Classification of Inverter

The inverters are used to convert DC power into AC power and are used in UPS (Uninterrupted Power Supply) for power backup, AC motor speed control, etc. The inverter classifies in different manners. The classification of an inverter is as listed below.

Nature of Source

According to the nature of an input power source, the inverters are classified into two types;

In voltage source inverter, ripple-free DC voltage source is used to supply input power.

In the case of a current source inverter, a voltage source is converted into a current source, and it is used to supply the input power to the inverter.

Configuration of Inverter

According to the arrangement of power electronics switches, the voltage source inverters are classified into three types;

• Series inverter
• Parallel inverter (push-pull inverter)
• Bridge inverter (Half-bridge/full-bridge)

Series Inverter

In a series inverter, the thyristors are connected in series. The basic circuit of a series inverter (operated by thyristors) is shown in the figure below.

In a thyristor-based series inverter, class A type of commutation is used, where SCRs turn off themselves; hence, this inverter is also known as a self-commutated inverter.

It is used in induction heating, ultrasonic equipment, and power supply for the cycloconverter.

The commutation circuit for this inverter is simple. In this inverter, it is possible to obtain sine wave output by making off time (Toff) zero.

Parallel Inverter

The parallel inverter needs a center-tapped transformer on the output side. A parallel inverter is also known as a center-tapped inverter and is also known as a push-pull inverter.

The basic configuration of resistive load thyristor-based parallel inverter is as shown in the figure below.

In a parallel inverter, the load waveform is not dependent on the load; instead, it is a simple and economical circuit and uses class C commutation.

Comparison Between Series and Parallel Inverter

 Parameter Series Inverter Parallel Inverter Position of commutating components In series with a load C is in parallel with a load Type of commutation Class A Class C or D Type of inverter Self-commutated Forced commutated Resonant circuit Necessary Not necessary Output transformer Not required Essential Possibility of core saturation Not possible Possible Output waveform Depends on load Independent of load Distortion in output High Low Feedback diode Not available Available Output frequency Not adjustable Adjustable Type of output waveform Sine Square

Bridge Inverter

Bridge inverters are classified into two types;

• Half-bridge inverter
• Full-bridge inverter

Half Bridge Inverter

For single-phase output, a half-bridge inverter required two switches. Thyristor-based single-phase half-bridge inverter is as shown in the figure below.

Here two thyristors S1 and S2, are used along with D1 and D2 connected across them. Instead of thyristor, any other power electronics switches can be used according to the application, like; IGBT, MOSFET, Transistor.

In a half-bridge inverter, the DC supply is divided into two halves each. Let say, V volt DC supply is given to the inverter. The output voltage will be V/2 voltage.

Full Bridge Inverter

The circuit diagram of a single-phase thyristor-based full-wave inverter is as shown in the figure below.

As shown in a circuit diagram, for a single-phase full-wave inverter, we need four switches. Two switches conduct simultaneously.

In a full-bridge inverter, the output voltage is the same input voltage.

Nature of Output Waveform

The output of an inverter is AC. And there are different types of AC signals. It provided that the most used AC signal is a sine wave. But it can be square, quasi square, or PWM signal.

The filters are used to convert sine wave signals from other shapes of signals. The percentage of harmonics frequency components largely depends on the shape of output waveforms.

According to the shape of AC signals, the inverters are classified as below.

• Square wave inverter
• Quasi-square wave inverter
• PWM inverter
• Sine Wave inverter

The shape of the above signal is as shown in the figure below.

Type of Commutation

There are different types of techniques used for the commutation of power electronics switches.

• Forced commutation
• Line commutation
• Self-controlled (self-commutation)

Forced Commutation

To turn off conductive thyristors or SCRs, different types of forced commutation circuits are used. Out of that, two popular technics are;

• Auxiliary commutation (Mc Murry Bedford circuit)
• Complementary commutation (Mc Murry circuit)

Commutation circuits are designed to successfully commutate the thyristors. The power consumed by the commutation circuit is must be as minimum as possible.

The type of commutation can be voltage or current commutation. In a commutation circuit, the components must be optimized to improve the overall efficiency of the inverter.

Line Commutation

The line commutation technique used mains AC voltage; this technique is used for thyristors.

This type of inverter operates in inversion mode (α > 90˚).

Self-Controlled Commutation

Self-controlled commutation is also known as simple self-commutation. This type of commutation technique is used in power semiconductor devices like; power BJT, power MOSFET, or IGBT.

These devices do not need any external commutation circuitry to turn off, so the inverter uses these devices are known as the self-commutation technique.

Used Power Semiconductor Devices

In an inverter, we can use any power semiconductor device as a switch to contract an inverter.

The devices used in the inverter are; SCR, IGBT, MOSFET, transistors. And according to that, the inverters are classified as;

• Thyristorized Inverters (Using SCR)
• Transistorized Inverters (Using Transistor)
• MOSFET based inverters
• IGBT based inverters

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