Cycloconverters – Types, Working and Applications

Power supplies can be classified into two broad categories: AC power supply and DC power supply. Since only AC power can be generated economically, most electrical machines run on AC power. However, the standard voltage and frequency from generating stations may not suit certain industrial machines. This is where power electronic converters like inverters and cycloconverters become essential for frequency and voltage conversion.

In this comprehensive guide, we'll explore cycloconverter types, working principles, circuit diagrams, applications, and advantages to help you understand this critical power electronics component.

What is Cycloconverter? - Detailed Explanation

A cycloconverter (also called cycloinverter) converts constant voltage, constant frequency AC waveform to another AC waveform of different frequency by synthesizing the output waveform from segments of the AC supply without an intermediate DC link.

Key Characteristics of Cycloconverters:

• Direct frequency conversion: No DC link stage required
• High efficiency: Typically 95-98% due to direct conversion
• Bidirectional power flow: Can operate in both motoring and regenerative modes
• Variable output frequency: From 0 Hz to 1/3 of input frequency
• Four-quadrant operation: Complete speed and torque control

The conversion process uses power electronic switches like thyristors (SCRs), IGBTs, or MOSFETs arranged in positive and negative converter groups. Each group conducts during alternate half-cycles, enabling bidirectional power flow and precise frequency control.

How Does Cycloconverter Work? - Step by Step Working Principle

Understanding cycloconverter working principle is crucial for power electronics engineers. The operation involves controlled switching of thyristors to synthesize desired output frequency from input AC supply. If you are completely new to thyristors check out our article on what a thyristor is and how it works. 

Cycloconverter Working Process:

1. Input AC Supply: Three-phase or single-phase AC at fixed frequency (50/60 Hz)
2. Positive Converter Group: Conducts during positive half-cycles of desired output
3. Negative Converter Group: Conducts during negative half-cycles of desired output
4. Control Circuit: Generates firing pulses based on desired output frequency
5. Output Synthesis: Combines segments to create variable frequency AC output

Firing Angle Control in Cycloconverters:

The output frequency and voltage are controlled by varying the firing angle (α) of thyristors. Key control parameters include:

• Firing Angle (α): Controls output voltage magnitude
• Switching Frequency: Determines output frequency (fo = fin × switching ratio)
• Pulse Width: Affects harmonic content and power quality

Mathematical Relationship:

Output Frequency: fo = (m/n) × fin
Where: m = number of output cycles, n = number of input cycles, fin = input frequency

Why do we need Cycloconverters?

Okay, now we know that Cycloconveters convert AC power of fixed frequency to AC Power of variable Frequency. But why do we need to do that? What is the advantage of having an AC power supply with variable Frequency?

The answer to this question is Speed Control. Cycloconveters are extensively used for driving large motors like the one used in Rolling mills, Ball mills Cement kils etc. The out frequency of a Cycloconverters can be reduced upto to zero which helps us to start very large motors with full load at minimum speed and then gradually increase the speed of the motor by increasing the output Frequency. Before the invention of Cycloconverters, these large motors has to be unloaded completely and then after starting the motor it has to be loaded gradually which results in time and man power consumption. If this interested you, you should also check out Variable Frequency Drives (VFDs), both VFDs and cycloconverters serve a similar purpose, that is, they use frequency control for motor speed. 

Types of Cycloconverters

Based on the output frequency and number of phase in the input AC power source the Cycloconverters can be classified as below

1. Step-Up Cycloconverters

2. Ste-Down Cycloconverters

1. Single-Phase to Single-Phase Cycloconverter
2. Three-Phase to Single-Phase Cycloconverter
3. Three-Phase to Three-Phase Cycloconverter

Step-Up Cycloconverters: Step-Up CCV, as the name suggests this type of CCV provide output frequency greater than that of input frequency. But it is not widely used since it not have much particle application. Most application will require a frequency less than 50Hz which is the default frequency here in India. Also Step-Up CCV will require forced commutation which increases the complexity of the circuit.

Step-Down Cycloconverters: Step-Down CCV, as you might have already guessed it well.. just provides an output frequency which is lesser then the input frequency. These are most commonly used and work with help of natural commutation hence comparatively easy to build and operate. The Step-Down CCV is further classified into three types as shown below we will look into each of these types in detail in this article.

Basic Principle behind Cycloconverters:

Although there are three different types of Cycloconverters, the working of them are very similar except for the number of power electronic switches present in the circuit. For instance a single phase to Single Phase CCV will have only 6 power electronic switches (SCR’s) while an Three Phase CCV might have upto 32 switches.

