POWER ELECTRONICS APPLICATIONS
POWER CONVERTERS : AC PHASE ANGLE CONTROL
AC-AC power conversion at fixed frequency is often referred to as ac regulation. Source voltage reversal
provides for natural / line commutation (turn-off ) of triacs, alternistors and inverse parallel (back-back)
pairs of scr's which are the most commonly used control devices.The control devices are referred to in
literature as
A.C. Switches
w
w
A.C. Contactors
w
A.C. Regulators
Control Methods
There are 3 basic methods of control
w
PHASE ANGLE CONTROL (P.A.C.)
w
TAP CHANGING CONTROL
w
INTEGRAL CYCLE Control
The output voltage waveforms characterise the three methods and some derivatives, as shown
SWITCH
PHASE ANGLE CONTROL
INTEGRAL CYCLE
PAC + INTEGRAL CYCLE
HALF CYCLE SEQUENCE
TAP CHANGING
TAP CHANGING + PAC
PHASE ANGLE CONTROL
EET307 POWER ELECTRONICS
AC-AC CONVERTERS
1
Prof R T KENNEDY
Topologies
A number of possible topologies with differing performance characteristics are available for phase angle
ac power control, as shown in table
TOPOLOGY
CONTROL RANGE
COMMENTS
Half to full power control as Apart from the limited control
only the positive half cycle range the large DC component
is controllable.
is a disadvantage.
LOAD
~
control
LOAD
control
~
control
Full wave control from zero Replacement of the inverse
to full power
parallel scr's by the triac
eliminates the need for gate
isolation.
LOAD
~
control
Full wave control from zero w The common cathode scr
to full power
arrangement avoids the need
for gate isolation
w scr's reverse voltage limited
to rectifier forward voltage.
but
w > cost
w < efficient
w < output voltage(rectifier VF )
LOAD
~
control
ac LOAD
~
Full wave control from zero The need for gate signal
to full power
isolation requires using pulse
transformers or opto- isolators,
and is a disadvantage.
Full wave control from zero w Flexible system:
to full power
ac & dc loads
w better scr utilization
but
w least efficient (2 VF )
w least output voltage (2 VF )
dc
load
c
o
n
t
r
o
l
EET307
POWER ELECTRONICS
AC-AC CONVERTERS
2
Prof R T KENNEDY
PHASE ANGLE CONTROL WAVEFORMS : RESISTIVE LOAD
VT1ak
iT1ak
iin(t)
in
iout
iT2ak
Vout
vin (t )
VT2ak
  90o
 
  90o
Vin o
 
IT1,ak
Vin
o


IT2,ak o
 
Iin
o
VT1,ak
o
VT2,ak
Vout
o
o
Current and voltage waveforms for the single phase ac controller
w
w
w
EET307
scr's only subjected to reverse voltage when both devices are off
scr's are subjected to a very rapid rate of rise of current,
rate of rise of scr voltage is low.
POWER ELECTRONICS
AC-AC CONVERTERS
3
diak
dt
dvak
  VP,in
dt
Prof R T KENNEDY
Harmonics
An indication of the quality of the ac output voltage is given by the total harmonic distortion
Individual harmonics are however useful if there is a need to include output filters
The input current harmonics, which are of significant importance in respect to the current EMC
legislation are identical to the voltage harmonics.
Symmetrical gate trigger delay  in each half cycle to avoid dc components results in only the odd
harmonics.
1.1
1.05
1
0.95
0.9
IRN
0.85
0.8
0.75
IFRN
0.7
0.65
0.6
0.55
0.5
0.45
0.4
0.35
IH3RN
0.3
0.25
0.2
IH5RN
0.15
IH7RN
0.1
FIGURE B2-5
TABLE B2-7
IH9RN
0.05
0
0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
160
170
180

EET307
POWER ELECTRONICS
AC-AC CONVERTERS
4
Prof R T KENNEDY
PHASE
IT1,ak
Vin ANGLE CONTROL WAVEFORMS : SERIES RESISTIVE -INDUCTIVE LOAD
VT1ak
iT1ak
iin(t)
in
iout
iT2ak
VR
VT2ak
vin (t )
Vout
VL
  60o
Vin o
IT1,ak o
IT2,ak

 
o
VR
  209.7 o

R 2  (L) 2


 L 
tan 1 
 R 
  149.7 o

o
Iin
  30o
L
Z

2


 
 
o
Vout
o
VL
o
VT1,ak o
VT2,ak
w
w
w
EET307
o
scr's only subjected to reverse voltage when both devices are off
diak
, than the resistive circuit
dt
dv
scr's are subjected to a very rapid rate of rise of voltage, ak
dt
scr's are subjected to a lower rate of rise of current,
POWER ELECTRONICS
AC-AC CONVERTERS
5
Prof R T KENNEDY