发电机和电气设备毕论文外文翻译.docx

Generator andElectrical EquipmentsGeneratorIntroduction Electric generators convert mechanical energy to electrical energy, which is moreeasily transmitted to remotely located points of application. The first large electric generating systems used direct-current (dc) generators, mainly because direct current wasbetter understood than alternating current(ac). However, dc generators are limited togenerating power at relatively low voltages, largely due to problems at their commutates. As power networks developed, higher and higher voltages were required to transmitlarge blocks of power over longer and longer distances. Electric transformers can easilychange the normally low voltage generated to the high voltages needed for efficient powertransmission, and of course, transformers only work on alternating current. Ac generators, or alternators as they are commonly called, are so much simpler mechanically, somuch more efficient, and require so much less maintenance than dc machines that alllarge generating plants output alternating current today. Although de transmission lines cantransport extremely large blocks of power very efficiently over long distances, the power isalways generated as alternating current, transformed to the voltage required, rectified andtransmitted as direct current, and then inverted back to alternating current at the point ofapplication.Mechanical Energy The mechanical energy for driving the generator must be derived from a source withenough reliability and capacity to make it economically feasible to develop and transmitthe energy electrically to the point of use. A small water supply running only duringexceptionally wet years or located at a great distance from electrical consumers wouldprobably not be suitable. Mechanical energy sources which cannot be moved, such ashydraulic turbines or even wind machines, must have the cost of transporting the energyproduced (among other factors) taken into account when overall costs are calculated.