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SENSORLESS FIELD ORIENTED CONTROL OF BRUSHLESS ...

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Preliminaries<br />

In reading the literature on motor control a number of different motor configurations will be<br />

encountered, along with terminology that describes them. While some of that material is only a<br />

few decades old, the “motor scene” has changed significantly from the presentation contained in<br />

most textbooks. Therefore, some preliminaries about modern motor types and construction are<br />

presented to give the reader some perspective as to where BPMS motors fit into the overall<br />

picture. In addition, this information is requisite to understanding the various schemes that have<br />

evolved to control these motors.<br />

Motor Structures<br />

Both linear and rotational motors are in common use in servo applications. A rotational motor<br />

consists of a physical member that is stationary (the stator) and a physical member (the rotor) that<br />

rotates with respect to the stator. There are two general configurations of rotational motors: radial<br />

flux and axial flux. Axial flux motors [66] are an active area of research but are not nearly as<br />

common as radial flux motors in servo applications. A comparison between the two is shown in<br />

Figure 2.1.<br />

Figure 2.1 – Radial- and axial- flux motors.<br />

Of the radial flux motors, there are two common configurations: inner-rotor and outer-rotor.<br />

There exist motors that have a rotor that is both inside and outside of the stator but they are not in<br />

common use. The inner-rotor configuration offers the possibility of a smaller overall motor<br />

diameter, lower rotor inertia, and possibly better winding heat dissipation. These characteristics<br />

make it suitable for applications that require small size, low mechanical time constant, and high<br />

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