The Analysis of Induction Motors with Voltage Control by Symmetrically Triggered Thyristors

The application of thyristor switching to induction motor speed control has resulted in a number of unconventional supply systems. One such technique, which has been successfully employed in a number of applications, is variable voltage control by means of symmetrically triggered thyristors in the stator phases of a wye-connected machine. In this paper, an analytic method for predicting the steady-state performance of such a system is presented. The solution is obtained in closed form without the necessity for iteration to obtain the proper boundary conditions. In addition, it is shown that the symmetry of the solution permits an additional reduction in the computation time. Since matrix techniques are utilized throughout the analysis, the equations can be easily implemented into a digital computer program. Hence, the method is well suited to the evaluation of proposed motor designs when used in conjunction with thyristor voltage control. Measured torque-speed characteristics of a typical drive system are included and the results compared to an analytical solution. It is demonstrated that a computed solution will favorably predict the performance of an actual system.