1-1ECE 320 Energy Conversion and Power Electronics Spring 2009 Instructor: Tim Hogan Chapter 4: Concepts of Electrical Machines: DC Motors (Textbook Sections 3.1-3.4, and 4.1-4.2) Chapter Objectives DC machines have faded from use due to their relatively high cost and increased maintenance requirements. Nevertheless, they remain good examples for electromechanical systems used for control. We’ll study DC machines here, at a conceptual level, for two reasons: 1. DC machines although complex in construction, can be useful in establishing the concepts of emf and torque development, and are described by simple equations. 2. The magnetic fields in them, along with the voltage and torque equations can be used easily to develop the ideas of field orientation. In doing so we will develop basic steady state equations, again starting from fundamentals of the electromagnetic field. We are going to see the same equations in ‘Brushless DC’ motors, when we discuss synchronous AC machines. 4.1 Geometry, Fields, Voltages, and CurrentsThe geometry shown in Figure 1 describes an outer iron frame (stator), through which (i.e. its center part) a uniform magnetic flux, φˆ, is established. The flux could be established by a current in a coil or by a permanent magnet for example. In the center part of the frame there is a solid iron cylinder (called rotor), free to rotate around its axis. A coil of one turn is wound diametrically around the cylinder, parallel to its axis, and as the stator and its coil rotate, the flux through the coil changes. Figure 2 shows consecutive locations of the rotor and we can see that the flux through the coil changes both in value and direction. The top graph of Figure 3 shows how the flux linkages of the coil through the coil would change, if the rotor were to rotate at a constant angular velocity,ω. ()tωφλcosˆ=(4.1) Figure 1. Geometry of an elementary DC motor.
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