Induction Motors for Crane Applications

The paper describes a control system for 4-quadrant operation of induction motors with characteristics suitable for crane hoist duty. The control system is based on a combination of stator-voltage control and rotor chopping. A thyristor stator-voltage regulator is used to provide a means of motor-speed closed-loop control. The regulator automatically reverses the phase rotation for plug braking or provides d.c. to the stator for injection braking as determined by load conditions. The rotor chopper operates with a duty cycle that is a predetermined function of rotor speed. Analysis of the chopper action in the rotor is used to determine the optimum function required to develop the maximum possible torque per stator ampere over the operating speed range. Different control functions are found to be necessary for motoring and d.c.-injection modes of operation. Analysis of the system and experimental results are presented showing successful 4-quadrant operation.