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Electric Machinery - Chapter 6: Problems

tài liệu Electric Machinery - Chapter 6: Problems sau đây. Tài liệu dành cho các bạn học chuyên ngành Điện. Tham khảo nội dung tài liệu để nắm bắt nội dung chi tiết. | 348 CHAPTER 6 Polyphase Induction Machines load on the motor the rotor speed decreases resulting in larger slip increased induced rotor currents and greater torque. Examination of the flux-mmf interactions in a polyphase induction motor shows that electrically the machine is a form of transformer. The synchronously-rotating air-gap flux wave in the induction machine is the counterpart of the mutual core flux in the transformer. The rotating field induces emf s of stator frequency in the stator windings and of slip frequency in the rotor windings for all rotor speeds other than synchronous speed . Thus the induction machine transforms voltages and at the same time changes frequency. When viewed from the stator all rotor electrical and magnetic phenomena are transformed to stator frequency. The rotor mmf reacts on the stator windings in the same manner as the mmf of the secondary current in a transformer reacts on the primary. Pursuit of this line of reasoning leads to a singlephase equivalent circuit for polyphase induction machines which closely resemble that of a transformer. For applications requiring a substantially constant speed without excessively severe starting conditions the squirrel-cage motor usually is unrivaled because of its ruggedness simplicity and relatively low cost. Its only disadvantage is its relatively low power factor about 0.85 to 0.90 at full load for four-pole 60-Hz motors and considerably lower at light loads and for lower-speed motors . The low power factor is a consequence of the fact that all the excitation must be supplied by lagging reactive power taken from the ac source. One of the salient facts affecting induction-motor applications is that the slip at which maximum torque occurs can be controlled by varying the rotor resistance. A high rotor resistance gives optimum starting conditions but poor running performance. A low rotor resistance however may result in unsatisfactory starting conditions. However the design of a .