Контрольная работа №1(Вариант №1
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Notes: 1enclosure – кожух, корпус, вид исполнения двигателя 2open – dripproof – каплезащитное исполнение 3splash – proof – брызгонепроницаемое исполнение 4reset – возврат 5single – pole toggle switch – однополюсный рычажный выключатель Вариант №3. Manual Electric Drive Controls. Switches. Every electric circuit must have some type of switch. Therefore a switch is the most basic of all electrical controls. Its purpose is to open and close an electric circuit. It must start and stop the flow of electricity, and, in effect, it does this by connecting and disconnecting wires. The switch contacts always become a part of the electric wire that is used for supplying electricity to the appliance in question. Therefore, when the switch contacts are open, the wire is effectively broken, and when the contacts are closed, the wire is made to be continuous again. In addition to having contacts that open and close, a' switch must have a second mechanical feature: The contacts must open and close quickly. This is known as the snap-action feature of a switch. A switch would operate without this feature, but it would not last very long. There is an arc when the flow of electricity is started and another arc when the flow is stopped. The latter arc causes the switch contacts to become pitted, burned, and worn. The greater the arc, the greater is the wear on the contacts. If the contacts open and close very quickly, the amount of arcing will be greatly reduced. For this reason a mechanical means of-causing the switch contacts to snap open or closed must be incorporated in the switch mechanism. There is an exception to this requirement in the case of a mercury switch, for its contacts do not move. The quick opening and closing is achieved by a sudden movement of a pool of mercury. Approved nomenclature relative to the pole and throw designation of a switch is shown by illustration in Fig. 6. The AC Relay. A relay is a type of switch having contacts which are operated by magnetism. The contacts of the relay open and close circuits just the same as if the contacts had been operated by hand. The magnetism is created by allowing an electric current to pass through a coil of wire, and the magnetism is removed when the flow of current through the coil is stopped. A switch, separate from the relay, is used to connect and disconnect the coil from the source of power. The principle of operation of a relay is illustrated in Fig. 7. The separate switch which connects the relay coil to the source of power may be any type of manually operated switch, or it may be an automatically operated type. The magnetism created by the coil causes certain metal parts to snap together. The movement of one of the metal parts operates the contacts of the relay. The electrical input to the appliance is controlled by the relay contacts. When the coil of Fig. 7 is connected to the voltage source, the end moves upward and the contacts close. The movable contact is insulated from the movable metal part. Although the relay contacts of Fig. 7 are normally open and close when the coil is energized, they could be arranged to be normally closed, and then would open when the coil was magnetized. Most applications use the normally open type of contact. A further variation in the contacts is obtained by using double-pole-switch contacts (four-contact button) instead of the single-pole type illustrated in the figure. The current rating of the contacts is very important, and proper selection depends upon the current flow to the appliance which is to be controlled. The main advantage in using a relay is that the switch and wires which control the current flow to the relay coil do not conduct the current to the appliance. Since the current required for energizing the coil is quite small (less than 1 amp), the wire and switch can have low-current ratings and proportionately low costs. The large appliance current is conducted through the contacts of the relay. Also the switch may be placed at a distant position from the appliance since long lengths of small-size wire between switch and coil do not cause voltage drop in the wires leading to the appliance. This type of arrangement is .well adapted for controlling motors, heaters, or large lighting loads from remotely located small-size switches. Pushbuttons.1 The momentary-contact manually operated pushbutton is the most common type of remote control device for starting and stopping electric motors. It is also used for .operating the electric-relay type of circuit. This control is not the same as the pushbutton wall switches2 that-are sometimes used in the home for controlling lights. That type of pushbutton is not momentary contact; Instead its contacts remain open or closed until the opposite button is pressed. Another difference is that the pushbutton wall switch is a complete control in itself, whereas the momentary-contact pushbutton requires other equipment in performing its controlling function. The standard-duty pushbutton control for farm use is usually the