фазный ротор, получать, взаимодействовать, противодействовать, постоянно вращаться, сводить на нет.
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