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IV. Вставьте в предложения нужное слово.1. In a shunt-connected motor, the field is connected directly across the voltage source and is therefore... of variations in load and armature current (dependent, independent). 2. If the load on the motor increases, the motor... (speeds up, slows down). 3. When a shunt motor is started, the starting current is... (small, constant). 4. The reduced back-e.m.f. allows the armature current to... (decrease, increase). 5. For this reason... are considered constant-speed motors (series motors, shunt motors). V. Задайте вопросы к подлежащему и его определению с вопросительным словом what (что, какой). Образец. 1. Shunt motors are considered constant-speed motors. – What is considered constant-speed motors? 2. The amount of torque varies with the strength of the main field and the armature current. – What varies with the strength of the main field and the armature current? 3. Heavier torque is needed to drive the increased load. – What is needed to drive the increased load? – What torque is needed to drive the increased load?
1. In a shunt-connected motor the field is connected directly across the voltage source. 2. The field is independent of variations in load. 3. The reduced back-e.m.f. allows the armature current to increase. 4. So the developed torque varies directly with the armature current. 5. The starting current is small, when a shunt motor is started. VI. Дайте краткие ответы на вопросы (парная работа). Образец смотри в упражнении VII (page 82). 1. Is the torque developed by the motor caused by the force arising from interaction of the magnetic field around the armature coils with the main field? 2. Does the torque developed vary directly with the armature current? 3. Do the armature current and torque decrease when the motor speeds up? 4. Is the variation of speed from "no-load" to "full" load only about 10 per cent of the no-load speed? 5. Are shunt motors used where constant speed under varying load is desired?
VII. Расскажите о двигателе с параллельным возбуждением, употребив данные слова и словосочетания: in a shunt motor, the field, directly, the voltage, source, variations in load and armature current, if, the load, motor, reducing the back-e.m.f., when, the starting current, small, where constant speed, under varying load, is connected across, is independent of, increases, slow down, is started, is, are used, is desired.
Text 3 Series Motors The series motor has its field connected in series with the armature and with the load. The field coil consists of a few turns of heavy wire; and since the entire armature current flows through it, the field strength varies with the armature current. If the load increases, the motor slows down and the back-e.m.f. decreases, which allows the current to increase and to supply the heavier torque needed. The series motor runs very slowly with heavy loads and very rapidly with light loads. If the load is completely removed, the motor will speed dangerously and fly apart; for the current required is very small and the field very weak, so that the motor cannot turn fast enough to generate the amount of back-e.m.f. needed to restore the balance. Series motors must never be run under no-load conditions, and are therefore seldom used with belt drives from which the load can be removed. You can see, also, that series motors are variable-speed motors – that is, their speed changes a great deal when the load is changed. For this reason series motors are seldom used when a constant operating speed is needed; and they are never used where the load is intermittent – in other words, where the load changes frequently, or is put on and taken off while the motor is running. The torque – the turning force – developed by any d.c. motor depends on the armature current and the field strength. In the series motor, the field strength itself depends on the armature current, so that the amount of torque developed depends doubly on the amount of the armature current flowing. When the motor speed is low, the back-e.m.f. is, of course, low and the armature current is high. This means the torque will be very high when the motor speed is low or zero, such as when the motor is starting. Thus the series motor has a high starting torque. Because of its high starting torque the d.c. series motor must never be started unloaded. For if there is no opposing torque on starting, the motor will accelerate furiously and race to a dangerously high speed. The armature may disintegrate, with the coils and commutator segments flying out and causing damage. There are special jobs which require a heavy starting torque, and the high rate acceleration this heavy torque allows. Such applications are cranes, electric hoists. The motors used in these machines are usually series motors because the loads here are very heavy at start and then become lighter once the machine is in motion. Words and Word Combinations
E x e r c i s e s I. Прочитайте и переведите слова и словосочетания: series motor, heavy wire, increase – increases, allow – allows, dangerously, weak, fast enough, restore, never, seldom, variable speed, a great deal, intermittent, frequently, doubly, unloaded, on starting, special jobs, applications. II. Найдите в тексте эквиваленты следующих слов и словосочетаний: сила поля, ток якоря, с большими нагрузками, быстро, опасно, достаточно быстро, восстановить, редко, значительно, другими словами, часто, из-за, при пуске, требовать. III. Выберите и вставьте нужное слово в данные предложения. 1. The field coil consists of a... turns of heavy wire (many, few). 2. If the load increases the motor... (stops, slows down). 3. The series motor runs very... with light loads (rapidly, slowly). 4. For this reason series motors are seldom used when a constant operating... is needed (current, speed). 5. The... developed by any d.c. motor depends on the armature current and the field strength (load, torque). 6. When the motor speed is... the back-e.m.f. is of course, low and the armature current is high (low, zero). IV. Определите, от каких имен прилагательных образованы следующие наречия. Переведите эти пары слов на русский язык: rapidly, dangerously, frequently, furiously, highly, slowly, heavily. V. От полученных в упражнении IV имен прилагательных образуйте степени сравнения. Переведите все формы на русский язык. Образец. 1. warm – warm er, (the) warm est; 2. interesting – more interesting – (the) most interesting.
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