TOPICS ON ELECTRICITY AND MAGNETISM
|
| Electric Motors and Generators They are rotary machines that turn either mechanical energy into electrical energy (generators or dynamos) or electrical energy into mechanical energy (motors). Primarily, an electric generator operates on the principle of electromagnetic induction, that is, it runs because of a conductor moving through a magnetic field, or the latter moving past a conductor, induces an electromotive force (e.m.f.) in the conductor. On the other hand, electric motors work due to a force exerted on a current-carrying conductor when it is placed at right angles to a magnetic field. We could say that almost all electric energy utilized today is produced by means of electric generators, run hydraulically , nuclear or by steam. Apart from all the applications in the fields of lighting and heating, the vast amount of this energy is changed into useful work by different types and sizes of electric motors in every sphere of human activity. Nowadays, D.C. generators power is only in some cases utilized to supply the electrical energy distributed in large cities for general use; A.C. has largely supplanted it. The main use for D.C. power is in combination with the D.C. motors employed in industry and transportation. In unipolar generators, the direction of the induced e.m.f. is always the same, hence a commutator is not necessary. These machines can be operated at very high speed, so they are adaptable to turbine drive. Their basic disadvantage is the low value of e.m.f. per disk or cylinder. Even at very high speeds, the induced e.m.f. has a maximum of forty to fifty volts, so that twelve or more disks or cylinders in series are required to get a 500 volt output; besides, there is the difficulty of conducting the current from the rotating members at high speed with the corresponding brush friction. Finally, the machines are dear and heavy for their output. The greatest difficulty in the operation of D.C. machinery is commutation. It limits severely the output to which D.C. generators can be built and prevents the operation of large machines at very high speed. In order to get sparkless commutation, the current and therefore the induced e.m.f. in the coil undergoing commutation must be zero during the period of commutation. Two factors prevent the e.m.f. from being zero. Under load the armature current generates a magnetic field transverse to that produced by the poles and sets up flux in the commutator zone; the coil turns that undergo commutation cut this flux having an e.m.f. induced in them. Besides, the current in the armature coils generates a magnetic field linking the coils. Once it reverses, in the brief time of the commutation period, an e.m.f. self-induction takes place. In both cases, the e.m.f., though small, acts in the short-circuited coil of low resistance producing a large current.Electric rotating machinery may be classed into three groups: those that generate or operate on D.C.; those generating or operating on A.C. and finally, converters. This group includes rotating devices which change A.C. into D.C. or viceversa, or which change the frequency of A.C. Motors The armature of the simplest motor consists of a coil of many turns of insulated wire wound in longitudinal slots in a soft iron cylinder, which is mounted between the curved pole pieces of a strong magnet. The clearance between the cylinder and the magnet poles is made as small as possible because air spaces in a magnetic circuit always tend to reduce the magnetic flux. The armature is fixed on a central shaft for rotary purposes and a device known as commutator is incorporated in order to reverse the current through the armature coils at the proper moment. On the surface of this commutator there are two segments of copper or brass insulated from each other by small strips of ebonite. Carbon brushes press lightly against the copper segments by means of springs and as the armature and commutator revolve, a brush makes contact first with one segment and then with the other. The two brushes are connected to the external circuit so that current enters one brush, flows through the armature coils and leaves at the other brush. This principle is the same for larger motors but the core of the armature in the latter is made of suitably shaped laminations of soft iron separated from each other by thin layers of insulation and pressed together. This reduces heat losses due to eddy currents. Some types of electric motors are equipped with removable brushes. These carbon brushes are under spring tension and provide current for the commutator of the motor. The spring is held in a small opening by means of a plastic cap. Occasionally these brushes become worn to such a degree that the spring cannot supply enough pressure to make a good electric contact between brush and commutator, and the motor fails to work. A new brush can be easily installed by removing the cap and pulling out the spring and old brush. It may be possible to stretch the spring enough to get the proper amount of tension. If the contact between the brushes and the commutator is not good, there will be a great deal of sparking and an unpleasant smell while the motor is running. A brush that is partially broken or worn unevenly can cause this condition. We must change the brush for a new one. If a new brush is not available we can resurface the old one by sanding it with very fine sand paper. We must not emery because the particles may burn out the bearings. As the commutator is curved, the face of the brush must be curved out slightly to fit closely against the commutator. According to the way field coils are connected with the armature coils, D.C. motors may be classified into series-wound, shunt-wound and compound-wound motors. Series-wound motors have a large starting torque and they are used for trams, trains and all those motors which require a big pull for starting. Applications of Electricity Electricity is the source of an endless number of applications. First of all, as it has a very valuable heating effect we can make a great use of it in many ways: electric fires and toasters; irons and cookers, and similar appliances such as kettles and grills and many others that contribute to make life easier for us. The heating power of electricity is not only used in the home but also in industry to an enormous extent in electric furnaces of various sorts: muffle furnaces for tempering tools; induction furnaces for melting brass and similar alloys, and furnaces which employ the intense heat of the electric are. The idea of heat is also related to that of electric light because much of our lighting depends on the intense heating of short lengths of fine wire to such a point that they glow with a bright light, which can be switched on and off at will. The most familiar light is the ordinary incandescent bulb lamp but there are forms of lighting which are much more efficient, for instance the arc lamp, a source of light used where intense illumination is required; the search lights, cinematograph projectors, etc. Thus, we see how heat and light can be obtained by the use of electricity. Let us add now that we can use it for producing power applying it to motors and generators extensively employed in almost every aspect of industry. Electricity is related to almost all our daily activities through the use of the electric bell; the telephone, whose messages cross short and long distances; the radio which constitutes a quick means of spreading news as well as a source of culture and amusement; the telegraph used in the past to transmit messages by means of electric impulses sent through wires; the refrigerator that preserves our food and keeps our drinks cool, and television that transmits visible moving images by means of electromagnetic wireless waves. Besides, there are some electrical processes of great importance in industry and life. The first is welding, that is the joining of metals by raising their temperature enough to melt and fuse them together. Another electrical process of real interest in the field of industry is electroplating which provides an example of the chemical effect of a current. Quite different from these two applications are the X-rays or Rontgen rays, a form of radiation penetrating many substances impervious to ordinary light. We could go on speaking about the uses of electricity without finishing, so unsuspected the possibilities of its applications are .
|