Pulsed width modulation method for power transforming apparatus

In a power transforming apparatus which is capable of applying a voltage, current and frequency of an alternating current to a load circuit, detecting a voltage and frequency supplied to the load circuit, and performing a pulse width modulation with respect thereto, a pulse width modulation method using the power transforming apparatus, comprising the steps of:

• a first step for computing a phase voltage command in accordance with a phase of a voltage applied to the load circuit;
• a second step for performing a three-phase pulse width modulation with respect to the phase voltage command when the voltage and frequency applied to the load circuit are below a predetermined value;
• a third step for computing new phase voltage commands which have the same phase as the phase of a current when the voltage and frequency applied to the load circuit are the predetermined value;
• and
• a fourth step for performing a pulse width modulation so that a switching operation having the maximum absolute value of the current among the new phase voltage commands obtained in the third step becomes idle.