Page 139 - Industrial Power Engineering and Applications Handbook

Static controls and braking of motors 6/119
The uncontrolled rectifier units are simple rectifiers additional feedback inverter unit would alw he es\ential.
and use power diodes universally. The rectification as shown in Figures 6.31-6.33. depending upon the
obtained is uncontrollable and provides only a fixed d.c. configuration of the converter unit.
voltage output from a fixed a.e. supply. Thc diodes have
no control over their switching instants. These rectifier Now It is possible that at some locations there is no ax. source
units are used extensively to provide a fixed d.c. source available. such as for battery-operuted lifts and motor vehicles.
to an inverter unit when being employed to control an Such applications may also call for a variable d.c. source. When it
a.c. machine. Since there is no switching of diodes is so. it can be achieved with the use of a chopper circuit which
involved, there arc no voltage surges across the diodes. uses the conventional semiconductor devices. The devices are
There is thus no need to provide a snubber circuit across switched at high repetitive frequencies to obtain the required variation
in the output voltage as with the use of a phase-controlled thyristor
the diodes to protect them asainst such surges, as discussed rectifier. A typical chopper circuit is shown in Figure 6.15. usinp
later. diodes and a controlled unidirectional semiconductor switch. which
When, however. a variable d.c. voltage output is can be a thyristor or an ICIRT.
required, controlled rectifiers are used. Now the diodes
are replaced by one or two phase-controlled thyristors Relevance of different quadrants of a
(SCRs) in each phase, one thyristor per phase for a half converter unit
wave and two per phase in anti-parallel for a full wave
rectification. The required voltage variation is achieved Depending upon the mode of operation of a motor, the
by adjusting the delay time of the gate-firing pulse (a) to type of converter unit can be decided. For simplicity, the
each thyristor unit. Thyristor circuits are possible up to operation (conduction) of a motor can be reprewnted by
any rating by arranging them in series-parallel four quadrants as illustrated in T;lble 6.4.
combinations. With the advances in this technology, it is
possible to achieve a total matching of switching sequences Quadrant I Both V and Tare positive. The machine
of all thc thyristors to ensure an accurate and fully cohesive can be run only in one direction (say. forward). Brak-
operation (switching of all thyristors at the same instant). ing operations are possible. It is a converter mode
Unlike diode, a thyristor does not turn ON automatically and either a half-wave or a full-wave rectifier can be
at the instant it becomes forward biased but requires a used.
gate pulsc at its gate terminal to switch it OK. This is Quadrant I1 Now T is in the reverse direction and
obtained through a control circuit which is a part of the the machine can be run in the reverse direction. Braking
rectifier unit. The gate-firing pulse is provided at the and regeneration are possible. For regeneration an
appropriate instant to each thyristor in each switching additional bridge will be essential as discussed later.
cycle. positivc half to negative half, to obtain the required For current to flow in either direction. a full-wave
voltage. The switching is automatic as the thyristors are rectifier will also he essential.
line commutated and at each half cycle the voltage Quadrant 111 Now both the voltage and the torque
waveform passes through its natural zero. The instants are in the reverse direction otherwise it is similar to
may bc delayed from 0" to 180" electrical to obtain the Quadrant I. The machine can now be run in the reverse
required infinite control in the output supply, as illustrated direction.
in Figure 6.23(b). The d.c. output is controlled by adjusting Quadrant IV Now the voltage alone is in the reverse
the delay time of the gate-firing pulse. A few voltage direction and the machine can be run in the forward
waveforms are illustrated in Figure 6.2Xb) at different direction. Braking and regeneration are possible. The
firing angles. Phase control of positive and negative half machine in the forward direction can generate, when
waves of each phase can be infinitely varied to meet any it is brought down from a higher speed to a lower
power demand. speed or when feeding a load going down hill etc. For
A half-wave rectifier is able to provide only a uni- regenera-tion. an additional bridge will be essential as
directional d.c. power source which may also contain noted above, and for the current to flow in either
many a.c. ripples (Figure 6.24(a)). A full-wave rectifier direction, a full-wave rectifier.
is employed to reduce such ripples, on the one hand, and
provide a d.c. power in forward as well as reverse The changeover from motoring to regeneration, 1.e.
directions, on the other. A fixed forward and reverse d.c. from Quadrant I to Quadrant 1V or from Quadrant I11 to
power is required for an inverter unit when it is employed Quadrant I1 or vice versa. is achieved by first bringing
to control an a.c. machine. Now an uncontrolled rectifier the torque to zero by ceasing the firing of one bridge and
unit is adequate as V and ,f control is obtained through commencing that of the other.
the inverter unit.
A controlled rectifier unit is necessary when it has to
control a dc. machine. which would call for a variable 6.9.2 Inverter unit
d.c. voltage. When the d.c. machine has to opcrate in
only one direction (quadrants I or 111) a half-wave The purpose of an inverter unit is to invert a fixed d.c.
controlled rectifier will be adequate and when the machine power to a variable ax. power which can be achieved in
has to operate in either direction. a full-wave controlled two ways:
rectifier will be essential.
For operations in quadrants I1 and IV it is essential to 1 Using IGBT devices These are the latest in the field
have an unrestricted flow of reverse power and hence an of static power control. They are easy to handle and