Wide-Range Speed Control of Three-Phase Squirrel-Cage Induction Motors Using Static Frequency Converters

An equipment is presented for speed control of three-phase squirrel-cage induction motors supplied through a thyristor converter directly from the mains (the European 50-Hz system) by controlling the frequency and voltage magnitude. Thus a continuous speed control system is achieved, corresponding to a frequency range of from 2.5 to 85 Hz. Speed control is at a constant torque, up to the motor frequency rating (50 Hz), and at a constant power over this frequency. The system configuration is described, a brief analysis of the control loop parameter adjustment is given, and the experimental results are discussed. Finally, the general characteristics of the method are presented.