ABSTRACT
Direct current (dc) machines operate on dc voltages and currents. Recall from Chapter 2 that time-varying currents flowing through properly placed windings can lead to generation of a rotating magnetic field. Direct currents do not vary with time, and it may seem puzzling at first glance how they may result in rotation. As will be shown in Section 3.2, a dc machine is inherently ac but is made to appear dc at its terminals by means of a mechanical rectifier called a commutator. Similar to ac machines, dc machines operate based on the laws of induction and interaction.
