Thyristor Rectifier

A power diode reetifier unit feeding a fixed d.e. power to an inverter unit to control an a.c. motor, or a thyristor rectifier unit, directly controlling a d.e. motor, both contain... 

Cuncnt imiisieiits A similar situation will arise wheti a switching ON operation of the rectifier unit occurs hen it is a thyristor rectifier. Under load conditions, the stored magnetic energy in the incoming supply system, which can be the feeding transformer and the line reactances similar to a fault condition discussed earlier, may cause a current transient which can be expressed by... 

Solid-State DC Drives. The controlled-thyristor rectifier and separate-field DC motor is the solid-state motor drive in greatest use. The combination provides control over at least a 10 1 speed range, plus an additional two to three times by field weakening. Depending upon the power level, the rectifier is operated directly from the AC supply lines, or via a transformer. Typical speed regulation of 2% can be accomplished with a single control system. The horsepower and speed limitations are set by the DC motor, not by the semiconductor rectifiers. The DC motor and rectifier can be combined to any required power level. 

Thyristor rectifier for variable voltage but constant frequency. 

Thyristor rectifier-inverter for variable voltage and variable frequency. 

A typical basic circuit is shown in Figure 14.3 which consists of two main parts, a three-phase bridge-connected thyristor rectifier and a three-phase bridge-connected thyristor inverter. 

The cellhouse is composed of 3 lines with a total 324 cells and 3 rectifier circuits. Figure 6 shows the outline of the facilities. The No. 1 line was 18 cells/row x 8 rows for a total of 144 cells a silicon rectifier with 32.5 kA 550V was used. The No. 2 line had the same facilities as the No. 1 line. The No. 3 line was composed of 18 cells/row x 2 rows for a total of 36 cells, and a thyristor rectifier of 140V 36 kA was used. Of these cells, 2 rows, 36 cells, were normally short circuited for cell maintenance. 

A thyristor rectifier uses four silicon semiconductors of alternating types in a pack. Figure 8.7 shows that without some outside influence the three junctions can not simultaneously be conductive. When the cathode side is negative, two junctions are conductive, but the center, or control, junction is not. Applying a current pulse to the interior P-layer neutralizes the bias in that layer and allows current to flow through the rectifier pack. 

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-... 

Static excitation system. This system consists of a static thyristor rectifier unit supplied from an external power source. The excitation current is connected to the rotor windings via conventional shprings. The static thyristor rectifier unit can be supplied on request. 

Current density can be increased without impairing the quaUty of the copper by polishing the cathode surface by brief periodic current reversals (PCR). Reversed current electrolysis, first developed for electroplating, was tested in 1952 for copper refining. Although good results were obtained, no suitable electrical equipment for current reversal was available. The thyristor-controUed siUcon rectifier, introduced in the 1960s, provided a means for... 

In addition to secondarv resistance control, other devices such as reactors and thyristors (solid-state controllable rectifiers) are used to control wound-rotor motors. Fixed secondary reactors combined with resistors can provide veiy constant accelerating torque with a minimum number of accelerating steps. The change in slip frequency with speed continually changes the effective reac tance and hence the value of resistance associated with the reactor. The secondaiy reactors, resistors, and contacts can be varied in design to provide the proper accelerating speed-torque curve for the protection of belt conveyors and similar loads. 

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,... 

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. 

Uncontrolled line side diode bridge rectifier. When a variable d.c. is required, it can be replaced by thyristors. Mechanical braking or non-regenerative braking-. 

Silicon-controlled rectifiers (SCR). These arc basictilly thyristors and unless specified, a thyristor will mean an SCR Triacs... 

NoU It is possible that at some loealioiis there is no a.e. source available, such as (or battery-operated lifts iirul motor vehicles,. Such applications may also call for a variable d.e. source. When it is so. it can be achieved with the use of a chopper circuit which uses the conventional semiconductor devices. The devices are switched at high repetitive frequencies to obtain the required variation in the output voltage as with the use of a phase-controlled lliyristor rectifier, A typical chopper circuit is shown in Ingure 6.2, i. using diodes and a controlled unidirectional semieonduetor switch, which can be a thyristor or tin IGBT. 

Figure 6.24(a) A few configurations of controlled rectifier units (for uncontrolled rectifier units the thyristors (SCRs) are replaced with diodes)... 

Loads of rectifier, thyristor, UPS (uninterrupted power supply) and battery chargers etc. 

Potential control rectifiers can also be constructed using thyristors. However, these produce strong high-frequency harmonic waves that can be transmitted to... 

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. 

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