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

Electric Motors Protection. Thermal Overload Relay. A thermal overload relay consists of two parts: (1) a small switch that is normally held either open or closed against the force of a spring by means of a catch and (2) a thermal device for tripping the catch. This thermal device is usually a bimetallic strip which is heated by current passing directly through the strip or by a heater coil surrounding the strip. The bimetallic strip consists of two thin layers of different metals welded together. The metals are chosen to have quite different temperature coefficients of expansion4, and, since the strip is firmly fixed at one end, the other end deflects as the temperature rises. The heater coil is in series with the circuit that is to be protected and is therefore designed to have a low resistance. It is provided with a shunt if the current is large. Such a device will not operate on momentary overloads, such as occur during the starting of a motor, but will operate if the overload persists long enough to heat up the bimetallic strip to the tripping point. The magnetically operated contactor switch of Fig. 17 can be made to give automatic overload protection if one inserts the contacts of a thermal overload relay in the circuit of the operating coil and connects its heater coil in series with the main circuit. Notes: 1pressure switch – переключатель напряжения 2application - применение 3overload - перегрузка 4coefficient of expansion – коэффициент расширения Вариант №3 Fuses1 A fuse is a piece of metal inserted in a circuit that is intended to melt and open the circuit before excessive currents have had time to damage the remainder of the circuit by overheating. The melting2 or blowing of a fuse is accompanied by аn arc3 and by spattering of 'the fused metal, so that it is generally advisable to mount the fuse in the center of a tube. Sometimes the tube is then filled with oil or a fireproof powder to quench the arc. A fuse has heat storage capacity and, as far as protection from excessive current is concerned, combines the functions of the thermal overload relay and the magnetic contactor switch. Fuses are available for currents up to 600 amp at voltages up to 600 volts and for currents up to 300 amp at voltages up to 34,500 volts and also for smaller currents at voltages up to 132,000. The higher voltages are alternating current only. Protection from Overloads and Short Circuits4. Electric circuits and machines must be protected from overloads and short circuits, and this is accomplished by automatically disconnecting the circuit or machine from the line when such conditions occur. An overload in general can injure5 a cable or machine only by overheating it to a temperature that will damage the insulation, and, since all materials have heat storage capacity, it follows that moderate momentary overloads do no damage. Therefore the protective equipment should not operate on such overloads. On the other hand, short circuits or other faults begin doing damage immediately and very rapidly. Therefore they should be disconnected as quickly as possible. The circuit breaker with instantaneous overload trip provides the most rapid and dependable protection against short circuits and is used extensively for the protection of the smaller and less important installations. The best protection against overloads is given by thermal overload relays controlling automatic switches, such as circuit breakers or magnetic contactor switches. If magnetic switches are required in any case for the normal operation of the motor, this method of protection is not expensive. In the case of small low-voltage motors fuses are often the only protection provided. They respond too quickly to momentary overloads, and this often means that they must have current ratings too high to provide adequate protection against moderate continuous overloads. Circuit breakers are often provided with an inverse-time-limit attachment. This is simply an adjustable dashpot that delays the movement of the tripping mechanism. The length of the delay is approximately inversely proportional to the current through the trip coil, so that the breaker responds slowly to moderate overloads but opens quickly on heavy short circuits. The delay feature makes it possible to obtain selectivity in the operation of circuit breakers that are in series on the same system. By adjusting the time delays, the breaker that is nearest to the fault is made to open first. This removes the fault, and therefore the other breakers do not open at all. In this way the interruption is localized. No-Voltage Release. Suppose that a motor is running at normal speed and that the power supply is interrupted owing to some trouble in the powerhouse or in the line; the motor will stop. If the power supply is now reestablished, with the starting arm still in the running position, there will be no starting resistance in series with the armature to limit the current. To take care of such a contingency, the starter is usually provided with a no-voltage release, as shown at M, Fig. 18. The starting arm is moved from the starting to the running position against the tension of the spring S and is held in the running position by the electromagnet M. the power supply is now interrupted, the motor will slow down and stop. The exciting current does not drop to zero as soon as the power supply is interrupted, because the voltage Eg generated by the motor is across the field circuit. That is, the motor has automatically become a self-excited generator with no prime mover to drive it. As it slows down, its voltage decreases and consequently its field current also decreases, which causes a further decrease in voltage. At about half speed the magnet M becomes too weak to hold the arm against the pull of the spring, and the arm is pulled back to the starting position. The resistance R may be omitted if the line voltage is low enough to make it feasible to design the coil of M for full-line voltage. Fig. 18. Starter with a no-voltage release for a shunt or compound motor. Another method of exciting the magnet M is to connect it in series with the field coils of the motor. When connected in this way, it not only acts as a no-voltage release but also protects the motor against the hazard of a break in the field circuit. If the field circuit is accidentally broken, the field current is interrupted, and the flux and back voltage drop approximately to zero, thus allowing a very large current to flow through the armature winding. The magnet M, however, releases the starting arm, which flies back to the starting position and interrupts this current before any damage is done. The no-voltage release of Fig.18 is the standard protection. It does not protect against a break in the field circuit, but all motors are protected by either circuit breakers or fuses, and the very large armature current resulting from a break in the field circuit causes the breaker of fuses to operate very quickly and disconnect the motor from the line. The chief objection to the no-field release is that most shunt motors are operated with field control or speed, and it is difficult to provide a magnet that will function properly over the whole range of field current. The no-field release is used chiefly in special applications where the chances of