Inverter : Single Phase Inverter and 3-Phase Inverter

The inverter is an electronic device or circuitry that changes direct current (DC) to alternating current (AC). In short, the Inverter is a DC-AC converter. It can be seen as an application for DC chopper. We can classify inverters into single-phase inverters and 3-phase inverters. We will see some inverter circuits and how they act.

1. Single Phase Inverter: Half Bridge, Full Bridge

Half Bridge

1-1. Half-bridge inverter circuit

1-2 Half-bridge inverter graph

 In figure 1-1, we can see the Half-bridge circuit. In this circuit, there are two rules of triggering: First, because of KVL, T1 and T2 can not be on at the same time. Second, KCL is satisfied by D1 and D2.
 A pair of a diode and a transistor acts like a switch. So In figure 1-2, if SW1 (T1 and D1) is on for the time duration 0<t<T/2, then V_o is V_s/2. On the other hand,  if SW2 (T2 and D2) is on for the time duration T/2<t<T, then V_o is V_s/2.



Assuming that it's a highly inductive load, in contrast with resistor load before, the current graph is not a longer square wave. In this case, the equations about max and minimum currents can be derived.

Full Bridge

In a Full-Bridge inverter, the load is charged to Vs instead of Vs/2

Control Logic 1: Inductive Load

2-1. Full-Bridge control logic 1

2-2. Full-Bridge inverter circuit

2-3. Full-Bridge inverter graph

In figure 2-1, during the time T1-T2, current flows through the load in the direction from left to right and it is charged. After T1 and T2 are off, the charged inductive load releases current. So from this, Diode D3 and D4 are on, which is the time D3-D4. The same thing happens during T3-T4 and D1-D2.

Control Logic 2: Inductive Load

2-1. Full-Bridge control logic 2

2-2. Full-Bridge inverter graph

This is a more complicated control logic than before. During the time T1-T2, current flows in the direction from left to right. For this time, the load is charged as control logic 1. After that, transistor 1 (T1) is still on. In this case, current doesn't flow through D4. So while transistor 1 and diode 3 are on, voltage becomes zero, but current remain the max current because we assumed that load is highly inductive. After T1 is off, load release current like control logic 1 and this time is D3-D4. The same thing happens in T3-T4, T3-D1, D1-D2.

2. 3-Phase Inverter

3-1. 3-Phase Inverter Circuit

3-2. Control Logic

Fig 3-1 shows 3 phase inverter circuit and Fig 3-2 is its control logic. As you can see, the period of each line in the inverter is 180 degrees. For the time during 0~60 degrees, the circuit can be drawn as Fig. 3-3. 

3-3. 0~60 degree

60~120 degree

From this, each voltage can be calculated as below. We can get phase voltage until 360 degrees in this way.

Below are the graphs of phase voltage and line voltage. Line voltage can be calculated as the difference of phase voltages.

Phase Voltage

Line Voltage

As a result, we can notice that DC voltage is changed to a similar form of AC voltage, which is similar to the sine graph. So this circuit acts as an inverter that we want.