Generator and Electrical EquipmentsGeneratorIntroduction Electric generators convert mechanical energy to electrical energy, which is more easily transmitted to remotely located points of application. The first large electric generating systems used direct-current (dc) generators, mainly because direct current was better understood than alternating current (ac). However, dc generators are limited to generating power at relatively low voltages, largely due to problems at their commutates. As power networks developed, higher and higher voltages were required to transmit large blocks of power over longer and longer distances. Electric transformers can easily change the normally low voltage generated to the high voltages needed for efficient power transmission, and of course, transformers only work on alternating current. Ac generators, or alternators as they are commonly called, are so much simpler mechanically, so much more efficient, and require so much less maintenance than dc machines that all large generating plants output alternating current today. Although de transmission lines can transport extremely large blocks of power very efficiently over long distances, the power is always generated as alternating current, transformed to the voltage required, rectified and transmitted as direct current, and then inverted back to alternating current at the point of application.Mechanical Energy The mechanical energy for driving the generator must be derived from a source with enough reliability and capacity to make it economically feasible to develop and transmit the energy electrically to the point of use. A small water supply running only during exceptionally wet years or located at a great distance from electrical consumers would probably not be suitable. Mechanical energy sources which cannot be moved, such as hydraulic turbines or even wind machines, must have the cost of transporting the energy produced (among other factors) taken into account when overall costs are calculated. Steam-turbine power plants, however, can be located near a coal seam, lumber mill, or a reliable source of cooling water to save on transportation costs. Some mechanical power may be obtained from sources more easily located near the point of utilization. Gas turbines and reciprocating gas or diesel engines fall into this category. Except for standby emergency power generators, even here it might be more economical to install large units and transmit the power to the point of use. Large power plants will generally have better operating efficiencies than small ones, and it may be desirable to locate a large plant near the center of use and then distribute the power generated outward, assuming the fuel supply is transportable.Each type of mechanical driver has its own peculiarities, and some have a sizable impact on the generator configuration. There are marked differences as to the engine output Speeds available, the speed pulsations possible, the chances of overspeed , etc.Normally, the generator shaft is horizontal and direct-connected to the driver. Sometimes speed-changing gear boxes are installed between a high-speed turbine and a lower speed generator. These allow the turbine to run at its most efficient speed, a speed that may be too high for the generator. Small hydraulic turbines usually have their shafts mounted horizontally; large hydraulic machines have their shafts direct-connected and vertically mounted. The generator may include special bearings to carry the thrust imposed by the water flowing through the turbine. Criteria like these for providing mechanical energy impose special designs on the generating machines.Basic Principle and Construction There are two quite distinct forms of modem alternator. While the principle of operation of each is the same, i.e. , the movement of magnetic poles past stationary coils, their constructions are very different. The reason for this is that each has been designed to match its prime mover, i.e., to suit the mechanical device that is to tap the two principal natural power resources-failing water, on the one hand, and steam, generated by heat from fossil fuels or nuclear fuels, on the other. To match the output of the turbo alternators, the water wheel generators must therefore be multi-polar and hence of large diameter and small axialength. There is a limit to the length of a turbo alternator, based largely on the mechanical considerations involved in supporting a large rotor mass between a bearing at each end. At 3 000 or 3 600 r/min (50 or 60 Hz) the rotor must be extremely well balanced and its surface smooth. With the lower speed water powered machine, such precautions can be relaxed with a view to making the larger rotor cheaper to make. The fundamental difference in shape between the rotors of the two types of machine is consequent upon the above considerations, but now a secondary difference is introduced by what could be termed