tailieunhanh - Chapter 3: Uncontrolled induction motor drives

In this chapter, operation of uncontrolled induction motor drives is exam-ined. We briefly outline methods of assisted starting, braking, and re-versing. Speed control by pole changing is explained, and we describe abnormal operating conditions of induction motors. | 3 UNCONTROLLED INDUCTION MOTOR DRIVES In this chapter operation of uncontrolled induction motor drives is examined. We briefly outline methods of assisted starting braking and reversing. Speed control by pole changing is explained and we describe abnormal operating conditions of induction motors. UNCONTROLLED OPERATION OF INDUCTION MOTORS In a majority of induction motor drives in industrial and domestic applications the control functions are limited to the tum-on and turn-off and in certain cases to assisted starting braking and reversing. When driving a load an induction motor is supplied directly from a power line and operates with fixed values of stator voltage and frequency. The speed of the motor is approximately constant motors with a stiff mechanical characteristic . with low dependence of load torque on the speed having been usually used. As already mentioned such a characteristic is associated with a low rotor resistance that is with low losses in the rotor. 43 44 CONTROL OF INDUCTION MOTORS Thus high-efficiency motors somewhat more expensive than standard motors are particularly insensitive to load changes. Clearly an uncontrolled motor drive is the cheapest investment but the lack of speed control carries another price. In many applications a large percentage of the electric energy is wasted because of that shortcoming. The most common induction motor drives are those associated with fluid transport machinery such as pumps fans blowers or compressors. To control the flow intensity or pressure of the fluid valves choking the flow are used. As a result the motor delivers full power a significant portion of which is converted into heat in the fluid. This situation is analogous to that of a car driven with a depressed brake pedal. Energy and money savings have been the major reason for the increasing popularity of ASDs which typically are characterized by short payback periods. Sensitivity to voltage sags constitutes another weakness of uncontrolled