tailieunhanh - ebookModern Control Technology--Components & Systems (2nd Ed.)p2

| CHAPTER 7 Direct Current Motors OBJECTIVES After studying this chapter you should be able to Explain the theory of operation of electric motors in general and DC motors in particular. Distinguish the characteristics of series-wound shunt-wound compound and permanent-magnet motors. Use the torque-speed curve of a motor to predict its performance. Select a DC motor based on mechanical requirements. Understand the operation of linear amplifier drivers for DC motors that incorporate power transistors IC amplifiers Darlington transistors and power MOsFeTS. Understand DC motor-speed control using pulse-width-modulation concepts. Understand operating a DC motor from rectified AC using silicon-controlled-rectifier circuits. Understand the operating principles of brushless DC motors. INTRODUCTION An indispensable component of the control system is the actuator. The actuator is the first system component to actually move converting electrical energy into mechanical motion. The most common type of actuator is the electric motor. Motors are classified as either DC or AC depending on the type of power they use. AC motors covered in Chapter 9 have some advantages over DC motors They tend to be smaller more reliable and less expensive. However they generally run at a fixed speed that is determined by the line frequency. DC motors have speed-control capability which means that speed torque and even direction of rotation can be changed at any time to meet new conditions. Also smaller DC motors commonly operate at lower voltages for example a 12-V disk drive motor which makes them easier to interface with control electronics. 295 296 CHAPTER 7 THEORY OF OPERATION The discovery that led to the invention of the electric motor was simply this A currentcarrying conductor will experience a force when placed in a magnetic field. The conductor can be any metal iron copper aluminum and so on. The direction of the force is perpendicular to both the magnetic field and the current Figure .

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