Page 144 - Industrial Power Engineering and Applications Handbook

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Dedicated transformer

AC 47"
I
supply 4% 3

Rectifier or converter unit
Converts a.c to d.c. and can be a ower diode fixed voltage for a.c. drives (phase controlled thyristor converter for d.c. drives)
D.C.link circuit connectingat06
Inverter unit
Inverts d.c. to variable a.c. and can be an IGBT or a thyristor (GTO) circuit.
Variable Vwith an VSI (Figure 6.28(a)) or variable I with a CSI (Figure 6.29), and variable fax. power output
Inductor L
(i) It is necessary to smooth a.c. ripples, whether it is a power diode or a thyristor rectifier. It also suppresses harmonics when, @is a
controlled rectifier, producing a.c. rip les and harmonics
(ii) Can be a large size inductor, when 6 is a current source inverter (CSI) (Figure 6.29)
(iii) Provides a short-circuit protection for a fault in d.c. link, by adding to its impedance
Charging capacitor, to hold the charge, by smoothing the output ripples and providing a near constant voltage source to the inverter circuit,
when it is a voltage source inverter. When the converter is a thyristor converter, a resistance R is also provided with C to make it suitable to
perform its duty under frequent thyristor switchings, by quickly discharging it through R. Now it becomes a snubber circuit, to also protect
the inverter devices from duldt.
(a) Current limiting reactor on I/C side, to control dildtduring switching of thyristor units when @is a phase controlled rectifier. Not
necessary when the unit is supplied through a dedicated transformer.
(b) Also required to limit dildt to the solid-state circuits when the source of supply is large and is protected by current limiting device
(Section 6.14, Figure 6.35).
Inverter unit (conventional name). Converts fixed ax. to variable ax.
Figure 6.26(a) Basic IGBT or thyristor (GTO) inverter unit

of much higher frequency on the natural voltage waveform.
Figure 6.27(b) is a simple block diagram for a PWM
scheme, the natural voltage being the voltage obtained
by the switching of the IGBTs. The carrier wave can be
of any shape, the frequency of which is altered, to obtain
the required degree of modulation, and hence the voltage,
while the amplitude is kept fixed. The amplitude is a
matter of scheme design. (For more detail refer to the
textbooks in the Further reading.) Generally, a triangular
wave is used as shown in Figure 6.27(b) to obtain a
more uniform sinusoidal voltage waveform. By Fourier
analysis we can establish the amplitude of voltage and
quality of waveform (distortions), and by controlling the
pulse widths through the frequency of the carrier wave,
we can decide the best modulation to obtain the required
amplitude and a near-sinusoidal output voltage waveform.
(For details of Fourier analysis, refer to a textbook.)
This is the most commonly used technique in the inverter
circuit to obtain the required Vlf pattern. It is also
economical and can be used to control multi-motor drives
through a single unit. Since the variation is based on
voltage, the inverter may be called a voltage source inverter
(VSI). To obtain an accurate Vlfcontrol, it is essential
that the voltage is maintained uniform (without ripples)
as much as possible. This can be achieved by providing
a capacitor across the d.c. link as shown in Figure 6.26(a).
The purpose of the capacitor is to hold the charge and
smooth the output a.c. ripples of each diode and hence
provide a near-uniform d.c. voltage. The charge retained
by a capacitor can be expressed by
du
Figure 6.26(b) A small rating IGBT inverter unit Q=C-
(Courtesy: Kirloskar Electric) dt