Complementary Commutation by Ashakoor

Comsats Institute of Information Science Technology Abbottabad

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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


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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

Class C Commutation


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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

Class C Commutation


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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

Class C Commutation


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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

Class C Commutation


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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

Class C Commutation


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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

Class C Commutation


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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

Class C Commutation


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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)

Class C Commutation


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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

Class C Commutation


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Q & A


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Documents

Complementary Commutation by Ashakoor