Методические указания к контрольным заданиям для студентов агробиологических и агроинженерных направлений заочной формы обучения

The Story of American Schools The first schools in America started in the 1600s. The Puritans, that is people who left England because of their religious beliefs, wanted each person in New England to know the Bible. So they organized schools to teach religion and basic subjects. But by the 19th century large numbers of children did not attend school. The problem of children's education started a great debate in America. There were three groups of people who had different ideas. One group said that young people should spend their time at home helping their families. As most Americans lived on farms there was always much agricultural work to be done. The second group, mostly businessmen, believed that children should work at factories. America's Industrial Revolution had begun, and this group knew that there would be many jobs in manufacturing. Some young people were already working at factories. They were children from 7 to 16 years old and their working day lasted up to 13 hours. The third group said that to help create a better society, young people should know how to write and express their own ideas. Therefore each state should develop a system of public schools, called free schools, or common schools. This idea had been supported by Thomas Jefferson, the third president, and later by Abraham Lincoln who said that education was very important for people. In 1839 Horace Mann, a Massachusetts-born educator, a lawyer by profession, opened the first common school in the United States. He devoted his life to this idea and soon a lot of common schools were opened throughout the state of Massachusetts. His example attracted national attention. Before long many states were doing what Massachusetts had done. The free school supporters had won the debate. Энергетический факультет Вариант № 1 Farm Electric Motors Selection According to Starting Requirements Single-phase electric motors1 are not inherently self-starting. Some special component part or style of winding must be incorporated into their design before they will start themselves and any attached load. Various electrical principles are used to accomplish this purpose, and all except one of the single-phase electric motors are named after the principle employed. This fact accounts for the following names: split-phase, capacitor-start induction run capacitor motor, two-value capacitor motor, repulsion motor2, repulsion-induction motor, repulsion-start induction motor, shaded-pole motor, and the one exception - the universal motor. Since different electrical-starting principles are employed, it is understandable that the motors could very likely have different abilities to start a load. That is exactly the situation. Therefore, the name not only indicates a certain starting principle but also designates the motor's ability to develop starting torque. Furthermore, since one of the motor's jobs is to start the load, the selection of a motor to perform this duty is made according to the name of the motor. Fig. 2. (a) Repulsion-start induction motor. This motor develops a very high starting torque. (b) Two-value capacitor motor also known as a capacitor-start capacitor-run motor. The capacitors are located in the base or in the end shields. Fig. 2. (a) Repulsion-start induction motor. This motor develops a very high starting torque. (b) Two-value capacitor motor also known as a capacitor-start capacitor-run motor. The capacitors are located in the base or in the end shields. The actual selection of a motor for starting a certain farm load is commonlyу made from the three types of motors shown in Figs. 1 and 2 and three-phase type 3of Fig. 3. The other types of motors are not as widely used, or are available only with their associated equipment. Machines that must be started with a part or all of their operating load attached, or machines which in themselves present a fairly large amount of resisting torque during the starting period, are said to be h a r d t о s t a r t. This category includes such machines as a meat grinder, the vacuum pump of a milking machine, Fig. 3. Three-phase squirrel-cage induction motor. It is used when three-phase service is available. This motor will not start or operate satisfactory on a single-phase system. Notice that the bearings of this particular motor are lubricated for life. a small air compressor, a piston-type water pump, and a large-diameter attic fan. The smaller sizes of feed grinders and conveyers are also classed as h a r d to start. Loads of this type require a motor that develops a high-starting torque. The capacitor-star induction motor and the three-phase induction (squirrel cage) motor fulfill this requirement. For the average farm a three-phase supply is not available so the capacitor-star motor is the suggested solution to the selection problem. The capacitor-start induction motor is commonly available in sizes ranging from 1.6 to 5 hp. However, loads in this category do not usually require a motor larger than 1 hp. This motor develops more starting torque than an equivalent-size split-phase motor and at the same time has less input current while starting. It has a greater initial cost than the split-phase motor, from one and one half to two times more, but other than starting, its operating characteristics are the same. Many farm machines offer a very large resisting torque when being started, since they must be started while completely