Thyristors rectifier converters

A modification of this basic drive system uses solid-state rectifiers and thyristors to convert the wound-rotor, variable-frequency slip power first to direct current and then to hne-frequency power (60 Hz in the United States). This in turn is fed back to the power system as useful energy. 

Diode bridge rectifier (converter) Inverter unit IGBT or thyristor, depending upon the size of machine,... 

In cathodic protection stations 220 V ac rectifiers are used with silicon diodes or steered thyristor rectifiers, the output parameters of which are automatically regulated. As a result of the development of electronics, field power transistors have been introduced and used for the construction of cathodic protection stations in the form of superacoustic transistor converters. Such stations have over approx. 30% energetic efficiency, much smaller dimensions and mass, they work quietly, and are easily regulated automatically over the range of out-... 

These are unidirectional and uncontrollablet static electronic devices and used as static switches and shown in Figure 6.14. A diode turns ON at the instant it becomes forward biased and OFF when it becomes reverse biased. By connecting them in series parallel combinations, they can be made suitable for any desired voltage and current ratings. Whether it is a transistor scheme or a thyristor scheme, they are used extensively where a forward conduction alone is necessary and the scheme calls for only a simple switching, without any control over the switching operation. They are used extensively in a rectifier circuit to convert a fixed a.c. supply to a fixed d.c. supply. 

The exciter is an AC generator with a stator-mounted field. Direct cur rent for the exciter field is provided from an external source, typically u small variable voltage rectifier mounted at the motor starter. Exciter oui put is converted to DC through a three-phase, full-wave, silicon-diode bridge rectifier. Thyristors (silicon-controlled rectifiers) switch the cur rent to the motor field and the motor-starting, field-discharge resistors These semiconductor elements are mounted on heat sinks and assembled on a drum bolted to the rotor or shaft. 

DC methods mostly use shunt or compound wound motors. Occasionally series wound motors are used when high torque at low speeds is required. These machines are fed with DC voltage derived from a three-phase AC source using a thyristor converter. The thyristor converter rectifies the AC into DC but with control over the magnitude of the average DC voltage. Thyristors are also called silicon controlled rectifiers . 

In this group there are several systems that use power electronics to provide a variable magnitude voltage at a variable frequency. Most of the systems use rectifiers and thyristors in the form of converters and inverters. 

Since 1960 the direct current for electrolysis has been provided exclusively by silicon rectifiers. A set of rectifiers can supply up to 450 000 A. Voltages up to 4.0 kV per diode are feasible, but usually for safety, a peak a.c. voltage of 1500 V, corresponding to a d.c. output of 1200 V, is not exceeded. Liquid cooling of the diodes permits a compact design, and self-contained equipment reduces leakage losses. Modem electrolysis cell plants also use continuously variable thyristor converters in place of silicon diodes [64]. 

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