И. Б. Абрамова О. П. Кириченко
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TurbinesThe turbine is a machine for generating mechanical power from the energy of the stream of fluid. Steam, hot air or gaseous products of combustion, and water are the most widely used working fluids. A steam turbine may be defined as a form of heat engine in which the energy of the steam is transformed into kinectic energy. It consists of the following fundamental parts: a) a casing or shell containing stationary blades: c) a set of bearings; d) a governor and valve system for regulating the speed and power of the turbine. The main types of steam turbines are axialflow turbines and radial-stage turbines. The reciprocating steam engine came into its own during the nineteenth century, when it found greatest use in mills, locomotives and pumping systems. The modern steam turbine, developed last century, is rapidly replacing the reciprocating engine for large installations. Gas is used as the working fluid in gas turbines. The basic theory underlying their design and their operating characteristics is identical with that for steam turbines. The energy of water is converted into mechanical energy of a rotating shaft in hydraulic turbines. Power may be developed from water by three fundamental processes: by action of its weight, of its pressure or of its velocity; or by a combination of any or all three. BoilersA boiler is a closed vessel in which water, under pressure, is transformed into steam by the application of heat. Open vessels and those generating steam at atmospheric pressure are not considered to be boilers. The furnace converts the chemical energy of the fuel into heat. The function of the boiler is to transfer this heat to the water in the efficient manner. Progress in steam-boiler development has been rapid. The first boilers were very crude affairs, as contrasted with our present-day standards. The greatest number of contributions have been made in the last half century. The field of application is diversified. Boilers are used for heating, supplying steam for processes, furnishing steam to operate engines, etc. Maintaining the correct boiler water level is the most important duty of the boiler operator. It is of the utmost importance that the manufacturer supply suitable and reliable devices for indicating the water level. Coal as well as liquid and gaseous fuels are used for boiler firing. The ideal boiler must be of correct design, sufficient steam and water space, and good water circulation. Electric motorsThere is a wide variety of d.c. motors. There are shunt motors, series motors, synchronous motors, induction motors, single-,two-,and three-phase motors. They are used to drive various machines. Direct-current motors are of three principal kinds, and are named according to the manner in which their field coils are connected to the armature. They are named respectively: series, shunt, and compound. In the series motors the field windings and armature are connected in series with each other. All the current which passes through the armature passes through the field coils. The field windings are therefore composed of a few turns of thick wire. Starting under heavy load, a series motor will take a large current to provide the huge torque required. The field coils of shunt motors are connected direct across the brushes, hence they have the full voltage of the mains applied to them. The shunt motor may by called a constant speed motor, and is suitable for driving machine tools, lathes, wood-working machines and any machines requiring a steady speed. A compound motor has both shunt and series field windings and therefore partakes of the nature of both types of motors. A. C. Electric MotorMotors for alternating-current circuits may by either single-phase or polyphase (two-or three-phase).They may again be divided into two kinds, named respectively:1.Synchronous;2.Non-or asynchronous, ordinarily called induction motors. The most widely used a.c. motor is the induction motor. It has two main parts: a)the stationary winding or stator, which sets up a rotating magnetic field, and b)the rotating part of the motor, i.e. the rotor. The rotor of a commercial a.c. motor consists of an iron core with large copper bars placed in sets around the circumference and connected at both ends to copper rings. This is called a squirrel-cage rotor. When a rotor is placed in a rotating magnetic field, a large current is induced in it. A.c. motors are exactly similar in construction to a.c. generators and may be called inverted alternators, since the same machine may be used as either a generator or motor. Synchronous motors are very suitable for large powers, where the machine can be started up without load, and once started run for long periods. For supplying direct-current power networks, the supply comes first from an alternating-current source and is converted to direct current by synchronous convertors or motor-generator sets. |
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