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Ebook Electric machinery (6th edition): Part 2

(BQ) Part 2 book "Electric machinery" has contents: DC machines, variable-reluctance machines and stepping motors, single-and two-phase motors, introduction to power electronics, speed and torque control. | DC M a c h i n e s p c machines are characterized by their versatility. By means of various combinations of shunt-, series-, and separately-excited field windings they can be designed to display a wide variety of volt-ampere or speed-torque characteristics for both dynamic and steady-state operation. Because of the ease with which they can be controlled, systems of dc machines have been frequently used in applications requiring a wide range of motor speeds or precise control of motor output. In recent years, solid-state ac drive system technology has developed sufficiently that these systems are replacing dc machines in applications previously associated almost exclusively with dc machines. However, the versatility of dc machines in combination with the relative simplicity of their drive systems will insure their continued use in a wide variety of applications. 7.1 INTRODUCTION The essential features of a dc machine are shown schematically in Fig. 7.1. The stator has salient poles and is excited by one or more field coils. The air-gap flux distribution created by the field windings is symmetric about the center line of the field poles. This axis is called the field axis or direct axis. As discussed in Section 4.6.2, the ac voltage generated in each rotating armature coil is converted to dc in the external armature terminals by means of a rotating commutator and stationary brushes to which the armature leads are connected. The commutator-brush combination forms a mechanical rectifier, resulting in a dc armature voltage as well as an armature-mmf wave which is fixed in space. Commutator action is discussed in detail in Section 7.2. The brushes are located so that commutation occurs when the coil sides are in the neutral zone, midway between the field poles. The axis of the armature-mmf wave then is 90 electrical degrees from the axis of the field poles, i.e., in the quadrature axis. In the schematic representation of Fig. 7.1 a, the brushes are shown in the .