Speed Control of Induction Motors Using Saturable Reactors [includes discussion]

Synopsis: With the progress in automation, there is a growing need for precise and stepless control of motor speeds in driving fans, pumps, conveyors, and all types of machines. Usually, d-c motors with field and/or voltage control, or induction motors driving the load through eddy-current clutches, are employed for this purpose. In this paper, another method of induction-motor speed control is considered; it employs feedback controlled saturable reactors in series with the motor windings. The performance of the reactor speed-control scheme is considered as a problem in power modulation, in which an electric energy source forms the input, and controlled torque and speed are the desired outputs. This method does not require a d-c power supply, or any commutators or power tubes. In addition to an essentially standard induction motor, the scheme only uses readily available reactors and control circuits. The direct current required for the control is only 1 or 2 % (per cent) of the motor rating, and can readily be supplied from amplistats, or equivalent devices, just as in the case of the clutch. Calculated and test data are given, showing that this method of speed control allows the speed of a wound-rotor induction motor to be set at any desired value between 100% forward and 100% backward, and to be held nearly constant at the set value, over a full range of torque from plus (motoring) to minus (braking), up to the maximum torque value.