Thyristors

General Introduction to Thyristors and its Applications

Thyristor is the general name given to a family of semiconductor devices having four layers with a control mechanism, although this term is most commonly applied to the SCR (silicon-controlled rectifier). This term is derived from thyratron and transistor because the device combines the rectification action of thyratron and control action of transistor. Thyristor stands for thyratron transistor.

The device was first developed at the Bell Laboratories in the USA and was commer­cially introduced by the General Electrical Company in the USA in 1957. The device appeared in the market under different names such as SCR, thyristor, thyrode etc. Since its inception the thyristor has come to stay as a basic building block in many industrial and power system applications. Its ability to be controlled, compactness, fast response, high reliability, better efficiency, large power handling capacity, high voltage and current ratings, good trigger sensitivity, static operation, large power gain, sturdy construction, long life, very little maintenance and low cost of fabrication—due to advancement in the field of fabrication—have given the thyristor a colourful reception in every field. Today thyristors are finding applications in the control of dc/ac motors; for the improvement of power factor; and as switching devices. They have become an integral part of HVDC transmission. They are so advantageous that our major steel plants had decided to mod­ernise their plants by replacing MG sets and mercury-arc rectifiers by thyristor converters. Thyristors have helped in further cost reduction and in the development of drive system by changing the emphasis from dc motors to ac motors. With cycle converters and invert­ers, the speed of an ac motor can also be controlled with ease and reliability. Apart from these main applications it finds use as a switching device, particularly in the improvement of power factor of transmission lines and mains. Thyristor can be used as a power switch­ing device with a power handling capacity ranging from a few watts to as high as 4 MW (2,500 A at 1,600 V). Some thyristors have a rating as high as 400 A, 10,000 V for use in hv ac transmission lines.

Thyristors, with its large number of advantages and tremendous control capabilities, have numerous applications and have completely replaced the electro-magnetic control systems. Thyristor basically serves two functions viz electronic switching and electronic control. Some of the applications of thyristors are listed below.

Applications of Thyristors

  • Speed control of dc and ac motors.
  1. As rectifier for conversion of ac into dc.
  2. As inverter for conversion of dc into ac
  3. As dc chopper or dc to dc converter for converting dc at one level to dc at another level.
  4. As cyclo-converter for converting ac of one frequency into ac of another frequency.
  5. Control of temperature, level, position and illumination.
  6. Power switches (dc and ac circuit breakers).
  7. HVDC transmission lines.
  8. Improvement of power factor in transmission lines.

More applications:

  1. As static switches.
  2. Control of induction heating.
  3. Relay control.
  4. Phase control.
  5. As special power supplies for aircrafts and computers etc.

SCRs and triacs having high voltage and current ratings are widely employed for power control applications whereas other members of thyristor family are employed for small power applications   and for switching in control and digital circuits.

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