Electronic speed control of motors

IN THE EARLY years of the electrical industry there was no acceptable a-c motor. After the development of the induction motor and the synchronous motor two machines were available which are unsurpassed in simplicity, reliability, and economy. These motors are, however, both inherently constant speed machines. In later years, a variety of a-c motors have been developed and a number of these have had adjustable or variable speed characteristics. There is, however, as yet, no adjustable-speed a-c motor so flexible as the Ward-Leonard combination, and there is still room for much improvement in adjustable speed a-c motors.