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bring more clarity to the subject, passing references to a V and f of the fixed a.c. input supply or I, and I, (phasor
d.c. machine are also provided. To make the discussion control) in the machine’s parameters and use these to
of static drives more complete different configurations perform a required duty cycle with very precise speed
of converter units are also discussed for the control of control.
d.c. motors.
In the past five decades solid-state devices such as 6.7.1 Power diodes
diodes, transistors and thyristors have attained a remark-
able status and application in the field of electronic power These are unidirectional* and uncontrollable? static
engineering. Diodes and thyristors were introduced in electronic devices and used as static switches and shown
the late 1950s, while the basic transistor (BJT -bipolar in Figure 6.14. A diode turns ON at the instant it becomes
junction triode) was introduced in 1948. In India they forward biased and OFF when it becomes reverse biased.
appeared much later (thyristors were introduced in the By connecting them in series parallel combinations, they
1970s and power transistors in the 1980s). This technology can be made suitable for any desired voltage and current
is now extensively applied to convert a fixed a.c. power ratings. Whether it is a transistor scheme or a thyristor
supply system to a variable ax. supply system, which in scheme, they are used extensively where a forward
turn is utilized to perform a required variable duty of a conduction alone is necessary and the scheme calls for
fixed power system or a machine. They are all semi- only a simple switching, without any control over the
conductor switching devices and constitute two basic switching operation. They areusedextensively inarectifier
families, one of transistors and the other of thyristors. circuit to convert a fixed a.c. supply to a fixed d.c. supply.
The more prevalent so far is discussed here to give readers
an idea of the use of this technology in today’s domestic
and industrial applications, power generation, distribution A (anode)
and their controls etc.
More emphasis is provided on the control of induction
motors. Research and development in this field is a il K(cathode)
continuous process and is being carried out by agencies
and leading manufacturers. This aims to advance and Figure 6.14 Circuit symbol for a power diode as a switch
optimize the utility of such devices by improving their
current-handling capability and making them suitable
for higher system voltages, switching speeds etc. There 6.7.2 The power transistor family
may be more advanced versions available by the time
this book is published and readers should contact the The solid-state technology in the field of transistors in
leading manufacturers for details of the latest technology. particular has undergone a sea-change, beginning in the
Diodes are purely static power switching devices and 1950s from the basic bipolar junction transistor (BJT) to
are used extensively with thyristor and transistor power the more advanced insulated gate bipolar transistor (IGBT)
schemes. Transistors are relatively cheaper and easy to by the 1990s. The following are some of the more
handle compared to thyristors. The latter are more prominent of the power transistor family that are com-
expensive and more complex as noted below. This text monly used in power circuits.
deals with the application of such devices for the control
of induction motors, which can now be employed to Bipolar junction transistors (BJTs)
perform variable duties through its stepless speed control
by close monitoring of load requirements during a These are the basic transistors (triodes) and are illustrated
particular process or while performing a specific duty in Figure 6.15. They are unidirectional and controllable
cycle. The controls are assisted by microprocessor-based, and are capable of handling large currents and high
open- or closed-loop control techniques, which can sense voltages and also possess high switching speeds (faster
and monitor many variables such as speed, flow of than thyristors). However, they require a high base current
material, temperature, pressure or parameters important due to the high voltage drop across the device, which
€or a process or a duty cycle. With these techniques, it is causes a high loss and dissipation of heat. This adverse
possible to achieve any level of automation. Open-loop feature of their characteristics renders them unsuitable
systems are used where high accuracy of controls and as power switching devices for efficient power conversion.
feedback is not so important and closed-loop where a Therefore they are generally used as electronic control
high degree of accuracy of control is essential. With devices rather than power devices in electronic control
solid-state technology it is now possible to utilize a circuits and are not produced at higher ratings.
conventional machine to perform a variable duty.
Transistors so far have been developed to handle currents Two-junction transistors or power Darlingtons
up to 2000 A and voltages up to 1200 V and are utilized
for low-capacity power requirements. Thyristors have These also have three terminals as illustrated in Figure
been developed up to 3000 A and voltages up to 10 kV
and are employed for large power requirements such as *A unidirectional switch is one that can conduct in only one direction
HV d.c. transmission and static VAr controls. With the and blocks in the reverse direction.
variety of such devices and their number of combinations, tA controllable switch is one that can be turned ON and OFF by
it is possible to achieve any required output variation in switching a control circuit ON and OFF.