Fixed and programmable automation

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12. AC MOTORS

Two basic types of motors are designed to operate on alternating current: synchronous motors and induction motors. The synchronous motor is essentially a three-phase alternator operated in reverse. The field magnets are mounted on the rotor and are excited by di­rect current, and the armature winding is divided into three parts and fed with three-phase alternating current. The variation of the three waves of current in the arma­ture causes a varying magnetic reaction with the poles of the field magnets, and makes the field rotate at a con­stant speed that is determined by the frequency of the current in the AC power line.

The constant speed of a synchronous motor is advan­tageous in certain devices. However, in applications where the mechanical load on the motor becomes very great, synchronous motors cannot be used, because if the motor slows down under load it will «fall out of step» with the frequency of the current and come to a stop. Synchro­nous motors can be made to operate from a single-phase power source by the inclusion of suitable circuit elements that cause a rotating magnetic field.

The simplest of all electric motors is the squirrel-cage type of induction motor used with a three-phase supply. The armature of the squirrel-cage motor con­sists of three fixed coils similar to the armature of the synchronous motor. The rotating member consists of a core in which are imbedded a series of heavy conduc­tors arranged in a circle around the shaft and parallel to it. With the core removed, the rotor conductors resemble in form the cylindrical cages once used to ex­ercise pet squirrels. The three-phase current flowing in the stationary armature windings generates a rotating magnetic field, and this field induces a current in the conductors of the cage. The magnetic reaction between the rotating field and the current-carrying conductors of the rotor makes the rotor turn. If the rotor is re­volving at exactly the same speed as the magnetic field no currents will be induced in it, and hence the rotor should not turn at a synchronous speed. In operation the speeds of rotation of the rotor and the field differ by about 2 to 5 per cent. This speed difference is known as slip.

Motors with squirrel-cage rotors can be used on sin­gle-phase alternating current by means of various ar­rangements of inductance and capacitance that alter the characteristics of the single-phase voltage and make it resemble a two-phase voltage. Such motors are called split-phase motors or condenser motors (or capacitor motors), depending on the arrangement used. Single-phase squirrel-cage motors do not have a large starting torque, and for applications where such torque is required, repulsion-induction motors are used. A repulsion-induction motor may be of the split-phase or condenser type, but has a manual or au­tomatic switch that allows current to flow between brushes on the commutator when the motor is start­ing, and short-circuits all commutator segments after the motor reaches a critical speed. Repulsion-induc­tion motors are so named because their starting torque depends on the repulsion between the rotor and the stator, and their torque while running depends on in­duction. Series-wound motors with commutators, which will operate on direct or alternating current, are called universal motors. They are usually made only in small sizes and are commonly used in household ap­pliances.

13. ENGINEERING AS A PROFESSION

Electrical and Electronics Engineering

Electrical and electronics engineering is the largest and most diverse field of engineering. It is concerned with the development and design, application, and manu­facture of systems and devices that use electric power and signals. Among the most important subjects in the field are electric power and machinery, electronic cir­cuits, control systems, computer design, superconduc­tors, solid-state electronics, medical imaging systems, robotics, lasers, radar, consumer electronics, and fibre optics.

Despite its diversity, electrical engineering can be di­vided into four main branches: electric power and ma­chinery, electronics, communications and control, and computers.

Electric Power and Machinery

The field of electric power is concerned with the de­sign and operation of systems for generating, transmit­ting, and distributing electric power Engineers in this field have brought about several important developments since the late 1970s. One of these is the ability to trans­mit power at extremely high voltages in both the direct current (DC) and alternating current (AC) modes, reduc­ing power losses proportionately. Another is the real time control of power generation, transmission, and dis­tribution, using computers to analyze the data fed back from the power system to a central station and thereby optimizing the efficiency of the system while it is in op­eration.

A significant advance in the engineering of electric machinery has been the introduction of electronic con­trols that enable AC motors to run at variable speeds by adjusting the frequency of the current fed into them. DC motors have also been made to run more efficiently this way.

Electronics

Electronic engineering deals with the research, de­sign, integration, and application of circuits and devices used in the transmission and processing of information. Information is now generated, transmitted, received, and stored electronically on a scale unprecedented in history, and there is every indication that the explosive rate of growth in this field will continue unabated.

Electronic engineers design circuits to perform spe­cific tasks, such as amplifying electronic signals, add­ing binary numbers, and demodulating radio signals to recover the information they carry. Circuits are also used to generate waveforms useful for synchronization and timing, as in television, and for correcting errors in dig­ital information, as in telecommunications.

