Elevator speed control system

An elevator speed control system for controlling the rotational speed of an induction motor for driving an elevator car, comprising an induction motor, a thyristor means for controlling the voltage applied to said induction motor, a command speed signal generator for generating a command speed signal for the rotational speed of said induction motor, a tachometer generator coupled to said induction motor to generate a speed signal representing the actual rotational speed of said induction motor, a subtacter coupled to both said command speed signal generator and said tachometer generator to produce a difference signal between the output signals from both said generators, phase shifter means coupled between said subtracter and said thyristor means and responsive to said difference signal from said subtracter to produce firing signals for said thyristor means, said induction motor, said tachometer generator, said substracter, said phase shifter means and said thyristor means being coupled in a closed loop circuit, and a nonlinear circuit connected in said closed loop circuit for producing an output voltage approximately proportional to the square root of the input voltage applied thereto and offsetting the nonlinear characteristic of the induction motor, whereby the loop gain of said closed loop circuit is maintained constant regardless of the input thereto.