Abstract :
During the last ten or fifteen years a great deal of work has been done toward perfecting methods for varying the speed of induction motors. Each of these methods has certain disadvantages, which have precluded the general adoption of any one of them for practical use. The method most generally applied for varying the speed is that of inserting resistance in the secondary circuit. This method of speed regulation, however, has the one great disadvantage that, for a certain load and speed a certain amount of resistance is required; and as soon as the load changes, the resistance must be changed in order to maintain the same speed. When the load is taken off the motor will return to its synchronous speed. It is also evident that such speed regulation can be obtained only by a great sacrifice of efficiency, due to the high ohmic losses in the secondary circuit.
