Computer-Aided Thyristor Overload Current Ratings for Motor Drive and Other Applications Requiring Repetitive Overloads

The wide usage of the silicon-controlled rectifier in high-power applications is due, to a large degree, to the inherent overload capability of thyristors. Motor drive applications, for example, are characterized by complex load duty cycles and short-time overloads when accelerating the motor. Welding cycles, zero-crossfiring power control, and pulsewidth-modulated inverters are other examples where the overload capability of the silicon-controlled rectifier is important. The many types of overload conditions that can occur in thyristor applications are defined and the procedures and equations utilized are described. The complexity of such equations dictate that even with the utility of the electronic computer some approximations are required. The accuracy of these approximations is analyzed and the range of reasonable accuracy established. Due to the large number of variables of thyristor overload ratings the presentation of overload capability curves is difficult. The presentation of short-time, occasional, and complex overload ratings independent of ambient temperature, as presented, is a significant step in providing the industry with accurate and useful overload information on thyristors.