I. Составьте предложения из данных слов.
1. Acts, the rotor, like a magnet, by the rotating stator field, and, is attracted. 2. Are, synchronous motors, not self-starting. 3. An external source, power, of, require, the rotor, does not, in the induction motor. 4. Very little, with normal load changes, varies, slip.
II. Предложите заголовки ко всем абзацам текста.
III. Одним предложением ответьте на вопрос:
About what motors is this text?
Text 5
Testing Field Coils
Use an ohmmeter for testing for open and internally shorted field coils. Disconnect the field coil leads from the armature so as to avoid parallel circuits in testing, and place the ohmmeter across the field leads. If the ohmmeter reads infinity, there is an open circuit somewhere in the field winding. The open-circuited coil can be detected by testing each coil individually. The coil with the open circuit should be disconnected from other coils and replaced.
The armature resistance of a d.c. machine is normally so low that an ordinary ohmmeter will not be able to measure it. Even if the armature has a few shorted turns, the ohmmeter will read practically zero.
If the armature has an open-circuited turn, the ohmmeter will also read zero because of the numerous parallel paths. Therefore special equipment must be used to test armature.
N o t e s
| the ohmmeter will read practically zero | омметр будет показывать практически ноль |
E x e r c i s e s
I. Составьте предложения из данных слов.
1. Can be, the open-circuited coil, by, each, individually, detected, testing, coil. 2. Of, a d.c. machine, low, normally, resistance, the armature, is. 3. Must, to test, special, be used, armature, equipment.
II. Одним предложением выразите главную мысль первого абзаца текста.
III. Письменно изложите краткое содержание текста на английском языке (не более 3 предложений).
Text 6
Electromechanical Devices
Relays
Youlearnt when studying d.c. motors that a starting resistance is reguired to limit the starting current of a motor to a safe value.
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Starting resistors are therefore always incorporated into starting boxes, which are either manual or automatic. In the manual starting box, a button is pressed to start, stop or reverse the motor. Automatic circuits making use of what are known as "relays" then take over and do the rest.
Relays offer many advantages over the use of manually-operated switches. They make possible the control by a low voltage circuit of a high voltage (high current) circuit. For example, in the relay a high voltage circuit could be connected across terminals. This circuit could be energized by applying a low voltage to the relay coil. Similarly, the high voltage circuit could be opened by removing the low voltage from the relay coil. Since the operator does not directly close or open the high voltage circuit, he is protected from high voltage shock, and from a possible burn due to arcing at the switch.
Since the relay coil circuit is operated at low voltage and current, the switch for the relay circuit can be located at a point distant from the relay. By this means, an operator can turn equipment on or off from a remote location. For example, a wireless operator can key a transmitter located in another part of a ship or building, by using a "morse" key and a relay. Whenever the operator presses the key, the relay in the transmitter is energized, and its contacts turn on the high voltage to the transmitter.
N o t e s
| making use of what are known as "relays" | используя так называемые "реле" |
| take over | берут это на себя |
| do the rest | делают все остальное |
E x e r c i s e s
I. Соедините два предложения, употребив одно из предложенных в скобках слов.
1. Starting resistors are always incorporated into starting boxes. They are either manual or automatic (which, what). 2. Relays offer many advantages over the use of manually-operated switches. They make possible the control by a low voltage circuit of a high voltage circuit (because, if). 3. The operator is protected from high voltage shock. He does not directly close or open the high voltage circuit (whether, since). 4. A wireless operator can key a transmitter. It is located in another part of a ship (where, which). 5. The relay in the transmitter is energized. The operator presses the key (when, which).
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