Performance improvement in AC-AC SCR controller

Standard use of AC SCR controllers is for starting induction motors. However, the reduction of the starting torque has limited its application to no load starters. This paper uses a novel technique to expand the range of applications of the AC SCR controllers. The technique increases the frequency in discrete steps until the line frequency is reached. These discrete steps are sub-multiples of the line frequency. The sub-frequencies are generated by the omission or inclusion of line frequency half cycles. The sequence voltages are predefined for each sub-multiple of the frequency. In order to obtain positive torque for all the sub-frequencies the system is properly unbalanced. This strategy allows a start similar to the one achieved by the inverters, and additionally allows low speed motor operation with high torque. The system is complemented with a control of constant impedance for minimization of the reactive power when the motor operates in sub-load. Performance testing of the actual system has shown good results, confirming it, as an optional and cheaper alternative for starting-loaded induction motors with low RMS current.