loaded, or under conditions which cause even greater loads than normal. Such machines as large air compressors or refrigeration compressors, small feed grinders (up to 1 hp), certain elevator conveyers, and many of the larger water pumps fall into this category. The capacitor-start motor described in the previous section may be satisfactory for these jobs. However, the repulsion-start induction-type motor develops more starting torque and is best adapted for the h a r d e s t-t o-s t a r t loads. The repulsion-start motor develops about 20 per cent more starting torque than the capacitor-start motor, and even then it requires less input current. The lower value of input current is very important since it means less voltage drop in the lines which serve the motor. This line-voltage-drop factor validates still further the selection of a repulsion-start induction motor for the very-hard-starting loads. The repulsion-start induction motor has more parts than the capacitor-start motor and may be 7 to 12 per cent more expensive in the smaller sizes, but the cost from 1.2 hp and upward is usually the same. This motor has brushes and a commutator which may require occasional attention and, in general, a variety of sizes are not readily available in many electrical stores. The larger stores usually stock the motor in sizes ranging from 1.6 to 10 hp. Its principal advantage is that it has the highest starting torque per input ampere of any of the single-phase induction motors. Larger loads such as ensilage cutters, large feed mills, conveyers, gutter cleaners4, mixers, and blowers are usually hard or very hard to start, and require motor sizes such as 1.5, 2, 3, and 5 hp. This type of load is separated from the others since the driving motor must develop a large running torque and consequently will also develop a large starting torque. One type of motor which was previously described, the repulsion-start type, will also be satisfactory for these larger loads. In addition, two other types, the capacitor-start capacitor-run motor and the three-phase induction motor (squirrel-cage type), are quite well adapted for these applications. The capacitor-start capacitor-run type (also termed a two-value capacitor motor) has essentially the same starting characteristics as the capacitor-start motor but has better running performance. It has a higher efficiency and a higher power factor and is in effect an improved capacitor-start motor for the larger-horsepower sizes. The cost is approximately the same as that of the repulsion-start motor, and the motor is normally available in sizes from 1.2 to 10 hp. For sizes over 1 hp the three-phase induction motor is the least expensive of the three types. It has about the same starting torque as the capacitor type and is the most rugged, reliable, and satisfactory motor of the group. This motor is highly recommended if three-phase power is available. It is manufactured in sizes ranging from 1.6 hp upward. Notes: 1single-phase motor – однофазный мотор 2repulsion motor – репульсионный мотор 3three-phase motor – трехфазный мотор 4gutter cleaner - канавоочиститель Вариант №2 Farm Electric Motors Selection According to the Surroundings After having determined the horsepower rating of the motor for driving the load and the type of motor that will satisfactorily start the load, the remaining decisions involve those selections which are related to the location and surroundings in which the motor will be operated. Enclosures1. The enclosure, or housing, is most important in protecting the working parts of the motor. Frequently motor failures occur or the life of the motor is greatly reduced because the type of enclosure was not given proper consideration. There are six standard types of enclosures, but unless the farm motor application is most exceptional, only three of these types need to be considered. These are the open-dripproof, the splashproof, and the totally enclosed types. The most common type of enclosure for electric motors is the open-dripproof type2. The motors of Figs. 1 and 2 have this type of enclosure. It is applicable for locations in which the atmosphere is relatively free from foreign particles or splashing liquids. It should not be selected if the motor is to operate near a water spray, in the rain, or in areas containing lint, dust, or metallic or grain particles. In general, if the atmosphere bothers the operator, it is certain to be too much for this type of enclosure. The ventilation openings are near the base of the enclosure and provide for air circulation which cools the motor's windings. If excessive amounts of foreign particles, water, or oil are pulled inside through these openings, they destroy the insulation on 'the windings by causing overheating of the wires. The foreign particles g. 4. The motor wittua splash- Fig. 5. The totally enclosedfmotor proof type of enclosure. is adequately protected from wa- ter, dust, and lint. may also get into the bearings and cause excessive wear. Regardless of this limitation, the open-dripproof enclosure is suggested for general-purpose use around the farm. The splashproof type3 of enclosure provides more" protection against water and dust than does the dripproof type. This enclosure is especially well suited for the dairy farm and for processing rooms where washing of the equipment is required. It is also installed outdoors but should be covered when not in use. The housing protects the motor against water and particles, with the exception of the small amounts I hat may