Design of Polyphase Generators and Motors

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McGraw-Hill, 1912 - Electric machinery - 265 pages
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Page 207 - In view of these explanations, it is obvious that the impedance of the end rings will gradually decrease from a high value at starting to a low value at synchronism. Now we are more free to make use of this phenomenon in the case of synchronous motors than in the case of induction motors, for as already stated, when the synchronous motor is run at full speed, the squirrel cage is utterly inactive (except in serving to minimize "surging...
Page 206 - So soon as synchronism has been brought about, the squirrel-cage system is relieved of all further duty; and it is consequently immaterial whether it is designed for high or for low resistance. Consequently with the synchronous motor we are completely free from the limitations which embarrass us in designing hightorque induction motors. In the case of the synchronous motor, we can provide any reasonable amount of torque by making the squirrel-cage system of sufficiently high resistance. One difficulty,...
Page 202 - Figs. 108 and 109 are drawn curves which give a rough indication of the way in which the power-factor of the polyphase induction motor varies with the speed for which the motor is designed. While qualitatively identical conclusions will be reached by an examination of the data of the product of any large manufacturer, the quantitative values may be materially different, since each...
Page 211 - ... (Curve B), while the synchronous motor starts with high torque, the torque falls away much more rapidly; and when the torque has fallen to the value necessary to overcome the friction of the motor, the speed is several per cent below synchronous speed, and the application of the continuous excitation, if it suffices to pull the rotor into synchronism, will do so only at the cost of an abrupt and considerable instantaneous drain of power from the line. In the third case, (Curve C), there are present...
Page 4 - No matter what the shape of the curve ayb may be, the EMF passes through one alternation when the coil is moved a distance equal to that from the center of one pole to the center of the next. If the coil C be...
Page ii - McGraw,Hill BookGompany Electrical World The Engineering andMining Journal Engineering Record Engineering News Railway Age Gazette American Machinist...
Page 202 - ... powerfactor of the polyphase induction motor varies with the speed for which the motor is designed. While qualitatively identical conclusions will be reached by an examination of the data of the product of any large manufacturer, the quantitative values may be materially different, since each manufacturer's product is characterized by variations in the degree to which good properties in various respects are sacrificed in the effort to arrive at the best all-around result. It is for this reason...
Page 252 - On the Production of Rotary Magnetic Fields by a Single Alternating Current.
Page 203 - The precise circumstances of any particular case will require to be considered, in order to decide whether or not this alternative is preferable. While it has long been recognized that the inherent simplicity and robustness of the squirrel-cage induction motor constitute features of very great importance and justify the wide use of such motors, nevertheless we must not overlook the fact that induction motors are less satisfactory the lower the rated speed. The chief disadvantage of a low-speed induction...
Page 210 - It will now be agreed that the properties of synchronous motors lend themselves admirably to the provision of high starting torque. In addition to the means previously described whereby a synchronous motor may have not only high starting torque but may also automatically run close up to synchronous speed, so as to fall quietly into synchronism immediately upon the application of the field excitation from the continuous-electricity source, there is also available a phenomenon described by Mr. AB Field,...

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