ABSTRACT
We now consider the last of the three basic types of rotating electromagnetic (EM) machines, the so-called “DC machine.” Reflecting on the nature of rotating machines, a coil rotating in a magnetic field, or a field rotating in a coil, inherently produces an AC voltage. One would then suspect that the production of a constant, or “DC” voltage, would be somewhat more complicated, which in fact it is. It is of historical interest that DC machines were the first invented, mainly because the only available electrical source was the battery. Prior to about 1960, DC machines were widely used throughout industry, and served virtually all applications that required wide range speed control. As power electronic technology matured, it became practical and cost-effective to implement full range speed control of AC motors, and DC motors declined in importance, a trend that continues till today. DC machines are also somewhat more expensive, and require more maintenance than their AC equivalents. Nonetheless, there remain many applications for which a DC machine is ideally suited, and their continued investigation is justified in any comprehensive study of EM machines.
