Cycloconverters: Design, Working, Types & Applications

Overview

In this post, we will learn about Cycloconverters & its Design, Working Principle, Types & Applications.

A cycloconverter is a type of power electronic converter that can convert a constant-frequency alternating current (AC) to a different frequency AC. The input to a cycloconverter is typically a three-phase AC supply, and the output can be either single-phase or three-phase, at a frequency that is different from the input frequency. Cycloconverters are used in a variety of industrial and commercial applications, including electric motor drives, adjustable-speed drives, and power generation systems.

Cycloconverters are classified into two main types: single-phase and three-phase. Single-phase cycloconverters are used to convert single-phase AC to a different frequency AC, while three-phase cycloconverters are used to convert three-phase AC to a different frequency AC. Cycloconverters are also classified into two main categories based on the method of frequency conversion: step-down and step-up.

Cycloconverters are well known for their ability to operate at variable speeds and are used in many industrial drives where variable-speed operation is required, such as in fans, pumps, and conveyors. They are also used in electric traction systems.

Overall, Cycloconverters are a powerful and versatile solution for changing the frequency and voltage of AC power and play an important role in a wide range of industrial and commercial applications.

Cycloconverters Working Principle

The working principle & design of Cycloconverters are simple to understand. Cycloconverters are divided into two parts or simply it is made of two converters in which one converter works as a positive converter (P converter) and second converter works as a negative converter(N converter). The firing angle of both converters is set such that:

α_P +α_N = π

Where;

α_P = Firing angle of positive converter

α_N = Firing angle of negative converter

The firing angle of both converters is set such that it will change the output frequency (either high or low). The P – converter and N – converter is constructed by making the series of connection of variable alternating supply and diode together. The output voltage of both converters becomes equal in the P-type converter and N-type converter of the ideal cyclo-converter.

The output voltage at the terminal of the cyclo converter is either of the alternating voltage or of the converter because each converter has equal alternating voltage amplitude, phase as well as frequency. The power either flows from load to converter or converter to load by controlling the firing angle of each converter. The unidirectional property of the P – converter and N – converter gives the result in the P – converter working in a positive half cycle of alternating supply and N – converter working in a negative half cycle of alternating supply that is provided to it as input. It is independent of the phase between load voltage and loads current.

Characteristics of Cycloconverters

phase-controlled SCR (Silicon Controlled Rectifier) switching devices can be used throughout the range of CCVs (Current Controlled Voltage Source.), low cost, low-power TRIAC-based CCVs are inherently reserved for resistive load applications. The amplitude and the frequency of the converter’s output voltage are variable.

The output to input frequency ratio of a three-phase CCV must be less than about one-third (1/3) for circulating current mode CCVs or one-half for blocking mode CCVs. The Output waveform quality improves as the pulse number of switching-device bridges in phase-shifted configuration increases in CCV’s input.

Types of Cycloconverters

Based on frequency, there are two types of cycloconverters.

1: Step-down Cycloconverters

A step-down cyclo-converter is a device that steps down the fixed frequency power supply input into a lower frequency. It is similar to a step-down transformer that provides the output frequency less than that of input which is given as fo < fi. In the step-down cyclo-converter, the output frequency is limited to a fraction of the input frequency (below 20Hz in case 50Hz supply frequency). In this case, no separate commutation circuits are needed as SCRs (silicon-controlled rectifiers) are line commutated devices.

2: Step-Up Cycloconverters

A Step-up cyclo-converter is a single phase to single phase device which converts input AC frequency to a different output frequency with is higher than the input frequency. Single-phase to single-phase implies that both the input power and output power are single phases. It provides the output frequency more than that of the input so the equation is given as fo > fi.

In step-up cycloconverters, forced commutation circuits are needed to turn OFF SCRs (silicon-controlled rectifiers) at the desired frequency. Therefore a majority of Cycloconverters are of step-down family. The Step-down Cycloconverters circuits are further divided into the following types:

Single-phase to single-phase cyclo-converters
Three-phase to single-phase cyclo-converters
Three-phase to three-phase cyclo-converters

In addition to the frequency control, the cyclo-converter output voltage can be varied by applying the phase control technique. These can be used to provide fixed frequency output from variable frequency input value or variable frequency output from fixed frequency input. It is mainly used in very high power, low-speed AC motors and traction systems, especially low-frequency three-phase to single-phase systems.

Applications of Cycloconverters

Cycloconverters has wide range of applications. Some applications where Cycloconverters can be used are:

Grinding Mills
Heavy Washing Machines
Mine Winders
HVDC Power lines
Aircraft Power supply
SVG (Static VAR Generators)
Ship Propulsion system

Advantages

Efficiency is high compared to other converters that are available.
Capable of power transfer in bi-directions.
AC power at one frequency is converted to a lower frequency in a single conversion.
If SCRs fail, the cyclo-converter operates with a distorted output.
Power transfer is possible from supply to load and vice versa at any power factor.
Dynamic response is very good.
Smooth low-speed operation is achieved.