In the years since the first edition of Electric Machinery Fundamentals was published, there has been a rapid advance in the development of larger and more sophisticated solid-state motor drive packages. The first edition of this book stated that de motors were the method of choice for demanding variable-speed applications. That statement is no longer true today. Now, the system of choice for speed control applications is most often an ac induction motor with a solid-state motor drive. DC motors have been largely relegated to special-purpose applications where a de power source is readily available, such as in automotive electrical systems.