Controller with two modes of braking induction motors

A motor controller having improved firing means, a high-torque, two-speed, three-phase induction motor, thyristor means, said motor being connected to a three-phase power line through said thyristor means, said improved firing means controlling phases of firing said thyristor means, said motor controlling speed of a wide range of loads having torque in either forward or reverse direction with respect to the direction of running of said motor, said firing means including a firing circuit for developing ignition voltage, a ramp control means for developing a curve of voltage corresponding to a desired speed, a tachometer connected to said motor for developing a voltage proportional to actual speed, and a comparator having an output supplying the difference between said voltages for said desired speed and said actual speed, control switching means responsive to a stop command for connecting said motor to said thyristor means successively for regenerative and for reverse-running braking, firing timing means connected between said output of said comparator and said firing circuit for each phase of said power line to control times of firing, resetting means for each phase connected between said power line and said firing timing means,

• the improvement comprising a phase-shift circuit connecting each of said resetting means to a respective phase of said power line, each of said phase shift circuits delaying resetting of said firing timing means for a substantial portion of each one-half cycle occurring after respective alternate zero crossings of the respective phase of said power line, said alternate zero crossings being the zero crossings terminating those one-half cycles during which said thyristor means usually provides the greater amount of operating current while said motor is running, the delaying of each of said resettings permitting the initiation of firing even within the respective one-half cycles immediately following said alternate zero crossings such that only current that lags is conducted through said thyristor means and is controlled by said firing timing means in intervals of short enough duration to ensure smooth stopping of loads by reverse-running braking precisely at predetermined locations and also to ensure smooth starting of even those loads having torque in the direction of the running of said motor.