What is a Three Phase Inverter : Working & Its Applications

An inverter is a power electronic device, used to change the power from one form to other like DC to AC at the necessary frequency & voltage o/p. The classification of this can be done based on the source of supply as well as related topology in the power circuit. So these are classified into two types (voltage source inverter) and CSI (current source inverter). The VSI type inverter has a DC voltage source with less impedance at the input terminals of an inverter. The CSI type inverter has a DC current source with high impedance. This article discusses an overview of a three-phase inverter like a circuit, working and it’s applications.

What is Three Phase Inverter?

Definition: We know that an inverter converts DC to AC. We have already discussed different types of inverters. A three-phase inverter is used to change the DC voltage to three-phase AC supply. Generally, these are used in high power and variable frequency drive applications like HVDC power transmission.

In a 3 phase, the power can be transmitted across the network with the help of three different currents which are out of phase with each other, whereas in single-phase inverter, the power can transmit through a single phase. For instance, if you have a three-phase connection in your home, then the inverter can be connected to one of the phases.

Working Principle

A three-phase inverter working principle is, it includes three inverter switches with single-phase where each switch can be connected to load terminal. For the basic control system, the three switches operation can be synchronized so that single switch works at every 60 degrees of basic o/p waveform to create a line-to-line o/p waveform including six steps. This waveform includes a zero voltage stage among the two sections like positive &negative of the square-wave. Once PWM techniques based on the carrier are applied to these waveforms, then the basic shape of the waveform can be taken so that the third harmonic including its multiples will be canceled.

Single Phase Inverter

These inverters are available in two types like full-bridge type and half-bridge type

The full-bridge type inverter circuit mainly used to change DC to AC. This can be achieved through  the opening and closing of the switches within the right sequence. This kind of inverter includes four dissimilar operating states where these switches work on closed switches.

The half-bridge type inverter circuit is the basic building block in a full-bridge type inverter. This inverter includes two switches where each type of switch includes capacitors that have output voltage. Additionally, these switches complement each other, because if the first switch is turned ON then the remaining switch will be turned OFF.

Three Phase Inverter Design/Circuit Diagram

The circuit diagram of a three-phase inverter is shown below. The main function of this kind of inverter is to change the input of DC to the output of three-phase AC. A basic 3 phase inverter includes 3 single phase inverter switches where each switch can be connected to one of the 3 load terminals.

Generally, the three arms of this inverter will be delayed with 120 degrees angle to generate a 3 phase AC supply.
The switches used in the inverter have 50% of ratio and switching can be occurred after every 60 degrees angle. The switches like S1, S2, S3, S4, S5, and S6 will complement each other. In this, three inverters with single-phase are placed across a similar DC source. The pole voltages within the three-phase inverter are equivalent to the pole voltages within the half-bridge inverter with a single phase.’

The two types of inverters like the single-phase and three-phase include two conduction modes like 180 degrees conduction mode and 120 degrees conduction mode.

180° Conduction Mode

In this conduction mode, each device will be in conduction with 180° where they are activated at intervals with 60°. The output terminals like A, B, and C are connected to the star or 3 phase delta connection of the load.

The balanced load for three phases is explained in the following diagram. For 0 to 60 degrees, the switches like S1, S5 & S6 are in conduction mode. The load terminals like A & C are linked to the source on its positive point, whereas the B terminal is associated with the source on its negative point. Furthermore, the R/2 resistance is available among the two ends of neutral & the positive whereas R resistance is available among the neutral & the negative terminal.

In this mode, the voltages of load are given in the following.

VAN = V/3,

VBN = −2V/3,

VCN = V/3

The line voltages are given in the following.

VAB = VAN − VBN = V,

VBC = VBN − VCN = −V,

VCA = VCN − VAN = 0

120° Conduction Mode

In this type of conduction mode, every electronic device will be in a conduction state with 120°. It is apt for a delta connection within a load as it results within a six-step kind of waveform across one of its phases. So, at any instant, only these devices will conduct every device that will conduct at 120° only.

The connection of ‘A’ terminal on the load can be done through the positive end whereas the B terminal can be connected toward the negative terminal of the source. The ‘C’ terminal on the load will be in conduction is known as the floating state. Also, the phase voltages are equivalent to the voltages of load which is given below.

Phase voltages are equal to line voltages, so

VAB = V

VBC = −V/2

VCA = −V/2

Three Phase Inverter Applications

The applications of this type of inverter include the following.

• These inverters are utilized in variable frequency drive applications
• Used in high-power applications like HVDC power transmission.
• A three-phase square wave inverter is used in a UPS circuit and a low-cost solid-state frequency charger circuit.

Thus, this is all about an overview of a three-phase inverter, working principle, design or circuit diagram, conduction modes, and its applications. A 3 phase inverter is used to convert a DC i/p into an AC output. It includes three arms which are usually delayed through 120° of an angle to produce a 3 phase AC supply. The switches in an inverter have a 50% of ratio & switching happens after each T/6 of the time with 60° of angle interval. Here is a question for you, what are the different types of inverters available in the market?