Three Phase Inverter

THREE PHASE INVERTER

A three-phase inverter converts a DC input into a three-phase AC output. Its three arms are normally delayed by an angle of 120° so as to generate a three-phase AC supply. The inverter switches each has a ratio of 50% and switching occurs after every T/6 of the time T 60°angleinterval. The switches S1 and S4, the switches S2 and S5 and switches S3 and S6 complement each other.

The figure shows a circuit for a three phase inverter. It is nothing but three single phase inverters put across the same DC source. The pole voltages in a three phase inverter are equal to the pole voltages in single phase half bridge inverter.

Inverters have two modes of conduction -

1. 180° mode of conduction and

2. 120° mode of conduction.

180° mode of conduction

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.

The load voltages are gives as follows;

V_AN = V/3,

V_BN = -2V/3,

V_CN = V/3

The line voltages are given as follows;

V_AB = V_AN - V_BN = V,

V_BC = V_BN - V_CN = -V,

V_CA = V_CN - V_AN = 0

Waveforms for 180° mode of conduction

120° mode of conduction

In this mode of conduction, each electronic device is in a conduction state for 120°. It is most suitable for a delta connection in a load because it results in a six- step type of waveform across any of its phases. Therefore, at any instant only two devices are conducting because each device conducts at only 120°.

The terminal A on the load is connected to the positive end while the terminal B is connected to the negative end of the source. The terminal C on the load is in a condition called floating state. Furthermore, the phase voltages are equal to the load voltages as shown below.

Phase voltages = Line voltages

V_AB = V

V_BC = -V/2

V_CA = -V/2

Waveforms for 120° mode of conduction