Thyristors rectifiers using

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. 

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. 

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. 

Another popular rectifier circuit is the full-controlled three-phase full wave rectifier. This circuit is more expensive because six thyristors are used. However, the form factor is much better, about 1.01, and the ripple current is 360 Hz. The higher frequency makes it easier to filter the ripple current. The half-controlled three-phase bridge rectifier circuit may require armature current smoothing reactors to reduce the ripple current. Another problem associated with the non-uniform DC input to the motor is the commutation. The motor must commutate under a relatively high degree of leakage reactance. 

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-... 

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. 

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. 

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. 

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

Ni or Au coated rounds (W). They serve as supports for silicon single-crystal plates used in transistors, diodes, thyristors, and rectifiers. 

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. 

The applied voltage must be reduced almost in proportion to the frequency so that the flux remains almost constant. The control of the flux is achieved by using a frequency sensing circuit to fire the rectifier thyristors. As the frequency is reduced the X-to-R ratio of the complete circuit is reduced and therefore the shape of the torqne-speed curve becomes less peaked. Figure 14.4 shows the... 

Thyristors used in rectifier and inverter bridges are usually of two types. The first type is a three-terminal semiconductor that can only be turned on by a control or firing signal applied to its... 

Modem chargers use fast acting and accurate electronic devices to control the desired output characteristics. The rectifying device can be diodes or thyristors. 

Chromium plating from chromic acid baths is more sensitive to the source of current than most other processes, sufficiently so for commercial operators to use at least three-phase rectifiers as a rule, and to take precautions against any temporary break of current during voltage regulation. A recent investigation showed that the ripple introduced by thyristor control of rectifiers was detrimental to chromium electrodeposits. 

In the case of rectifying elements, the total installed cost of a thyristor system is usually less than that of one based on diodes. Operating costs also are lower. Maintenance costs, as we saw in Section 8.3.1.2B, are lower because of the use of static, solid-state control devices rather than the mechanical switchgear of a tap changer. Rectification efficiency might be expected to suffer from the loss of part of the sine wave before firing, but this loss usually is comparable to the inefficiencies of the regulating transformers and the saturable reactors in a diode system. 

This circuit uses only three thyristors and four diodes. The drawback of this rectifier circuit is its high ripple current the typical form factor is 1.05, with the ripple current frequency 180 Hz. 

Chopper Drives. The high-speed switch (chopper) characteristics of phase-controlled rectifiers (thyristors) can be used to create adjustable-speed dc drives. A constant-voltage rectifier or a dc bus is used as the incoming supply. When the first thyristor is turned on by a reference-level signal, the dc motor is connected to the constant-voltage bus. When motor speed rises above the reference level, a second thyristor is turned on, shutting off the first, and the motor... 

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