Voltage compensating apparatus and method for 3-phase inverter employing four switches

A voltage compensating method for a 3-phase inverter using four switches, comprising the steps of:

• detecting a zero-crossing point of an input AC voltage at a sampling twice that of the input AC voltage;
• reading voltages respectively across upper and lower DC link capacitors charged with input DC voltage rectified from the input AC voltage, for thereby checking voltage ripples involved in the capacitor voltages;
• controlling a delay time from the zero-crossing point, based on the checked ripples;
• and
• generating a triac control signal, thereby switching on a triac supplying the input DC voltage to the upper and lower DC link capacitors when the controlled delay time elapses.

In a 3-phase inverter coupling one phase of a 3-phase electric load to a connection node between upper and lower DC link capacitors connected to each other in series and adapted to respectively charge an input DC voltage applied thereto, remaining phases of the 3-phase load being coupled to respective switch legs of a B4 inverter stage, a voltage ripple compensating method comprising the steps of:

• checking the input DC voltage;
• reading a voltage at a series connection node between the upper and lower DC link capacitors;
• checking a ratio between the DC input voltage and the voltage at the connection node;
• controlling application of the input DC voltage to the upper and lower DC link capacitors in accordance with a DC offset corresponding to the checked voltage ratio based upon a triangular wave comparison algorithm for compensating voltage ripple in the charged in the upper and lower DC link capacitors.