enter at an angle upward from the floor. The over-lead and sides are completely shielded. The splashproof enclosure is shown in Fig. 4. The totally enclosed type of enclosure affords the most reliable protection for the motor of any of these types. (No air is circulated through the motor since there are no external openings, but the cooling is accomplished by direct radiation and by convection.) The totally enclosed motor should be used in many places around the farm, but so far its use is not too common. It is a good selection for the driving motor of a feed grinder or similar machine where the atmosphere is filled with dust and small pieces of grain, and it is also adapted to areas subjected to water sprays. A motor having a totally enclosed housing is shown in Fig. 5. Overload Protection. An overload means that the amount of current flowing to the motor is greater than the value of current marked on the nameplate of the motor. A motor must have current in order to produce torque, but the current also produces heat. The insulation on the motor's windings is not injured if the motor temperature is within the rated limits (usually 40°C above room temperature) but is damaged by higher temperatures. It is only logical that proper protection against excessive current be provided before operating the motor. Excessive current flows to a motor owing to any one of- the following reasons: the connected load is too great or becomes jammed; the belt is too tight; the bearings are worn or need lubrication; the input voltage to the motor is too low; the (V-type) pulleys are cut of line; alignment of the bearings is faulty owing to unequal tightening of the end shields or base. It may appear that these items can be avoided, but it is quite unlikely that they could all be avoided over a period of years. There are four types of overload protection for farm electric motors. One of these is installed by the manufacturer and is known as built-in-overload protection. Most types and sizes of motors for farm use are available with this protection. It is primarily suggested for motor sizes of 1 hp and less and is available in two styles - the automatic reset and the manual reset4. The automatic reset stops the motor in case of an overload and starts it again after it has cooled. This reset is not used for motors driving machines around which people are working, as there is a possibility of someone attempting to clear or clean the machine just as it starts again. Motors equipped with a manual-reset built-in overload are restarted by pressing a small button on the motor frame. These overload controls operate on the bimetallic-strip principle, and a certain length of time for cooling is necessary before the motor can be restarted. A second type of overload protection is the time-delay fuse. It is the cheapest of the four types so far as initial cost is concerned but must be replaced after it has performed its function. The correct ampere size for the time-delay fuse is obtained by multiplying the motor-nameplate current value by 1.15 or by selecting a fuse rating which exactly corresponds with the nameplate value. The manually operated motor starting switch is a very excellent type of overload control for motor sizes up to 1 hp. A similar switch with a larger frame is manufactured for sizes up to 3 hp. This overload control consists of a metal enclosure, a switch, bimetallic strips or a solder-and-ratchet wheel mechanism and a heater coil. The ampere rating of the heater coil or strip is selected by multiplying 1.15 by the motor-nameplate current value. The motor current flows through the heater coil, and the coil is designed to supply the necessary amount of heat to trip the switch mechanism if excessive current flows to the motor. The overload heater coil upon installation its ampere-rating tab should be retained and fastened to the switch, either in the switch lever or on the coil itself. The life of the coil is indefinite, but owing to the wide variety of types and sizes, replacements are not usually stocked locally, so it is well to include a spare with the initial order. The fourth type of overload protection for farm electric motors is the magnetic starting switch. The overload protection is once again gained as a result of heat generated by a heater coil or strip. When a predetermined amount of heat is being developed by the heater, the overload contacts open, thereby interrupting the flow of current to the main coil of the switch. Deenergizing the main coil breaks the contacts to the motor. The magnetic switch is operated with pushbutton control or with a single-pole toggle switch5. The pushbutton control is available as an integral part of the magnetic-switch enclosure or as a separate unit which can be remotely located for the convenience of the operator. It is always good practice to have a switch instead of using the plug cap of the attached cable as a switch. The larger the motor size, the more necessary a switch becomes, and one should be used for all sizes from 1.2 hp upward. The magnetic starting switch is especially suggested for the 3- and 5-hp motors and is recommended for use with the 1.5 - and 2-hp sizes. It is a very satisfactory type of switch and provided for no-voltage protection as well as protection against excessive current. Compared to the contacts of a double-pole manually operated switch, the fast-operating, positive-acting contact points of a magnetic switch perform much better and last longer.

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