High starting torque for AC SCR controller

A control strategy is proposed for AC thyristor controllers. The main feature of the proposed technique is that motors can start with high torque, while using an economical design. This allows the use of AC thyristor controllers for a wide range of applications, where they have not been used before. The method employs the AC thyristor controller as a discrete frequency inverter that increases the frequency until the frequency of the line is reached (60Hz in this case). These discrete frequencies produced by the control are sub-multiples of line frequency. They are generated by omission or inclusion of line frequency half cycles. Voltage sequences are predefined for each sub-multiple of line frequency. In order to obtain positive torque for all these frequencies, the system is unbalanced. The proposed control is simulated (using EMTP), and built. Comparisons are done with the traditional AC thyristor starter. Results show reduced RMS starting current, increased starting torque and the possibility to operate at low speeds.