The bare minimum for a Cycloconverter is shown above. It will have a Switching circuit on either side of the Load, one circuit will function during the positive half cycle of the AC power source and the other circuit will function during the negative half cycle. Normally the switching circuit will be demonstrated using SCR as power electronic device, but in modern CCV you can find the SCR’s being replaced by IGBT’s and sometimes even MOSFETS.

The switching circuits will also need a control circuit, which instructs the Power electronic device when to conduct and when to turn off. This control circuit will normally be a Microcontroller and might also have a feedback from the output to form a closed loop system .The user can control the value of output frequency by adjusting the parameters in the control circuit.The diodes in the above diagram are used to represent the direction of flow of current. The positive switching circuit always sources current into the load and the negative switching circuit always sinks current from the load.

Single Phase to Single Phase Cycloconverters

The Single Phase to Single Phase CCV is very rarely used, but to understand the operation of a CCV it should be first studied so that we can understand the Three Phase CCV. The Single Phase to Single Phase CCV has two pairs of full wave rectifier circuit, each consisting of four SCR. One set is placed straight while the other is placed in anti-parallel direction as shown on the picture below. This type of configuration is called a H-Bridge circuit, you can read more on H-Bridge Motor Driver Circuit if you are interested. 

All the gate terminals of the SCR’s will be connected to a control circuit which is not shown in the circuit above. This control circuit will be responsible for triggering the SCRs. To understand the working of the circuit let us assume that he input AC supply is of 50Hz frequency and the Load to be a pure resistive load and the firing angle of the SCR (α) to be 0°. Since the firing angle is at 0° the SCR when turned on will act like a diode in forward direction and when turned off will act like a diode in reverse direction. Let us analyse the wave form below to understand how frequency is stepped down using a CCV

The waveform of the supply voltage frequency is denoted by Vs and the wave form of the output voltage frequency is denoted by Vo. Here we are trying to convert the supply voltage frequency to 1/4^th of its value. So to do that for the first two cycles of the supply voltage we will use the positive Bridge rectifier and for the following next two cycles we will use the negative bridge rectifier. Thus we have four positive pulses in the positive region and then four in the negative region as shown in the output frequency waveform Vo. The current waveform for this circuit will be the same as voltage waveform since the load is assumed to be purely resistive. Although the magnitude of the waveform will change based on the value of resistance of the load.

The output frequency is represented using the dotted line on the Vo waveform, since it changes polarity only for every two cycles of the input waveform the output frequency with 1/4^th of the input frequency, in our case for an input frequency of 50Hz the output frequency will be (1/4 * 50) around 12.5Hz. This output frequency can be controlled by varying the triggering mechanism in the control circuit. 

Three Phase to Single Phase Cycloconverters

The Three Phase to Single Phase CCV is also similar to the Single Phase to Single Phase CCV, but in here the input voltage is a 3 Phase supply and the output voltage is a Single Phase supply with variable frequency. The circuit also looks very similar except we will need 6 SCR in each set of Rectifier since we have to rectify the 3 Phase AC voltage. 

 

Again the gate terminals of the SCR will be connected to the control circuit for triggering them and the same assumptions are made again to understand the working easily. Also there are two kinds of Three Phase to Single Phase CCVs , the first type will have a half wave rectifier for both Positive and Negative Bridge and the second type will have a full-wave rectifier as shown above. The first type is not used often because of its poor efficiency. Also in a full-wave type both the bridge rectifiers can generate voltages in both the polarity, but the positive converter can supply current (source) only in the positive direction and the negative converter can drain current only in negative direction. This allows the CCV to operate in four Quadrants. These four quadrants are (+V, +i) and (-V, -i) in rectification mode and (+V, -i) and (-V,-i) in inversion mode.

Three Phase to Three Phase Cycloconverters

The Three Phase to Three Phase CCV are the most used ones since they can drive Three Phase loads like motors directly. The Load for a Three Phase CCV will normally be a Three Phase Star connected load like the stator winding of a Motor. This converters take in Three Phase AC voltage with fixed frequency as input and provides Three Phase AC voltage with Variable frequency.

There are two types of Three Phase CCV, the one which has half wave converter and other with full wave converter. The Half wave converter model is also called as 18-thyristor Cycloconverters or 3-pulse Cycloconverters. The full wave converter is called as 6-pulse Cycloconverters or 36-thyristor Cycloconverters. A 3-pulse Cycloconverter is shown in the picture below

Here we have six sets of Rectifiers of which two is allocated for each phase. The working of this CCV is similar to single phase CCV except here the rectifiers can rectify only half the wave and the same happens for all the three phases

Frequently Asked Questions (FAQs) about Cycloconverters