16-4-1-1
The Effects of the
s
L
and the
d
C
on the Waveforms and the
THD
As Fig. 16-9 shows, power is drawn from the utility supply by means of a pulse
of current every half-cycle.
The larger the “base” of this pulse during which the
current flows, the lower its peak value and the lower the total harmonic distortion.
This pulse-widening can be accomplished by increasing the ac-side inductance
s
L
, as shown in Fig. 16-10a, by carrying out a parametric analysis using a
t
3
t
2
t
1
v
d
(
)
dr
s
i
i
=
i
dr
v
s
ω
t
0
i
s
i
s
Figure 16-9 Current and voltage waveforms for the full bridge diode rectifier.
1
1
/ 2
t
T
+

16-15
computer program such as PSpice™ (see Reference [2]).
For the same power
transfer, waveforms are shown for five values of
s
L
.
Increasing
s
L
decreases the
THD
; however, it also decreases the average dc-output voltage, as shown in Fig.
16-10b.
Another parameter under the designer’s control is the value of the dc-bus
capacitor
d
C
.
At its minimum, it should be able to carry the ripple current (in
dr
i
and in the current drawn by the switch-mode converters discussed in Chapter 4)
and keep the peak-to-peak ripple in the dc-bus voltage to some acceptable value,
for example less than 5 percent of the dc-bus average value.
Assuming that these
constraints are met, the effect of
d
C
is shown by means of parametric analysis in
Figs. 16-11a and 16-11b, which show that the lower the value of
d
C
, the lower
the
THD
and the higher the ripple in the dc-bus voltage, respectively.
In practice, it is almost impossible to meet the harmonic limits specified by the
IEEE-519 by using the above techniques.
Rather, the remedial techniques that
will be described in section 16-5 are needed to meet the harmonic specifications.
Ti me
300ms
310ms
320ms
330ms
I ( Rs )
- 40A
0A
40A
decreasing
d
C
Figure 16-11 Variation in
C
d
(a) input current; (b) output voltage.
(a)
(b)
Ti me
300ms
310ms
320ms
330ms
V( p, n)
100V
150V
200V
250V
decreasing
d
C
(a)
Figure 16-10 Effect of variation in
L
S
(a) input current distortion; (b) the output
voltage.
Ti me
70ms
80ms
90ms
100ms
I ( Rs )
- 20A
0A
20A
increasing
s
L
Ti me
85ms
90ms
95ms
100ms
v( p, n)
125V
150V
175V
200V
increasing
s
L
(b)

16-16
16-4-2 Three-Phase Diode-Rectifier Bridge
It is preferable to use a three-phase utility source, except at a fractional kilowatt,
if such a supply is available.
A commonly-used full-bridge rectifier circuit is
shown in Fig. 16-12a.
To understand the circuit operation, the rectifier circuit can be drawn as in Fig.
16-12b.
The circuit consists of a top group and a bottom group of diodes.
Initially, the effects of
s
L
and
d
C
can be ignored.
At least one diode from each
group must conduct for the input current to flow.
In the top group, all diodes
have their cathodes connected together.
Therefore, the diode connected to the
most positive voltage will conduct; the other two will be reverse biased.
In the
bottom group, all diodes have their anodes connected together.
Therefore, the
diode connected to the most negative voltage will conduct; the other two will be
reverse biased.

#### You've reached the end of your free preview.

Want to read all 464 pages?

- Spring '13
- Ha Tan
- Wind, Electric motor, Variable-frequency drive, Electric Drives