complementary commutation by ashakoor
TRANSCRIPT
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CLASS C COMMUTATION of SCR
Complementary Commutation or Parallel Capacitor Commutation
POWER ELECTRONICS (EEE – 338)
COMSATS Institue of Information Technology, Abbottabad
Comsats Institute of Information Science Technology Abbottabad
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GROUP Group Members
Muhammad Umar Masood SP09-BEE-034
Asim Zeb SP09-BEE-036
Ubair Naeem SP09-BEE-004
Instructor Sir Mohsin Shehzad
Power Electronics ( EEE – 338)
Class C Commutation
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INTRODUCTION SCR can be made ON by
Thermally excitation High A-K voltage High dv/dt Light Triggering (in LASCR) Gate Triggering
SCR
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COMMUTATION Commutation
It is a process of turning off a conducting thyristor
When the SCR is ON, it can be turned off by Reversing the A-K voltage Reducing the Anode current (IA) below holding current (IH)
Holding Current The holding current is the minimum current which must
pass through the SCR in order for it to remain in the 'ON' state
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COMPLEMENTARY COMMUTATION Circuit Diagram
V
R 1 R 2
T 1 T 2
IL
iC
C
a b
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CIRCUIT OPERATION Mode 0 (Initial-state of circuit)
Initially, both the thyristors (T1 & T2) are OFF State of circuit is
T1 OFF, T2 OFF, Vc=0
Mode 1 When a triggering pulse is applied at the gate of
T1, the thyristor T1 is triggered. Two circuit current starts flowing through the
circuit ( load current-IL & charging current-Ic. There paths are IL ---- V+ -- R1 -- T1 -- V- Ic ---- V+ -- R2 -- C+ -- C- -- T1 -- V-
V
R 1 R 2
T 1 T 2
IL
iC
C
a b
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CIRCUIT OPERATION (contd…)
Mode 1 (contd…) State of circuit is
T1 ON, T2 OFF, Vc = Vs
Mode 2 Triggering pulse is applied to T2 and it becomes ON As soon as T2 is ON, the negative polarity of C is
applied to anode of T1, and simultaneously positive polarity is applied to the cathode of T1
This creates negative voltage across T1 and it turns OFF
Charging path now becomes V+ -- R1 -- C+ -- C- --T2(A-K) -- V-
V
R 1 R 2
T 1 T 2
IL
iC
C
a b
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CIRCUIT OPERATION (contd…)
Mode 2 (contd…) Circuit state is
T1 OFF, T2 ON, Vc = -Vs
Mode 3 When again T1 is triggered, the discharging
current of capacitor turns OFF the T2. State of circuit is
T1 ON, T2 OFF, Vc = Vs
V
R 1 R 2
T 1 T 2
IL
iC
C
a b
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WAVEFORMS
Ig1
Ig2
IR1
t
t
t
Gate Pulse of T1
Gate Pulse of T1
Gate Pulse of T2
V/R1
Td = R1 . C
V
R 1 R 2
T 1 T 2
IL
iC
C
a b
(V+Vc) / R1
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WAVEFORMS (contd…)IT1
tcVT1
Vc(a-b)
-Vc(a-b)
t
t
t
t
V/R1 V / (R1 || R2)
V/R2
IT2
tc
+V
-V
toff Time Constant = R1 / C
V
R 1 R 2
T 1 T 2
IL
iC
C
a b
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DESIGN CONSIDERATION Expression for circuit turn off time
When T1 is conducting, C is charged to Edc through R2
Now, when T2 is triggered, a voltage twice the d.c. supply voltage Edc is applied to the R1C series circuit , so the current through circuit is
Voltage across T1 is
For making T1 off, the capacitor value must be equal to ET1
Class C Commutation
V
R 1 R 2
T 1 T 2
IL
iC
C
a b
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DESIGN CONSIDERATION (contd…) Let t = toff when Ec = 0
toff or tc time should be greater than turn off time of thristor T1
Similarly, when T2 is commutated tc = 0.6931 R2 C and this time should be greater than turn off time of T2
(A)
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Refrences
Books: Power Electronics , 1/e by Sivanagaraju ( Chapter # 02) Power Electronics by Singh, K B Khanchandani (Chapter # 02)
Website: http://www.slideshare.net/prathikm/thyristor-commutation-
techniques
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Q & A
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