Power Inverters: What Are They & How Do They Work?

What is an Inverter?

An inverter (or power inverter) is a power electronics device which used to convert DC voltage into AC voltage. Although DC power is used in small electrical gadgets, most household equipment runs on AC power. Hence we need an efficient way to convert DC power into AC power.

The inverter is a static device. It can convert one form of electrical power into other forms of electrical power. But it cannot generate electrical power. Hence the inverter is a converter, not a generator.

It can be used as a standalone device such as solar power or back power for home appliances. The inverter takes DC power from the batteries and converts into AC power at the time of the power failure.

A power inverter used in the power system network to convert bulk DC power to AC power. i.e. It used at the receiving end of HVDC transmission lines. This inverter is known as a grid-tie inverter.

How Does an Inverter Work?

Let’s understand the working of an inverter by an example. One bulb connected with a battery. It makes a close path. Hence the current will flow through the bulb.

The bulb has two terminals that are ‘A’ and ‘B’. The positive and negative terminal of the battery is connected with ‘A’ and ‘B’ terminal respectively and the bulb will glow.

Now, change the terminals of the battery. The bulb will glow in this condition also. So, what is the difference in both cases?

Here, one thing is different and that is the direction of the AC current.

Now imagine that you can rotate the battery at 50 or 60 rpm. What will happen? The direction will change 50 or 60 times. This is similar to AC power. And the frequency is 50 or 60 Hz.

This is just to understand the working principle of an inverter. Practically, inverter never works like this and it doesn’t have rotating parts.

The inverter uses the power electronics switches like IGBT, MOSFET. The number of switches depends on the type of inverter.

Let’s take a circuit diagram of a single-phase full-bridge inverter to understand the working.

There are four switches. A DC source connected with the switches and load.

When switch S1 and S2 are ON, S3 and S4 OFF, the direction of current through the load are positive in this condition. It gives a positive half cycle of the AC output.

Now, switch S3 and S4 is ON, S1 and S2 OFF. The current flowing in the opposite direction. It gives a negative half cycle of the AC output.

The ON and OFF time of switches decides the output frequency. The output of the inverter is a square wave. The filters used to generate a sine wave.

Types of Inverter

According to the Output Waveform

There are three types of inverters.

• Square wave inverter
• Modified sine wave inverter
• Sine wave inverter

Square Wave Inverter

This is the least used but simplest type of inverter. The output waveform of this inverter is a square wave. The home appliances and most of all equipment that works on AC, designed for the sine wave.

It converts the straight DC signal to a phase-shifting AC signal. But the output is not a pure AC signal. This is the cheapest type of inverter.

If you connect the equipment with a square wave inverter, it makes more losses. The equipment may get damaged in the worst case.

These types of inverters use to generate sine wave inverters by using filters (e.g. active low pass filters).

Modified Sine Wave Inverter

This inverter is also known as quasi wave inverter. This inverter generates the signal near to the sine wave. But it cannot generate the smooth sine wave.

A modified sine wave inverter creates some pauses before phase shifting. It doesn’t shift phase directly from positive to negative like a square wave.

The construction of this inverter is more complex than the square wave inverter but simpler than the sine wave inverter.

Sine Wave Inverter

This is the most efficient and complex type of inverter. It generates the pure sine wave which is a similar waveform of the grid power. All AC equipment designed to work on the sine wave signal.

A sine wave can be generated from the square wave inverter by modifying the output waveform.

This inverter made the least losses. But the cost of this inverter is very high. This type of inverters is widely used in residential and commercial applications.

According to the Type of Load

There are two types of AC power; single-phase and three-phase. Therefore, there are two types of load. And according to that, there are two types of inverters:

• Single-phase inverter
• Three-phase inverter

Single-phase Inverter

If the load is a single-phase, the inverter used to run the load that is the single-phase inverter. There are two types;

• Half-bridge inverter
• Full-bridge inverter

Single-phase Half-bridge Inverter

Two thyristors (S1 and S2) connected with two feedback diodes (D1 and D2) as shown in the below circuit diagram.

