Thyristor and diode controlled variable voltage drives for 3-phase induction motors

The steady state and transient operation of thyristor and diode controllers for variable voltage control of three-wire 3-phase induction motors is considered. Throughout the analysis the state-space form of system representation is used, which facilitates the application of switching constraints at the stator (primary) terminals. The steady state solution for constant speed was obtained in a closed form in terms of state transition matrices with the motor represented in stationary d-q-axes. The results of experimental studies on a motor show good correlation with theoretical results. It was found that the losses in thyristor and diode controlled motors are mostly due to operation at high slips, and that time harmonic currents produce very little additional losses, especially at high speed. Compared with an inverse-parallel thyristor pair in each supply line the thyristor-diode or thyrode connection has an economic advantage but results in rather worse copper losses, mainly due to a predominant second harmonic component of current. The start-up transient currents, torques and speed of a 3-phase induction motor were obtained for sinusoidal, thyristor and thyrode control. A comprehensive computer-aided analysis gave good agreement between measured and calculated current transients. The thyrode controller resulted in larger peak currents and larger negative shaft torques than the corresponding thyristor controller.