Prior to the 1960s, circuits consisted of separate elec­tronic devices — resistors, capacitors, inductors, and vacuum tubes — assembled on a chassis and connected by wires to form a bulky package. The electronics revo­lution of the 1970s and 1980s set the trend towards inte­grating electronic devices on a single tiny chip of silicon or some other semiconductive material. The complex task of manufacturing these chips uses the most advanced technology, including computers, electron-beam lithog­raphy, micro-manipulators, ion-beam implantation, and ultraclean environments. Much of the research in elec­tronics is directed towards creating even smaller chips, faster switching of components, and three-dimensional integrated circuits.

Communications and Control

Engineers work on control systems ranging from the everyday, passenger-actuated, such as those that run a lift, to the exotic, such as systems for keeping spacecraft on course. Control systems are used extensively in air­craft and ships, in military fire-control systems, in power transmission and distribution, in automated manufac­turing, and in robotics.

Computers

Computer engineering is now the most rapidly grow­ing field. The electronics of computers involve engineers in design and manufacture of memory systems, of cen­tral processing units, and of peripheral devices. The field of computer science is closely related to computer engi­neering; however, the task of making computers more «intelligent» (artificial intelligence), through creation of sophisticated programs or development of higher level machine languages or other means, is generally regarded as the aim of computer science.

One current trend in computer engineering is micro­miniaturization. Engineers try to place greater and greater numbers of circuit elements onto smaller and smaller chips. Another trend is towards increasing the speed of computer operations through the use of parallel processors and superconducting materials.

Mechanical Engineering

Engineers in this field design, test, build, and oper­ate machinery of all types; they also work on a variety of manufactured goods and certain kinds of structures. The field is divided into (1) machinery, mechanisms, materials, hydraulics, and pneumatics; and (2) heat as applied to engines, work and energy, heating, ventilat­ing, and air conditioning. The mechanical engineer, therefore, must be trained in mechanics, hydraulics, and thermodynamics and must know such subjects as metallurgy and machine design. Some mechanical en­gineers specialise in particular types of machines such as pumps or steam turbines. A mechanical engineer de­signs not only the machines that make products but the products themselves, and must design for both economy and efficiency. A typical example of modern mechani­cal engineering is the design of a car or an agricultural machine.

Safety Engineering

This field of engineering has as its object the preven­tion of accidents. In recent years safety engineering has become a specialty adopted by individuals trained in other branches of engineering. Safety engineers develop methods and procedures to safeguard workers in hazard­ous occupations. They also assist in designing machin­ery, factories, ships and roads, suggesting alterations and improvements to reduce the possibility of accident.

In the design of machinery, for example, the safety en­gineer try to cover all moving parts or keep them from accidental contact with the operator, to put cutoff switches within reach of the operator and to eliminate dangerous sharp parts. In designing roads the safety engineer seeks to avoid such hazards as sharp turns and blind intersections that lead to traffic accidents

14. AUTOMATION IN INDUSTRY.

Automated production lines

An automated production line consists of a series of workstations connected by a transfer system to move parts between the stations. This is an example of fixed automation, since these lines are set up for long produc­tion runs, making large number of product units and running for several years between changeovers. Each station is designed to perform a specific processing op­eration, so that the part or product is constructed stepwise as it progresses along the line. A raw work part enters at one end of the line, proceeds through each workstation and appears at the other end as a completed product. In the normal operation of the line, there is a work part being processed at each station, so that many parts are being processed simultaneously and a finished part is produced with each cycle of the line. The various opera­tions, part transfers, and other activities taking place on an automated transfer line must all be sequenced and co­ordinated properly for the line to operate efficiently.

Modern automated lines are controlled by program­mable logic controllers, which are special computers that can perform timing and sequencing functions required to operate such equipment. Automated production lines are utilized in many industries, mostly automobile, where they are used for processes such as machining and pressworking.

Machining is a manufacturing process in which metal is removed by a cutting or shaping tool, so that the remain­ing work part is the desired shape. Machinery and motor components are usually made by this process. In many cases, multiple operations are required to completely shape the part. If the part is mass-produced, an automated transfer line is often the most economical method of pro­duction. Many separate operations are divided among the workstations.

Pressworking operations involve the cutting and forming of parts from sheet metal. Examples of such parts include automobile body panels, outer shells of laundry machines and metal furniture More than one processing step is often required to complete a compli­cated part. Several presses are connected together in se­quence by handling mechanisms that transfer the par­tially completed parts from one press to the next, thus creating an automated pressworking line.

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