The supply voltage divides into two equal parts. The resistive load used to understand the working principle.

Mode-1

Thyristor S1 is ON and S2 is OFF during this mode. The current flowing path is V/2-S1-B-RL-A-V/2.

The current flowing through the load is B to A direction. And the voltage across the load is positive V/2. In this mode, the positive cycle of the output generates.

Mode-2

Thyristor S2 is ON and S1 is OFF during this mode. The current flowing path is V/2-A-RL-B-S2-V/2.

The current flowing through the load A to B direction. The voltage across the load is negative V/2. In this mode, the negative cycle of output generates.

Single-phase Full-bridge Inverter

In a full-bridge inverter, four thyristors and four feedback diodes used. One DC source applied to the circuit.

In a half-bridge inverter, one switch is in conduction at a time. And in a full-bridge inverter, two switches are in conduction at a time.

Mode-1

Thyristor S1 and S2 are ON and thyristors S3 and S4 are OFF during this mode. The current flowing path is V-S1-A-RL-B-S2-V.

The current flowing through the load is from A to B and make a positive half cycle.

Mode-2

Thyristor S3 and S4 are ON and thyristor S1 and S2 are OFF. The current flowing path is V-S3-B-RL-A-S4-V.

The current flowing through the load is from B to A and make a negative half cycle of output.

Three-phase Inverter

Generally, three-phase AC supply used in industries and the load is three-phase. In this case, a three-phase inverter used to run this load.

In a three-phase inverter, six diodes and six thyristors used. According to the conduction time of thyristor, this inverter divides into two types;

• 120-degree mode of operation
• 180-degree mode of operation

120-Degree Mode of Operation

At a time, two thyristors are in conduction. The conduction time for all thyristors is 120-degree. It means, a switch remains ON for 120-degree and OFF for the next 240-degree.

The shape of phase voltage is a quasi-square wave and the shape of the line voltage is three-stepped waveform.

180-Degree Mode of Operation

Three thyristors are in conduction at a time. The conduction time for all thyristors is 180-degree.

The shape of the line voltage and phase voltage is opposite to the 120-degree mode of operation. Here, for phase voltage, a waveform is a three-stepped wave and for line voltage, a waveform is a quasi-square wave.

In a 180-degree mode of operation, two thyristors of the common bridge are ON and OFF simultaneously. For example, in half cycle (180-degree) S1 is ON and the next half-cycle S4 is ON. So, at the same time, S1 is switching OFF and S4 is switching ON. Because of this simultaneous conduction, it is possible that the source may sort circuited.

This problem will not happen in a 120-degree mode of operation.

Applications of Inverter

Some of the applications of an inverter include:

• When the main power is not available, an uninterruptible power supply (UPS) uses battery and inverter.
• The power inverter used in the HVDC transmission line. It also used to connect two asynchronous AC systems.
• The output of the solar panel is DC power. The solar inverter used to convert DC power into AC power.
• The inverter produces variable output voltage by using a control unit (close-loop inverter). The speed of inverter controlled by supplying variable voltage. For example, it used in the refrigerator compressor motor, rail transport, induction motor speed control, electric vehicle.
• It can convert the low-frequency AC power to a higher frequency which used in induction heating.

Who Invented the Inverter?

Before the inverter was invented, a motor-generator set and rotary converter were used to convert DC power into AC power.

The engineering term inverter was first introduced by David Prince in an article titled “The Inverter” in 1925. In this article, Price defined the inverter as the inverse of a rectifier.

The term rectifier was in use for more than two decades prior to 1925. Rotary converters used as rectifier until the diode was not available. When it used for conversion of DC to AC, termed as “inverted rotaries”.

After the invention of power electronics switches, the new era of converters started. And increase applications of an inverter. Which leads to the advancement of an inverter.