UNIT III
AC voltage controller and
cycloconverter
4.1.1 Single-phase AC voltage controller
VT 1
VT2
u1
u1
io
O
uo
t
R uo
O
t
io
The phase shift range
(operation range of phase
delay angle):
O
u VT
t
0 ≤α≤ π
O
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t
2
• Resistive load, quantitative analysis
RMS value of output voltage
Uo 
 2U sint dt U

1

2
1

1
1
 
sin2 
2

(4-1)
RMS value of output current
Uo
R
RMS value of thyristor current
Io 
(4-2)
2
U1 1
1  2U 1 sin  t 
 sin 2


IT 
d

t

(
1


)

2  
R
R
2

2



Power factor of the circuit

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P UoIo Uo



S U1 I o U1
1
 
sin 2 
2

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(4-3)
(4-4)
3
Inductive (Inductor- resistor) load , operation principle
u1
VT 1
O
io
uG1
VT2
R
u1
t
uo
L
O
uG2
O
uo
t
O
io
t
O
t
t
The phase shift range:
φ ≤α ≤ π
uVT
O
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t
4
4.2 Other AC controllers
4.2.1 Integral cycle control—AC power controller
VT 1
VT2
u1
io
Conduction
uo angle =
2 U1
uo
R O 
M
2
M
2N
M
u1
uo,io
3
M
4
M
t
Line period
Control period = M *Line period
= 2
Circuit topologies are the same as AC voltage controllers.
Only the control method is different.
Load voltage and current are both sinusoidal when thyristors are conducting.
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4.3 Thyristor cycloconverters
4.3.1 Single- phase thyristor-cycloconverter
• Circuit configuration and operation principle
P
N
Z
uo
a p=
π
2
Output
voltage
Average
ap=0
output voltage

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a p=
π
2
t
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• Single- phase thyristor-cycloconverter
Modes of operation
uo
u o,io
O t1
iP
uP
uo
t2
t4
t3
t5
t
O
iN
uN
t
uo
uP
io
io
uN
t
uo
O
iP
P
N
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O
iN
t
O
t
Rectifi
Inver
cation
sion
blocking
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blocking
Rectifi
Inver
cation
sion
7
Typical waveforms
uo
t
O
io
O
t
1
3
2
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4
6
5
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• Modulation methods for firing delay angle
Calculation method
– For the rectifier circuit
u o  U d0 cos 
(4-15)
u2 u3 u4 u5 u6
u1
ωt
–For the cycloconverter
ap3 ap4
output
uo  U om sin ot
(4-16)
–Equating (4- 15) and (4-16)
U om
cos  
sin  o t   sin  o t
U d0
–therefore
(4-17)
  cos 1 ( sin  o t )
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(4-18)
us2 us3 us4 us5 us6 us1
uo
ωt
Principle of cosine
wave-crossing method
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Output voltage ratio
(Modulation factor)

150
Uom
120
(0    1)
Ud 0
/( º )
90
γ
60
30
0
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





γ






 3 2  t
2
2
Output voltage phase angle
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• The configuration with common input line
4.3.2 Three- phase thyristor-cyclo converter
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• The configuration with star-connected output
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Typical waveforms
Output voltage
200 t/ms
Input current with
Single-phase output
200 t/ms
Input current with
3-phase output
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200 t/ms
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• Input and output characteristics
The maximum output frequency and the harmonics in the output voltage
are the same as in single-phase circuit. Input power factor is a little
higher than single-phase circuit. Harmonics in the input current is a
little lower thanthe single- phase circuit due to the cancellation of some
harmonics among the 3 phases.
To improve the input power factor:
–Use DC bias or 3k order component bias on each of the 3 output
phase voltages
• Features and applications
Features:
–Direct frequency conversion—high efficiency
–Bidirectional energy flow, easy to realize 4- quadrant operation
–Very complicated—too many power semiconductor devices
–Low output frequency
–Low input power factor and bad input current waveform
Applications:
–High power low speed AC motor drive
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4.4 Matrix converter
• Circuit configuration
input
b
a
c
u
S1
S1
S1
1
2
3
S2
S2
S2
1
2
3
S ij
v
output
w
S3
S3
S3
1
2
3
a)
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b)
15
• Usable input voltage
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• Features
Direct frequency conversion—high efficiency can realize good input and
output waveforms, low harmonics, and nearly unity displacement
factor
Bidirectional energy flow, easy to realize 4- quadrant operation
Output frequency is not limited by input frequency
No need for bulk capacitor (as compared to indirect frequency converter)
Very complicated—too many power semiconductor devices
Output voltage magnitude is a little lower as compared to indirect
frequency converter.
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