Three-phase inverters are normally used for high power applications. The advantages of a three-phase inverter are:
• The frequency of the output voltage waveform depends on the switching rate of the switches and hence can be varied over a wide range.
• The direction of rotation of the motor can be reversed by changing the output phase sequence of the inverter.
• The ac output voltage can be controlled by varying the dc-link voltage.
The general configuration of a three-phase DC-AC inverter is shown in Circuit Diagram. Two types of control signals can be applied to the switches:• 180° conduction• 120° conduction
180 Degree conduction Mode: In this mode of conduction, every device is in conduction state for 180° where they are switched ON at 60° intervals. The terminals A, B and C are the output terminals of the bridge that are connected to the three-phase delta or star connection of the load.
The operation of a balanced star connected load is explained in the diagram below. For the period 0° − 60° the points S1, S5 and S6 are in conduction mode. The terminals A and C of the load are connected to the source at its positive point. The terminal B is connected to the source at its negative point. In addition, resistances R/2 is between the neutral and the positive end while resistance R is between the neutral and the negative terminal.