What is Inverter? A beginner Guide for Electrical & Electronic Eng Students

Introduction:

In this article, we’ll be explaining what an inverter is, what its function is and what its principle of operation is. You’ll find inverters in photovoltaic arrays to provide AC power for use in homes and offices. You will also find them in ‘variable frequency drivers’ to control the speed, torque and rotational direction of AC induction motors in fans, pumps, and compressors.  

What is an inverter?

Let’s go to the basics first. You’ve probably known the two different types of electrical power i.e. Direct Current (DC) and Alternating Current (AC). Direct current is the one you’d probably use for a basic flashlight, solar panels or batteries, and small electrical products with circuit boards. But when we talk about bigger household appliances, electricity works quite differently. The power supply from your wall outlet is based on Alternating current (AC) and is used to power up your house. Both types have their own limitations and so we often have to convert one form to the other in order to maximize their use. 

An inverter is a device that can do that for you: convert DC to AC. You can use an oscilloscope to look at the signal of both types of powers. DC will provide you with a current that essentially flows in one direction with maximum voltage; you’ll see one single, continuous, straight line. On the other hand for AC, you’ll see a sinusoidal wave-like pattern with the voltage alternating between the peak positive and peak negative points in one cycle. The current essentially travels ‘back and forth’. 

Working of an Inverter

To understand the working of an inverter, consider a circuit where a DC source is being used to power an AC device. To convert DC to AC there are four switches. The switches are paired in such a way that when switch 1 and 4 close, switch 2 and 3 will open and vice-versa. Therefore, the load will experience an alternating current although it is connected to a DC source.

When one pair of switches is open the current flows through the right-hand side of the AC device. Similarly, when the corresponding second pair of switches is open, the current flows through the left-hand side of the AC device. And so, the device will experience an alternating current. 

The device will not experience a sine wave; instead, the sudden switching will result in a square wave. This wave can actually damage the electrical device and so it needs to be smoothed out. For this engineers use certain electrical components such as diodes, IGBTs, and MOSFETs, etc. 

Use of IGBTs in Inverters: 

An IGBT (Insulated Gate Bipolar Transistor) is a three-terminal semiconductor device. Now consider the circuit shown below for a motor.

There is a DC source and an AC load, and a bunch of IGBTs connected to a controller. Each IGBT will receive a signal from the controller telling it when to open and close. The IGBT’s are paired together. 

When you power up the circuit, the controller will switch up a pair of IGBTs to allow current to pass through them for a limited period of time and so the motor will experience an alternating current. 

Use of Inverters to Control Motor Speed:

IGBTs open and close in a pulsating manner several times per cycle, which is also known as pulse width modulation.  The cycle itself is broken down into smaller segments and the controller tells the IGBTs how long to stay closed for each smaller segment. In this way, the IGBTs allow varying amounts of current to flow through the motor. 

As a result, the average power per segment results in a sine wave pattern. The more we divide the cycle into smaller segments, the smoother the sine wave will be. And so, the motor will experience a sine wave alternating current. 

The controller can then be used to control the amount of time a certain IGBT is open to control the corresponding frequency and wavelength of the signal which will control the motor speed, torque, and direction. 

Uses of Inverters: 

Inverters have many applications and use:

• They are used in tiny car adapters to the office, household applications, and large-grid systems.
• A common application is within photovoltaic arrays. 
• They are integrated into Variable Frequency Drivers to control the speed, torque, and direction of AC motors.
• They are used within HVAC systems for industrial and commercial properties. 
• They can be used as a UPS (Uninterruptible Power Supplies).
• They are used in solar power systems. 
• They can be used in centrifugal fans, mixers, pumps, conveyors, extruders, etc.