Thyristors control

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. 

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

Converts a.c to d.c. and can be ower diode fixed voltage for a.c. drives (phase controlled thyristor converter for d.c. drives)... 

Fully controlled thyristor inverter in anti-parallel to feedback energy... 

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. 

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. 

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. 

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

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]

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. 

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)... 

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... 

Controls are available in the range IGBTs 1600 V, 2000 A and thyristors 10 kV, 3000 A (ratings are only indicative) and can cover the entire voltage ranges and ratings of a motor. 

A fast VAr control is achieved through thyristor switching, which by itself is capable of a stepless variation. But switching of capacitors, which are switched in banks, is not steples.s. The SVCs may be of the following types. 

---