Thyristor Based CycloConverter and Its Applications

Cycloconverter is a frequency converter from one level to another, that can change AC power from one frequency to AC power at another frequency. Here, an AC-AC conversion process is done with a frequency change. Hence it is also referred as frequency changer. Normally, the output frequency is less than the input frequency. The implementation of the control circuit is complicated due to the huge number of SCRs. The Microcontroller or DSP or microprocessor is used in control circuits.


A cyclo-converter can achieve frequency conversion in one stage and ensures that voltage and the frequencies are controllable. In addition, the need to use commutation circuits is not necessary because it utilizes natural commutation. Power transfer within a Cycloconverter occurs in two directions.

There are two types of Cycloconverters

Step Up Cycloconverter:

These types use normal commutation and give an output at higher frequencies than that of the input.

Step Down Cycloconverter:


This type uses forced commutation and results in an output with a frequency lower than that of the input.

The cyclo-converters are further classified into three categories as discussed below.

Single phase to Single-phase

This Cycloconverter has two full wave converters connected back to back. If one converter is operating the other one is disabled, no current passes through it.

Three-phase to Single-phase

This Cycloconverter operates in four quadrants that are (+V, +I) and (−V, −I) being the rectification modes and (+V, −I) and (−V, +I) being the inversion modes.

Three-phase to Three-phase

This Cycloconverter is majorly used in AC machine systems that are operating on three phase induction and synchronous machines.

Introduction of Single Phase to Single Phase Cycloconverter using Thyristors

The Cycloconverter has four Thyristors divided into two Thyristor banks, i.e, a positive bank and a negative bank of each. When the positive current flows in the load, the output voltage is controlled by phase control of the two positive array Thyristors whereas, the negative array Thyristors are kept off and vice versa when negative current flows in the load.

Operational illustration of Single Phase Cycloconverter
Operational illustration of Single Phase Cycloconverter

The perfect output waveforms for a sinusoidal load current and various load phase angles is shown in Figure below. It is important to keep the non-conducting Thyristor array off at all times, otherwise, the mains could be short circuited via the two Thyristor arrays, resulting in waveform distortion and possible device failure from the shorting current.

An Idealized Output Waveforms
An Idealized Output Waveforms

A major control problem of the cyclo-converter is how to swap between banks in the shortest possible time to avoid distortion while ensuring the two banks do not conduct at the same time.

A common addition to the power circuit that removes the requirement to keep one bank off is to place a center tapped inductor called a circulating current inductor between the outputs of the two banks.

Both banks can now conduct together without shorting the mains. Also, the circulating current in the inductor keeps both banks operating all the time, resulting in improved output waveforms.

Design of Cycloconverter using Thyristors

This project is designed to control the speed of a single phase induction motor in three steps by using a Cycloconverter technique by Thyristors. An A.C Motors have the great advantages of being relatively inexpensive and very reliable.

Block Diagram of Thyristor Based CycloConverter
Block Diagram of Thyristor Based CycloConverter

Hardware Components Requirement

DC power supply of 5V, Microcontroller (AT89S52/AT89C51), Optoisolator (MOC3021), Single phase induction motor, Pushbuttons, SCR, LM358 IC, Resistors, Capacitors.

Zero Voltage Cross Detection

Zero voltage cross detection means the supply voltage waveform that passes through zero voltage for every 10msec of a 20msec cycle. We are using 50Hz AC signal, the total cycle time period is 20msec (T=1/F=1/50=20msec) in which, for every half cycle (i.e. 10ms) we have to get zero signals.

Zero Voltage Cross Detection
Zero Voltage Cross Detection

This is achieved by using pulsating DC after the bridge rectifier before being filtered. For that purpose, we are using a blocking diode D3 between pulsating DC and the filter capacitor so that we can get pulsating DC for use.

The pulsating DC is given to potential divider of 6.8k and 6.8K to deliver an output about 5V pulsating from 12V pulsating which is connected to the non-inverting input of comparator pin 3. Here, the Op-amp is used as a comparator.

The 5V DC is given to a potential divider of 47k and 10K which gives an output of about 1.06V and that is connected to inverting input pin no 2. One resistance of 1K is used from output pin 1 to the input pin 2for feedback.

As we know the principle of a comparator is that when the non-inverting terminal is greater than the inverting terminal, then the output is logic high (supply voltage). Thus the pulsating DC on pin no 3 is compared with the fixed DC 1.06V at pin no 2.

The o/p of this comparator is fed to the inverting terminal of another comparator. The non-inverting terminal of this comparator pin no 5 is given a fixed reference voltage, i.e., 2.5V taken from a voltage divider formed by resistors of 10k and 10k.

Thus we get ZVR (Zero Voltage Reference) detected. This ZVR is then used as input pulses to the Microcontroller.

ZVS Waveform
ZVS Waveform

Working Procedure of Cycloconverter

The circuit connections are shown in the above diagram. The project uses zero voltage reference as described above at pin no. 13 of the Microcontroller. Eight Opto – Isolators MOC3021 are used for driving 8 SCR’s U2 to U9.

4 SCR’s (silicon controlled rectifiers) used in full bridge is in antiparallel with another set of 4 SCR’s as shown in the diagram. Triggering pulses so generated by the MC as per the program written provides input condition to the Opto – isolator that drives the respective SCR.

Only one Opto U17 driving the SCR U2 is shown above while all others are similar as per the circuit diagram. SCR gets conducting for 20ms from the 1st bridge and next 20ms from the 2nd bridge to get the output at a point no – 25 & 26, the total time period of one AC cycle of 40ms which is 25 Hz.

Thus F/2 is delivered to the load while switch 1 is closed. Similarly, for F/3 the conduction takes place for 30ms in the 1st bridge and next 30ms from the next bridge, such that a total time period of 1 cycle comes to 60ms which in turn in F/3 while switch -2 is operated.

The Fundamental frequency of 50Hz is available by triggering on a pair from the 1st bridge for 1st 10ms and for the next 10ms from the next bridge while both the switches are kept in “OFF” condition. The reverse current flowing in the gates of the SCR’s are Opto – isolator output.

Applications of Cycloconverter

Applications include Controlling the speed of AC machines like It is mainly used in electric traction, AC motors having variable speed and induction heating.

  • Synchronous Motors
  • Mill Drives
  • Ship propulsions
  • Grinding Mills

I hope you have clearly understood the topic of Cycloconverter, it is a frequency converter from one level to another, that can change AC power from one frequency to AC power at another frequency. If any furthermore queries on this topic or on the electrical and electronic projects leave the comments section below.

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