the field circuit being broken are unusually high, as for example in experimental laboratories. Notes: 1fuses - предохранители 2to melt – плавить(ся) 3arc - дуга 4short circuit – короткое замыкание 5to injure - повреждать Вариант №4 Conductors1 For all practical purposes the copper cable and the star-section steel rod are the best forms of conductors, although there is no serious objection to the use of a tubular conductor. Cable has the advantage of being flexible and therefore easily installed. Since it may be purchased in lengths as great as 1,000 feet, it need have few joints. If too loosely woven, it may not have the required stiffness. Steel rods make a mechanically strong, durable job. The 10-foot lengths must be screwed together and special tools are required for making bends. Recently aluminium conductor has come into use, but care must be taken to avoid electrolytic corrosion where the aluminium comes in contact with iron, copper, or zinc. Such contacts should be avoided u possible. According to the code the use of copper, copper-covered steel, or copper-alloy fittings is not permissible when an aluminium conductor is used. Aluminium should not be used for ground connections, since it corrodes in the ground. Copper should be used in such exposures. The light weight and flexibility of aluminium, however, are advantageous юг conductors above ground. According to the Code, copper cable2 or other copper conductor must have a weight of not less than 3 ounces per foot galvanized steel about 5 ounces, and aluminium about 1.6 ounces. Excellent lightning conductors are made with galvanized steel or iron or steel with an integral copper coating. Painting conductors above ground does not detract from the value of the conductor, and their life may be increased by painting as soon as serious corrosion is in evidence Aluminium conductors and fittings used in seacoast areas, subject to the corrosive action of salt air, should be protected by painting or other means immediately upon installation. Conductors forming the earth connections should not be painted, since the electrical resistance of the ground is thereby much increased, whereas the resistance should be as low as practicable. Air Terminals. Air terminals usually consist of three parts: (1) the point, (2) the elevation rod or vertical conductor, to the upper end of which the point is attached, and (3) the tripod support. Sometimes only a point (not less than 10 inches high), footed direct to the roof conductor so as to be inconspicuous, is used. The points are the objects in the protective conducting system that receive the lightning discharge, thus preventing damage to the building itself. When the elevation is short erect position of the air terminal is maintained by firm attachment to the horizontal conductor on the roof, but, for heights of 18 inches to 5 feel galvanized-iron tripod supports are often used. The supports are frequently attached to the roof by galvanized nails. Screws are better, and where ready access3 may be had to the underside of the roof a firm attachment can be obtained by the use of bolts. Air terminals erected as described and illustrated are seldom damaged by wind or by snow and ice storms. The copper tube or so-called shell point is extensively used with the copper-tube elevation rod. When the point is solid for 3 inches or more from the end and the walls of the tube are not less than No. 20 (0.032 inch) gauge in thickness, they are sufficiently heavy to withstand the fusing effect of severe strokes. An alternative is the copper bay point, a solid, substantial fitting that is generally preferable. The bayonet point is also used with star-section steel elevation rods. Multiple points may be used without objection when the individual points are sufficiently heavy, but one large-size heavy point is enough for one air terminal and is less expensive. Where conditions causing corrosion are severe for example, at the top of chimneys, points and their associated conductors and fasteners if within 25 feet of the source of corrosive gases or fumes are lead covered. The elevation rod may be attached satisfactorily4 to the main conductor in several ways. The lower ends of tube elevation rods are so shaped that they can encircle a copper cable and be squeezed firmly into contact with it without the use of solder. A T-connector, designed to be inserted into the tube and made with finger grip that firmly engages the cable, may also be used. Star-section rods are provided with screw fittings5 for contact with the flanges of the conductor. Where end-to-end joints of the conductors are necessary, suitable solderless connectors may be used. When copper cable is used joints may be made with fittings of rugged construction that will permanently engage and connect the parts. Joints are almost entirely avoided in copper-cable installations, since the cable often can be run over a building from one corner to another diagonally opposite without a break. All joints must be mechanically strong and of low resistance without dependence upon the use of solder. Solder may be added to protect the joint from corrosion. Fasteners6 for securing the conductors to the walls and roof of a building should be spaced generally not more than 4 feet on the construction of the building. The principal aim is to obtain a neat and durable job with the conductor firmly held in place. Holes through the roof made by the fastener screws or nails must be made watertight. Notes: 1conductor – проводник 2copper cable – медный кабель 3access – доступ 4satisfactory – удовлетворительно 5fittings – осветительная арматура 6fasteners – крепеж Вариант №5 Galvanized-Steel Conductors If galvanized-steel, star-section rod with copper-bronze couplings is used, the rod shall be three-fourths inch in diameter. The conductor shall be run as directly as possible on the building with no sharp bends or loops, and in such manner the rod shall be used, and end-to-end joints shall be carefully screwed together. Where branches are attached to the main conductor, Y-connectors shall be used so that bends of at least 2-foot radius may be formed with the laterals. Copper-bronze or galvanized, malleable-iron screw fasteners or other fasteners of approved design shall be used. The fasteners shall be spaced not more than 4 feet apart, and screwed directly into wooden walls and roofs. Lead expansion shields shall be used for masonry and concrete walls. Holes through the roof made by the fasteners shall be rendered watertight by means of elastic roof cement. Star-section elevation rods shall be used with an inverted Y- or other approved connection to the main conductor. The diameter of the elevation rod shall be the same as the conductor. Solid, copper, bayonet air terminal points screwed to the upper end of the elevation rod shall be used. Air terminals shall be placed above ridges, gables, chimneys, and flat roofs, and shall be not less than 10 inches above the tops of chimneys, peaks, and pointed parts of the building. Galvanized-iron tripod supports for the air terminals shall be furnished and installed and fastened to the roof with galvanized-iron screws, bolts, or expansion screws, as the case requires. When bolts are used, a lock nut shall be provided or the end of the bolt upset to prevent loss of nut.

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