Variable-unbalanced-voltage control

SPEED CONTROL of wound-rotor induction motors where the torque is in the same direction as the mechanical rotation may be accomplished with moderate success merely by the use of adjustable resistance in the secondary or rotor circuit, provided the torque being developed by the motor is appreciable. Where the torque must be contrary to the rotation, for example, crane-hoist lowering, the problem of control at less than synchronous speed becomes much more complex, and special consideration in control design is required. Such refined subsynchronous speed control can be attained, provided the torque is in excess of possibly 35 per cent of full load, by exciting the motor stator with direct current and adjusting rotor resistance in the ordinary way. For those applications where the countertorque is less than 50 per cent normal, or absent altogether, satisfactory speed control is provided by application of adjustable unbalanced polyphase voltage to the motor primary. Neither of these systems is entirely satisfactory for the complete speed-torque range, since the former does not provide low torque at high speeds and the latter does not provide rated torque at slow speeds. Both require considerably greater input current than the motor would require to develop an equivalent countertorque with balanced polyphase voltage applied. It is possible, of course, to control the countertorque in the orthodox manner by an adjustable rotor resistor. This system has been used extensively, but is considered satisfactory only for certain applications involving predetermined loads.