Page 186 - Rashid, Power Electronics Handbook

11 Single-Phase Controlled Recti®ers 175

T~i
d These aspects have a negative in¯uence on both power
factor and power quality. In the last several years, the massive
i
Rect I d Rect II use of single-phase power converters has increased the
Rect. I
ac v ac problems of power quality in electrical systems. In effect,
d w
0 r
w r modern commercial buildings have 50% and even up to
Rect. II 90% of the demand originated by nonlinear loads, which are
composed mainly of recti®ers [1]. Today it is not unusual to
®nd recti®ers with total harmonic distortion of the current
a) b) THD > 40% originating severe overloads in conductors and
i
FIGURE 11.13 Single-phase dual-converter drive: (a) connection; and transformers.
(b) four-quadrant operation. Figure 11.14 shows a single-phase recti®er with a capacitive
®lter, used in much of today's low power equipment. The
input current is highly distorted due to the presence of the
As shown in Fig. 11.13, a better performance can be
®lter capacitor. This current has a harmonic content as shown
obtained with two recti®ers in back-to-back connection at
in Fig. 11.15 and Table 11.1, with a THD ¼ 197%.
i
the dc terminals. This arrangement, known as a dual converter
The recti®er of Fig. 11.14 has a very low power factor of
one, allows four-quadrant operation of the drive. Recti®er I
PF ¼ 0.45, due mainly to its large harmonic content.
provides positive load current i , while recti®er II provides
d
negative load current. The motor can work in forward power-
ing, forward braking (regenerating), reverse powering, and 11.2.2 Standards for Harmonics in Single-Phase
reverse braking (regenerating). These operating modes are Rectifiers
shown in Fig. 11.13b, where the torque T vs the rotor speed
o is illustrated.
R The relevance of the problems originated by harmonics in
line-commutated single-phase recti®ers has motivated some
agencies to introduce restrictions to these converters. The IEC
11.2 Unity Power Factor Single-Phase 1000-3-2 International Standard establishes limits to all low-
Rectifiers power single-phase equipment having an input current with a
‘‘special wave shape'' and an active input power P 600 W.
11.2.1 The Problem of Power Factor in Single- Class D equipment has an input current with a special wave
Phase Line-Commutated Rectifiers shape contained within the envelope given in Fig. 11.14b. This
class of equipment must satisfy certain harmonic limits,
The main disadvantages of classical line-commutated recti®ers shown in Fig. 11.15. It is clear that a single-phase line-
are that
commutated recti®er with parameters as shown in Fig.
i) they generate both a lagging displacement factor with 11.14a is not able to comply with the standard IEC 1000-3-2
respect to the voltage of the utility, and Class D. The standard can be satis®ed only by adding huge
ii) an important amount of input current harmonics. passive ®lters, which increases the size, weight, and cost of the

I [A] V[V]
15 400
Class D
Envelope
10 300
v s 200
i
s L
5 100
i
+ s
v C 0,015 t [s]
s 0 0 0,005 0,01 0,02 0,025 0,03 0
-100
-5
-200
-10
-300
-15 -400
a) b)
FIGURE 11.14 Single-phase recti®er: (a) circuit; and (b) waveforms of the input voltage and current.