tailieunhanh - POWER ELECTRONIC CONTROL OF INDUCTION MOTORS

Động cơ ba pha cảm ứng là thường xuyên sử dụng máy điện trong công nghiệp. Chúng được đặc trưng với chi phí thấp, độ tin cậy cao, hiệu quả cao, xây dựng đơn giản và, trong trường hợp động cơ điện cảm ứng lồng sóc, với các hoạt động hầu như bảo dưỡng miễn phí. Nếu hoạt động với nguồn cung cấp điện áp ba pha stato của tần số cố định và giá trị rms cố định, động cơ điện cảm ứng sẽ chạy ở tốc độ là rất ít phụ thuộc vào tải. Trái ngược với máy. | ENGNG3070 Power Electronics Devices Circuits and Applications 6. POWER ELECTRONIC CONTROL OF INDUCTION MOTORS Introduction Three-phase induction motors are the most frequently utilised electric machines in industry. They are characterised with low cost high reliability high efficiency simple construction and in the case of squirrel-cage induction motors with virtually maintenance-free operation. If operated with stator three-phase voltage supply of fixed frequency and fixed rms value induction motors will run at a speed that very slightly depends on loading. In contrast to DC machines where choice of methods of speed control and associated power electronic converters that are nowadays in use is rather limited there exists a variety of both speed control techniques and appropriate power electronic converters that are used in conjunction with three-phase induction motor drives. A three-phase induction machine requires three-phase AC supply at stator side. In a squirrel-cage type of induction machines this is simultaneously the only approachable winding. However in slip-ring induction machines the three-phase rotor winding may be approached as well. Thus the speed of an induction machine may be controlled by controlling the stator AC supply for both types of induction machines additionally speed may be controlled in slip-ring machines from the rotor winding side as well. Squirrel-cage induction machines are by far the most frequently used machines. It is for this reason that the following discussion will be predominantly devoted to speed control methods associated with alteration of the stator supply that are equally applicable for both types of induction machines. Only one method specifically aimed at slip ring machines will be looked at. If an induction machine is supplied with a voltage of frequency f then the so-called synchronous speed is determined with the frequency and the number of pole pairs P and is expressed in rpm as ns 60f P However an induction