two-button type. The function of each button is clearly labeled on the button or on the metal cover. Pushbutton use for circuit control is illustrated in Fig. 8. The buttons and their related contacts are held in position by springs. The buttons are usually made of bakelite or similar moulded insulation, and the contacts are silver or silver alloy. The current rating of the button contacts is small (1, 2, or 3 amp) since the appliance current does not flow through these contacts. Knockout holes for the wiring are provided in the metal or bakelite cases. The pushbutton control illustrated in Fig. 8 starts and stops the flow of current to a relay coil. The contacts of the relay, in turn, control the power input to the appliance. Which in this illustration is several electric strip heaters. When the start button is pressed downward, current flows from L1 through both pushbuttons, through the relay coil and back to L2. The contacts snap closed and the appliance is on. When the start button is released, a spring3 causes it to return to its normal position (the normal position is up ns shown in the figure). With the start pushbutton in the released position current cannot flow through its contacts, but notice that the relay contacts labeled x and у would be dosed and would conduct the current around the start button find on to the coil. When the stop button is pressed downward, the current flow to the coil is interrupted, the coil loses its magnetism, and the relay contacts open, thereby disconnecting the appliance. Notes: 1pushbuttons – кнопки, клавиши 2pushbutton wall switch – настенный кнопочный выключатель 3a spring – пружина Вариант № 4. Automatical Electric Drive Controls. Thermostats. A thermostat is a switch, but it is a special kind of switch. Its switch contacts must open and close automatically in response to1 changes in temperature. A thermostat 18 therefore an automatic electric switch used for temperature control. Classified according to usage, there are two general types of thermostats. The type for heating equipment has contacts which close when temperature decreases. When the temperature increases up to some predetermined value,2 the contacts automatically open and disconnect the heating equipment. If the temperature decreases a small amount (usually 1° to 4° F) below this predetermined value, the contacts close and the heating unit functions, raising the temperature bask to the original value. The other type of thermostat is for cooling and has contacts which close on a temperature rise. When the temperature increases up to a certain value, the contacts of the thermostat close and start an air-conditioning unit, a fan, or a refrigerator. When the temperature is back down to the desired value, the contacts open and stop the cooling equipment. Thermostats are undoubtedly the most widely used type of automatic control and already have found many applications on the farmstead. They are used in hot-water systems, electric hotbeds, electric brooders, electric incubators, refrigeration systems, crop driers, ventilating-fan cooling systems, electric stock waterers, electric ovens, and electric irons and for controlling the heating system of the farm residence and other farm buildings. Several different types of thermostats are shown in Fig. 14. It does not matter whether the thermostat is to be used for heating or for cooling; its switch contacts could be operated by the action of a bellows. A bellows is like a balloon in that It expands when the pressure inside is increased and contracts when the pressure is decreased. Actually, the bellows lengthens and shortens rather than changing in diameter. The change in the length of the bellows trips and resets the switch mechanism, and so to understand why the switch contacts open and close, it is necessary to know why the pressure inside the bellows changes in amount. The bellows3 is connected by a small metal tube to a metal bulb which contains liquid and gas. When the liquid inside the bulb is heated, it begins to boil (evaporate) and change into gas. The additional gas created by the boiling liquid causes an increase in pressure inside the bulb, the tube, and the bellows. The bellows will expand (get longer), owing to this increase in pressure. When the gas inside the bulb is cooled, it begins to condense thereby changing back into the liquid form. Less gas means less pressure inside the bulb, the tube, and the bellows.  