Thyristor Power Converters: Function, Applications, Advantages & Disadvantages

Thyristors are semiconductor devices that tend to stay ‘ON’ once turned ON, and tend to stay ‘OFF’ once turned OFF. A momentary event is able to flip these devices into either ON or OFF states, where they will remain that way on their own, even after the cause of the state change is taken away. Therefore, these devices are useful as ON/OFF switching devices just like toggle switches and can’t be utilized as analog signal amplifies. On the other hand, BJTs and IGBTs are non-hysteric devices, i.e. they don’t inherently ‘latch’ into a state after being stimulated by a voltage or current signal. For any given input signal at any given time, a transistor will exhibit a predictable output response as defined by its characteristic curve.

Thyristor Power Converters

Let’s consider a block diagram of a thyristor power converter below:

Thyristor power converter converts the available power from the source into suitable form to run the load or the equipment. For instance, the load may be a DC motor drive which requires DC voltage for its operation. As often the case, the available power supply is AC power supply. In our case, the thyristor power converter employed is an AC to DC power converter which converts the input AC power into DC output voltage to feed to the DC motor. Typically a measuring unit or an instrumentation unit is employed to measure and monitor the output parameters such as output voltage, the load current, the speed of the motor or temperature, and so forth. The control unit is used to control the output of the thyristor power converter so that to adjust the output voltage/current to the desired level to achieve optimum performance of the load or equipment. The signal from the control unit is used to adjust the phase angle/trigger angle of the thyristors in the power converter so as to vary the output voltage to the desired level.

Applications of Thyristor Power Converters

Thyristor power converters are widely used in the industry. Some key applications of thyristor power controllers include:

• Power control in metallurgical and chemical processes using arc welding, induction heating, melting, resistance heating, electrolysis, arc melting, and so on.
• Control of AC and DC motor drives in rolling mills, paper and textile mills, excavators, traction vehicles, etc.
• Power control in metallurgical and chemical processes using arc welding, induction heating, melting, resistance heating, electrolysis, arc melting, etc.
• Uninterruptible and stand by power supplies for critical loads such as computers, special high tech power supplies for aircraft and space applications.
• High voltage supplies for electrostatic precipitators and X-ray.
• Power conversion at the terminals of a HVDC transmission.

Advantages of Thyristor Power Converters

Some of the advantages of using thyristor power controllers include:

• Faster dynamic response as compared to the electromechanical converters.
• Lower acoustic noise when compared to electro-magnetic controllers, relays and contactors.
• High efficiency due to low losses in the Thyristors.
• Easy and flexibility in operation due to digital controls.
• Long life and minimal maintenance due to the absence of mechanical wear.

Disadvantage of Thyristor Power Converters

Despite their prevalent use in many applications, thyristor power controllers have some drawbacks, and they include:

• The thyristor AC to DC power converters and AC to AC power converters can operate at low power factor under some circumstances. Special steps are taken for correcting the line supply power factor, by installing power factor improvement equipment.
• All thyristor power controllers generate harmonics, due to the switching ON and OFF, these unwanted frequency components adversely affect the performance of the load connected to them. Additional the generated harmonics are injected into the power supply lines and thus adversely affect the other loads/equipment connected to the supply lines.
• Special protection circuits must be employed in thyristor power controllers in order to protect and safe guard the thyristor devices, this adds to the cost of the equipment.
• The thyristor power converters have no short time over loading capacity and therefore they must be rated for maximum loading conditions. This increases the cost of the equipment.

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