Cycloconverter: Applications & Types

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Key learnings:

Cycloconverter Definition: A cycloconverter (also known as a cycloinverter or CCV) is defined as a device that converts a constant voltage, constant frequency AC waveform to another AC waveform of a different frequency.
Function of Cycloconverters: Cycloconverters change AC to AC while altering the frequency, usually lowering it without any DC stages.
Types of Cycloconverters: The main types of cycloconverters are blocking mode cycloconverters and circulating current cycloconverters.
Single Phase Cycloconverters: Single phase to single phase cycloconverters use two full wave converters connected back-to-back to convert voltage.
Cycloconverter Applications: Cycloconverters are used in cement mill drives, rolling mills, ship propulsion, water pumps, washing machines, mine winders, and various industries.

What is Cycloconverter

A cycloconverter (also known as a cycloinverter or CCV) converts a constant voltage, constant frequency AC waveform to another AC waveform of a different frequency. A cycloconverter achieves this through synthesizing the output waveform from segments of the AC supply (without an intermediate DC link).

The main forms of electrical energy commonly available are constant DC (Direct Current) and constant AC (Alternating Current). Often though, we need to swap between AC and DC, change the frequency, or swap from constant to variable power. For these conversion purposes, several converters like inverters, rectifiers, DC choppers and cycloconverters are employed. The cycloconverters can in fact transfer AC power of a fixed frequency to the AC power of a different frequency (see figure 1).

A cycloconverter changes AC to AC, adjusting only the frequency, so it’s also called a frequency changer. Usually, the output frequency is lower than the input frequency. This process doesn’t involve DC stages. The control circuit is complex due to many SCRs (Silicon Controlled Rectifiers) and often uses a microcontroller, DSP, or microprocessor.

Types of Cycloconverters

There are two main types of cycloconverters:

Blocking mode Cycloconverters
Circulating current Cycloconverters

Blocking Mode Cycloconverters

Here, the positive converter will provide the necessary voltage when there is positive load current. At that time, the negative converter will be in the blocked condition. During the negative load current, the negative converter will provide the necessary voltage and at that time, the negative converter will be in the blocked condition. This type of operation is known as blocking mode operation and the converters which use this method of operation are termed as blocking mode cycloconverters. There is no need of intergroup reactors because only one converter is working at a time. Cost and size of these converters are low. These converters are most commonly used.

Circuiting Current Cycloconverter

In this type of converter, both converters operate simultaneously. This can cause a short circuit if both are enabled at the same time. To prevent this, an intergroup reactor (IGR) is placed between the converters. This setup creates a unidirectional circulating current.
Mainly there are two types according to the output frequency which are showing below

Step-up cycloconverters
Step-down cycloconverters

Step Up Cycloconverters

It can provide an output having the frequency greater than the input frequency by using line commutation.

Step Down Cycloconverters

It provides output having lower frequency than the input frequency by using forced commutation.
These types of converters are further classified into following:

Single Phase to Single Phase Cycloconverters

It consists of two full wave converters connected back-to-back. This setup is used to explain the basic operation of a cycloconverter.

Here’s how it works: If we need to output one-fourth of the input voltage, the positive converter operates during the first two cycles, providing current to the load and rectifying the input voltage. During the next two cycles, the negative converter operates, supplying current in the opposite direction. When one converter is active, the other is disabled, so no circulating current exists between them.

Three-Phase to Single Phase Cycloconverters

This type of converter can operates in four quadrants (+V, +I, -V, -I) in two modes (inverting and rectifying). Here; +V, +I are in rectifying modes and -V, -I are in inverting modes. Positive converters will provide positive current and negative converters will provide negative current to the load. At a time, one converter will operate. The circuit is shown below.

Three Phase to Three Phase Cycloconverters

These types of Cycloconvertes are formed by using 3 three-phase to single cycloconverters linked together to the load and is used in AC machine systems. Two basic configurations are wye and delta. Output of these converters can be connected in wye and delta. The phase of output voltages are 120^o shifted. The circuit diagram is shown below.