III. Заполните пропуски, выбрав нужный вариант.
1. The a.c. applied to the stator must ... a rotating field which will ... the rotor with it. 2. A two-phase induction motor has its stator ... ... of two windings ... at right angles to each other around the stator. 3. The voltages ... to phases A-AI and B-BI are 90 degrees out of phase. 4. The magnetic fields ... in the coils will be in phase with their respective currents. 5. From 180 to 360 degrees the magnetic field ... through a half-cycle and ... a revolution.
(rotates, applied, made up, generate, pull, placed, generated, completes)
IV. Найдите многокомпонентные определительные словосочетания в приведенных ниже предложениях. Переведите их на русский язык.
1. Three-phase a.c. applied to a three-phase symmetrically distributed winding generates a rotating magnetic field. 2. Two out-of-phase magnetic fields add together at every instant during their cycle. 3. A two-phase induction motor has its stator made up of two windings. 4. The resultant magnetic field rotates through another half-cycle, completes a revolution from 180 to 360 degrees.
V. Соедините парные простые предложения в одно сложноподчиненное, употребив союзы: since, how, which, if, whose.
1. The a.c. ... is applied to the stator must generate a rotating field. 2. You know ... three-phase a.c. applied to a three-phase winding will generate a rotating magnetic field. 3. ... the voltages are 90 degrees out of phase, the currents will be displaced by 90 degrees. 4. ... the magnetic fields generated in the coils will be in phase with their respective currents, the magnetic fields will also be 90 degrees out of phase with each other. 5. Two out-of-phase magnetic fields, ... coil axes are at right angles to each other, will add together.
VI. Расскажите об устройстве двухфазного асинхронного двигателя, пользуясь набором следующих слов и словосочетаний:
to be designed, to generate a rotating field, to be placed at right angles, to be out of phase, respective currents, to produce a resultant field, to complete a revolution, to excite a winding.
Text 7
Single-phase Motors
A single-phase induction motor has only one phase, and runs on single-phase a.c. This motor is extensively used in applications which require small low-output motors. The advantage gained by using single-phase motors is that in small sizes they are less expensive to manufacture than other types. Also they eliminate the need for three-phase a.c. mains. Single-phase motors are used in communication equipment, fans, refrigerators, portable drills, grinders, etc.
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Single-phase motors are divided into two kinds: (1) induction motors and (2) series motors. Induction motors use the squirrel-cage rotor and a suitable starting device. Series motors resemble d.c. machines in that they have commutators and brushes.
A single-phase induction motor has only a stator winding. This winding generates a field which can be said to alternate along the axis of the single winding, rather than to rotate.
When the rotor is stationary, the expanding and collapsing stator field induces currents in the rotor which generate a rotor field. The opposition of these exerts a force on the rotor, which tries to turn it 180 degrees from its position. But since this force is exerted through the centre of the rotor, the rotor does not turn.
But if something starts the rotor turning, the turning force in that direction is aided by the momentum of the rotor. The rotor will increase speed until it turns nearly 180 degrees for each alternation of the stator field. For remember that, since slip is necessary to cause an induced rotor current, at maximum speed the rotor must turn slightly less than 180 degrees every time the stator field reverses polarity.
Since the field due to the single-phase a.c. voltage applied to the stator winding is pulsating, single-phase a.c. induction motors develop a pulsating torque. They are therefore less efficient than three-phase or two-phase motors whose torque is more uniformly acting.
Words and Word Combinations
| run v | зд. работать, действовать |
| extensively adv | широко |
| low-output a | маломощный |
| gain v | получать, достигать |
| expensive a | дорогой, дорогостоящий |
| eliminate v | исключать, устранять |
| mains n | сеть |
| equipment n | оборудование |
| fan n | вентилятор |
| drill n | сверло, дрель |
| grinder n | шлифовальный станок |
| series n | последовательное соединение |
| suitable a | подходящий, соответствующий |
| resemble v | походить, иметь сходство |
| commutator n | коллектор |
| alternate v | чередовать(ся), перемежать(ся) |
| expand v | увеличивать(ся) в объеме |
| collapse v | сжимать(ся) |
| aid v | помогать, способствовать |
| uniformly adv | однородно; равномерно |
E x e r c i s e s
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I. Прочитайте слова и назовите их русские эквиваленты :
single-phase, extensively, application, to gain, expensive, to manufacture, to eliminate, equipment, refrigerator, to be divided, machine, to exert, necessary, slightly, to reverse, pulsating, therefore, efficient, uniformly, grinder.
II. Найдите в тексте эквиваленты следующих слов и словосочетаний :
работать на однофазном переменном токе, маломощный двигатель, получать преимущество, менее дорогостоящий, исключать необходимость, вентилятор, двигатель с последовательным соединением, пусковое устройство, переключатель, пытаться, сжимающийся, расширяющийся, увеличить скорость, немного повернуться, благодаря (чему-либо), действовать равномерно.
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