Fig. 14. Thermostats used on farms. The air switch shown on the left has a coiled bimetallic strip which operates a mercury switch. The bellows type with bulb and capillary tube, the bimetallic-strip type, and the wafer type are also pictured in this figure. Owing to the decreased pressure, the bellows contracts (gets shorter) .and allows the switch contacts to change position. It is significant to realize that the switch contacts are opened and closed in accordance with the temperature surrounding the metal bulb; therefore the location of the metal bulb is very important. The location of the thermostat itself is not important in controlling temperature, but the length of the tube which connects the bellows to the bulb must be considered when planning the various installations4. The amount of pressure required for expanding the bellows and operating the switch contacts is influenced by spring tension. The tension is adjustable and opposes the expansion of the bellows. The gas pressure inside the bellows will then have to be greater before the bellows can expand. Accordingly, the switch contacts will not be operated until the temperature increases, thus resulting in a greater gas pressure. Most metals expand when heated, but they do not expand the same amount. Two unlike metals which expand different amounts with an equal change in temperature provide the important mechanism in a bimetallic thermostat. The unlike metals are bonded together into one unit, and when the surrounding temperature increases, one metal expands more than the other, which causes the unit to bend. This bending action is used to trip and reset the switch contacts. Refer to Fig. 9 for an illustration of the bimetallic-strip principle, but real.ize that the bimetallic strips of a thermostat automatically reclose and do not need to be manually reset is do the contacts of the unit shown in this figure. Usually this type of thermostat is cheaper than the bellows type. It has fewer parts. However, it differs in application in that the thermostat itself must be placed in the location where the temperature control is desired. This placement is necessary since the bimetallic strip is the item that responds to temperature changes, and it is mounted inside the thermostat housing. In the bellows type the bulb responds to the changes, and it is separated from its associated thermostat housing. The wafer thermostat is the cheapest of the three types that are used on the farm in connection with electric heating. It consists of two hollow metal disks5, the switch mid its contacts, and a frame for holding the parts in place. The metal disks are filled with gas. When the gas is heated, the pressure increases and the disks expand. The expansion pushes the switch contacts open. When the disks are cooler, they contract and the switch contacts close. The thermostat is not calibrated in degrees, but this may be done locally. Like the bimetallic type, the entire thermostat must be located at that position where the temperature is to be controlled. Notes: 1in response – в ответ на… 2predetermined value – предопределенная величина 3the bellows – мембрана, сифон 4installation - установка 5hollow metal disk – полные металлический диск Вариант№ 5. PARAMETERS OF AN ELECTRIC CIRCUIT. Parameters of an electric circuit are made up of three circuit components: resistance, inductance and capacitance which determine the circuit characteristics.
The purpose of any circuit is to transfer electric energy from an electric generator battery of alternator3 to some other point of the circuit, where electric energy is converted to some other form of energy, such as heat, light or mechanical energy. It should be understood that energy is transmitted only when both electric and magnetic field exist around the сconductor. The amount of energy transfer depends on the intensities the electric and magnetic fields. ELECTRIC MOTORS. The electric motor is exceptionally well suited for farm jobs, as it is efficient in converting electric energy into mechanical energy, is easy to start and stop and can be controlled automatically. Its overload capacity makes it particularly important as a farm power unit since it can develop more than twice its rated power for periods of a few minutes without damage. Electric motors when properly selected and installed serve many years and require only periodic care. The electric motor has probably done more to make the farmer's standard of living equal to that of the urban resident than any other electric device including the electric lamp. There are many different sizes of electric motors. Why are these different sizes needed or, in other words, what is different about the various loads that causes one to select one of motor for this application and another for a different application? The answer is obtained by examining the characteristics of the various loads. Any equipment4 requiring an electric motor has moving parts. The parts represent an opposition to being moved, and in order for the motor to rotate it has to develop enough force to overcome all opposition to its rotation. The turning effort that the motor uses to overcome the opposition is called torque. The opposition is called countertorque or resisting torque. The amount of counter-torque depends upon the construction of the equipment and upon the amount of opposition added when the machine is performing its job. Equipment with large, heavy, moving parts require- more torque than equipment with lightweight parts. A 16-ft conveyor requires more torque when it is operating at а 450 angle than it does when it is horizontal. The more torque necessary) for turning the load, the larger is the horsepower rating5 of the motor required for the job. |
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