New PWM Technique Using a Triangular Carrier Wave of Saturable Amplitude

A new technique for a PWM inverter is described. By using a triangular wave of saturable amplitude, adjustment of the amplitude of the multiple reference waves can be avoided. Two typical circuits for inducing saturable triangular waves are illustrated, and a new strategy is proposed which uses the line-to-line voltages of a three-phase ac source as reference wave instead of the usual phase voltages. Two approaches which improve the performance of the PWM inverter and largely simplify the control circuit are developed. One depresses the occurrence of subharmonics without making use of the usual synchronization of the carrier wave with the reference waves; the other is a method for reducing the switchings of the power devices. A simple purely digital control circuit with improved performance has been constructed according to the new technique, and model PWM patterns for constant V/F control as well as their harmonic content are given.