II. Одним предложением выразите главную мысль третьего и четвертого абзацев текста.
III. Письменно изложите краткое содержание текста на английском языке (не более 10 предложений).
Text 7
Thermal Relay
Electrical and electronic equipments are usually protected against excessive currents by fuses and thermal relays which make use of the heating effect of a current.
A fuse is simply a piece of metal with a low melting point, inserted in series with a circuit which is to be protected. When the current through the circuit exceeds the maximum safe value, the fuse metal metls, breaking the circuit. Although a fuse is simple and unexpensive, it has the disadvantage that it is almost instantaneous in the action (when the current exceeds the maximum value, if only for an instant, the fuse blows). A fuse cannot be used to protect motors, therefore, because the starting current is many times higher than the operating current.
Thermal relays overcome this disadvantage. One type is made of two different metals which have different rates of expansion. The two metals are welded together, and the unit is called a "bi-metal strip". When a bi-metal strip is heated, it will bend by reason of the unequal rates of expansion of the metals. A strip is placed near a heating element through which the current to the motor passes. One end of the strip is fixed, and the other is free. When an overload current persists long enough, the free end bends, and opens a set of contacts which are in series with the motor. The motor circuit is broken and the motor stops. The bi-metal strip is held open by a catch, which must be manually reset.
Thermal relays are also used as a time-delay devices in some equipments. When a switch is closed, the heating element of the relay is connected to a voltage source, and the bending strip (after an appropriate interval) closes a pair of contacts.
One application of the thermal overload relay is in a common manually-operated starter used with a.c. motors.
When the start button is depressed, the contacts are closed, and the motor energized with three-phase a.c., rotates. If the motor is overloaded and draws excess current, the thermal relays operate and release the start switch. The contacts open, and the motor stops.
Remember, that the thermal relays must be re-set after you have checked the cause of the overloading.
Similarly, when the stop button is pushed, it releases the start button and the contacts open, stopping the motor.
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N o t e s
| "bi-metal strip" | биметаллическая лента |
E x e r c i s e s
I. Соедините два предложения, употребив одно из предложенных в скобках слов.
1. A fuse is a piece of metal with a low melting point. It is inserted in series with a circuit (which, what). 2. The fuse metal melts. The current through the circuit exceeds the maximum safe value (where, when). 3. The starting current is many times higher than the operating current. A fuse cannot be used to protect motors (because, although). 4. The motor is overloaded and draws excess current. The thermal relays operate and release the start switch (although, if). 5. The start button is depressed. The contacts are closed (when, because).
II. Одним предложением выразите главную мысль второго и третьего абзацев текста.
III. Письменно изложите краткое содержание текста на русском языке (не более 10 предложений).
Text 8
Review
Magnetic Relay
A magnetic relay is an electrically operated switch which consists of an electromagnet and a movable arm. When current flows through the coil, a magnetic field is set up which attracts the movable arm to the core of the magnet. This causes a set of contacts on the arm and relay frame to close, completing a circuit. When the coil is de-energized, the return spring pulls back the arm, and the relay contacts open.
The contacts can either be normally-closed or normally-open.
Thermal Relay
Thermal overload relays protect a circuit from overheating. One type consists of a bi-metal strip, in which two metals having different rates of expansion are welded together. When the strip is heated, it bends, causing a set of contacts to open. Other thermal relays can be used as time-delay devices.
Circuit Breaker
Circuit breakers are magnetic-type overload relays designed to protect circuits from overloads, voltage failure or current reversal. When the current through the magnetic coil of the breaker exceeds a certain value, the triggering mechanism is released, and the contacts are pulled apart, either by means of a weight or by spring action, thus opening the circuit.
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No t e s
| time-delay | время срабатывания (реле) |
Письменно изложите краткое содержание текста (не более 10 предложений) на английском языке.
Text 9
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