Cycloconverters: Introduction

                       Cycloconverters are basically AC to AC power converters. They convert constant voltage AC power to adjustable voltage, adjustable frequency AC power without any intermediate DC link.

The cycloconverter is defined as a frequency changer that converts ac power at one input frequency to output power at a different frequency with a one-stage conversion process.
In large power industrial applications (such as cement and ball mill drives, Rolling mill drives) thyristor phase-controlled cycloconverters are widely used.
The cycloconverters are used for Variable-speed, constant frequency (VFCS) power generation for aircraft 400Hz power supplies.

Types of cycloconverters:
(a) They are classified depending upon the number of phases of the output voltage as
1. Single phase cycloconverters
2. Three phase cycloconverters

1. Single phase cycloconverters
Single phase output may be obtained either from a single phase source or from a three-phase source. If the supply source is single phase it is called single phase to single phase cycloconverters. If the supply source is three-phase it is called as three-phase to single phase cycloconverters.

2. Three phase cycloconverters
In this case both the supply source and the output voltage are three-phase.

(b) Depends upon how the firing angle of the converter is varied, the they are classified as
1. Envelope cycloconverters
2. Phase controlled cycloconverters

1. Envelope cycloconverters
In this type, the firing angle of the component converters is not varied. It is kept constant normally at α=0° for the positive converter and α=180° for the negative converter during the positive half cycle of the voltage. Similarly α=180° for positive converter and α=0° for the negative converter during the negative half cycle of the voltage.

2. Phase controlled cycloconverters
In this type, the firing angle of the converter component is varied to vary both magnitude and frequency of output voltage.

Applications of Cycloconverters:

  1. Speed control of high power AC drives is achieved using these converters.
  2. Used in high frequency induction heating.
  3. Used in static VAR generation.
  4. As they are utilized for converting variable frequency input voltage to constant frequency output voltage, they are in use as power supply in the aircraft and space vehicles.
  5. They play a vital role in High Voltage DC(HVDC) Transmission for interconnecting two power grids operating at two different frequencies.

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