TABLE OF CONTENTS In Chapter 5, we stated that a generator is a machine that converts mechanical energy into electric energy. When a machine converts electric energy into mechanical energy, it is called a motor. There is no fundamental difference in either the construction or the operation of the two machines. In fact, the same machine may be used as a motor or a generator.
There are basically two types of motors: alternating-current (ac) motors and direct-current (dc) motors. An ac motor (discussed in Chapters 9 and 10) converts alternating (time-varying) power into mechanical power. When a machine converts time-invariant power into mechanical power, it is said to be a dc motor. This chapter is devoted to the study of dc motors.
When most of the power being generated, transmitted, and consumed is of the ac form, the use of a dc motor requires the installation of extra equipment for converting ac into dc. To justify the additional cost of a commutator on one hand and the installation of ac-to-dc converters on the other, a dc motor is put into service only when its performance is superior to that of an ac motor. By superior performance we simply mean that a dc motor is capable of doing what cannot be easily accomplished with an ac motor. For example, a dc motor can develop starting torque several orders of magnitude higher than a comparable size ac motor. A dc motor can operate at speeds that cannot be attained by an ac...
