AC to AC Converter: Working, Types and Applications
Table of Contents
AC to AC converter is used to change AC waveforms from one frequency at one magnitude to another frequency at a different magnitude. This conversion is primarily necessary for applications involving low frequency and variable voltage magnitude, as well as for controlling the speed of machines. We are aware that various loads operate with various power supplies, such as single-phase and three-phase supplies, and that the supplies can also be distinguished based on the voltage and frequency range.
What is AC to AC Converter?
For the operation of some specialized machines or devices, we need a specific voltage and frequency. AC to AC converters (also known as cycloconverters) are frequently used for induction motor speed control. We require some converters known as AC to AC converters in order to convert the actual power supply into the desired AC power supply.
Types of AC to AC Converter
Different types of AC to AC converters can be identified:
- AC to AC Converters with DC link
- Matrix Converters
- Hybrid Matrix Converters
Cycloconverters are frequently referred to as frequency changers because they can change the AC power’s magnitude in addition to converting it from AC power with one input frequency to AC power with a different output frequency. And Cycloconverters are chosen over DC links and numerous stages like AC to DC to AC that are costly and result in more losses. Depending on the ratings of the supply power being used, the cost of the necessary DC link will change.
The cycloconverter’s working principle can be seen in the above illustration, which demonstrates how the firing angle applied to the thyristors can alter the input wave frequency. We can obtain variable output frequency that can be step-up or step-down in relation to the input frequency by switching the positive and negative limb thyristors.
Cycloconverters are Classified into Different Types Based on Different Criteria
Cycloconverters have two parts, a positive part (also known as a positive converter) and a negative part (also known as a negative converter). Negative limb activity occurs during the negative half cycle, while positive limb activity occurs during the positive half cycle.
Classification of Cycloconverters Based on the Mode of Operation:
Blocking Mode Cycloconverters
Since only one limb, either the positive or negative limb, conducts at a time and the other limb is blocked, these cycloconverters do not require a limiting reactor. This is why they are referred to as Blocking Mode Cycloconverters.
Circulating Current Mode Cycloconverter
As both the positive limb and the negative limb conduct at the same time in these cycloconverters, a reactor is positioned to regulate the circulating current. Because there will be a circulating current in the system as a result of both limbs conducting simultaneously, the cycloconverter is also known as a circulating current mode cycloconverter.
Classification of Cycloconverters Based on the Number of Phases of Output Voltage
Single Phase Cycloconverters
These are again classified into two types based on the number of input phases.
1-Ø to 1- Ø Cylcoconverter
This cycloconverter changes the single-phase AC waveform’s magnitude and frequency at the input to a different magnitude and frequency at the output.
3-Ø to 1- Ø Phase Cycloconverter
A single-phase AC waveform with a different output frequency or magnitude is produced by this cycloconverter from a three-phase AC supply that has an input frequency and magnitude.
3-Ø to 3- Ø Phase Cycloconverter
The output of this cycloconverter is a three-phase AC waveform with a different output frequency or magnitude from the input three-phase AC supply’s frequency and magnitude.
Classification of Cycloconverters Based on the Firing Angle of Positive and Negative Limbs
The firing angle in this type of cycloconverter is fixed during the positive half-cycle for both the positive and negative half-cycles. The firing angle for a positive converter is set to α=0°, and for the negative half cycle, it is set to α=180°. Similar to a positive converter, a negative converter has its firing angle set to α=0° during the negative half cycle and α=180° during the positive half cycle.
Phase controlled Cycloconverters
By utilizing this type of cycloconverter, we are able to alter both the output frequency and output voltage. By altering the converter’s firing angle, both can be changed.
2. AC to AC Converters with a DC Link
As in this process, the AC is converted into DC by using the rectifier,. AC to AC converters with a DC link typically consist of a rectifier, DC link, and inverter. The DC link is used to store DC power after it has been converted into DC, and the inverter is used to convert it back into AC after that. The figure depicts an AC to AC converter circuit with a DC link.
AC to AC converters with a DC link is classified into two types:
Current Source Inverter Converter
One or two series inductors are placed between one or both limbs of the connection between the rectifier and inverter in this type of inverter. Here, a phase-controlled switching mechanism similar to a thyristor bridge is used as the rectifier.
Voltage Source Inverter Converter
In this kind of converter, the rectifier is a diode bridge, and the DC link is a shunt capacitor. Since the diode bridge causes less AC line distortion and low power factor than the thyristor bridge, it is preferred for low load applications. However, because the DC link passive component capacity requirement rises with the increase in power rating, AC to AC converters with a DC link are not advised for high-power ratings. High DC storage is required for storing large amounts of power, but these bulky passive components are expensive and inefficient because they result in higher losses during the AC to DC and DC to AC processes.
3. Matrix Converters
To increase the system’s reliability and efficiency by lowering the cost and losses of the DC-link storage element, matrix converters are used to convert AC to AC directly without using any DC links.
The bidirectional switches used in matrix converters, which are essentially nonexistent at the moment but can be created using IGBTs, are able to conduct current and block voltage from both polarities.
Matrix converters are again classified into different types based on the number of components used.
Sparse Matrix Converter
The operation of a sparse matrix converter is the same as that of a direct matrix converter,. But because fewer switches are needed here than with a direct matrix converter, the system’s reliability can be increased.
The sparse matrix converter needs 18 diodes, 15 transistors, and 7 isolated driver potentials.
Very Sparse Matrix Converter
When compared to a sparse matrix converter, the number of transistors is decreased while the number of diodes increases. As a result, higher conduction losses result from the increased number of diodes. The sparse/direct matrix converter and the very sparse matrix converter both perform similar functions. For a very sparse matrix converter, 30 diodes, 12 transistors, and 10 isolated driver potentials are needed.
Ultra Sparse Matrix Converter
These are used for low-dynamic variable speed drives because the input stage of the converter is unidirectional,. Which allows for an acceptable phase shift between the fundamental of the input current and the input voltage. Similar to this, the fundamental relationship between output voltage and current is 30°,. So these are primarily used in low-dynamic variable speed PSM drives. So For an ultra sparse matrix converter, 12 diodes, 9 transistors, and 7 isolated driver potentials are needed.
Hybrid Matrix Converter
Hybrid matrix converters are referred to as matrix converters that convert AC/DC/AC . And, like matrix converters, these hybrid converters do not use any capacitors, inductors, or DC links. If the voltage and current are both converted in a single stage,. The converter is referred to as a hybrid direct matrix converter. These are again divided into two types based on the number of stages they require for conversion. So A converter is referred to as a hybrid indirect matrix converter if voltage and current are converted in two distinct stages.
Cycloconverter using Thyristors
The goal of the cycloconverter project is to use thyristors. And the cycloconverter technique to control the speed of a single-phase induction motor. So Washing machines, water pumps, and vacuum cleaners all frequently employ induction motors, which are constant speed machines.
The circuit consists of an AC supply maintained at the cycloconverter. And a supply system (with a transformer, rectifier, and regulator to convert AC to DC). The mode selector and optoisolator are connected to the microcontroller. The motor is connected to the cycloconverter.
Induction motor speeds can be adjusted in three steps, F, F/2, and F/3. Slide switches connect the microcontroller, and by changing the status of these switches,. The microcontroller can deliver the proper triggering pulses to the dual bridge of the cycloconverter thyristors. So The frequency of the Cycloconverter’s output waveform can be changed by varying the triggering pulses. As a result, the single-phase induction motor’s speed can be controlled.
This article will discuss a few AC to AC converter, as well as their basic operation. So The majority of this equipment’s high-power converters are used in power electronic control applications.