Thyristor

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. 

Thyristors have been replacing saturable reactors they are small, efficient, and easily controlled by a wide variety of control systems. A modern crane control drive uses fixed secondary resistors and two sets of primaiy thyristors (one set for hoist, one for lower). With tachometer feedback for speed sensing, the control for the motor provides speed regulation and torque hmiting in both directions, all with static-devices. A wide variety of control systems is possible the control should be designed for the specific application. 

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,... 

Gate control in case of thyristor inverters only. 

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 latest in the field of static devices are MOS-controlled thyristors (MCTs), which are a hybrid of MOSFETs and thyristors. There is yet another device developed in this field, i.e. insulated gate-controlled thyristors (IGCTs). Implementation of these devices in the field of static drives is in the offing. 

The thyristor is a semiconductor device made of germanium or silicon wafers and comprises three or more Junctions, which can be switched from the OFF state to the ON state or vice versa. Basically it is a ptipn junction, as shown in Figure 6.20(a) and can be considered as composed of two transistors with npn and pnpjunctions, as illustrated in Figure 6.20(b). It does not turn ON when it is forward biased, unlike a diode, unless there is a gate firing pulse. Thyristors are forced commutated (a technique... 

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... 

MOS-controllcd thyristors (MCTs) and insulated gate-controlled thyristors (IGCTs) (discussed in Section 6.7.2). 

The use of SCRs in an inverter circuit is intricate because of the absence of a natural commutation. Now only a forced commutation is possible, as it is connected to a d.c. source which provides no current zeros and hence facilitates no natural commutation. A forced commutation calls for a separate switching circuit, which is cumbersome, besides adding to the cost. As a result of this feature, they are also called forced commutated thyristors. 

Unlike an SCR, which is unidirectional, a triac is a bidirectional thyristor switch and conducts in both directions. It can be considered as composed of two SCRs, connected back to back with a single gate, as shown in Figure 6.22(a). Since the thyristor now conducts in both directions there is no positive (anode) or negative (cathode) terminals. 

The triac may, however, have some limitations in handling frequencies higher than normal. In such cases, they can be simulated by using two SCRs in inverse parallel combinations as illustrated in Figure 6.22(b). Now it is known as a reverse conducting thyristor. An SCR has no frequency limitations at least up to ten times the normal. The required voltage and current ratings are obtained by series-parallel connections of more than one thyristor unit. 

Figure 6.22(b) Use of two SCRs in inverse parallel combination to simulate a triac (reverse-conducting thyristors)... 

Half wave similar to Figure 6.24(a) configunitions (b) and (c), using one diode per phase instead of a thyristor, or... 

Full wave (Figure 6.24(a)), configuration (a), using two thyristors in anti-parallel per phase or in the form of a centre-point configuration. 

Table 6.3 A brief comparison between a transistor and basic thyristor technology...

Once fired, a thyristor cannot be controlled. Ft requires a forced commutation to switch it off and (he gate control is quite cumbersome. To swdtch OFT-, (he conducting current Is reduced to less than its holding current. The commutation circuitry is ihcrctbre highly complex an[Pg.118]

There arc six thyristor firing circuits required for a 3 system, as there are (w o thyristors connecicd back to back each phase. The w holc scheme is therefore comple.x and less reliable. 

Power MOSFF.Ts and IGBTs can handle much higher switching frequencies, compared o a thyristor. In an a.c. motor control, fast switching is mandatory and therefore transistors are preferred. 

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