THE PROCESS OF TURNING ON OR OFF A POWER ELECTRONIC DEVICE.

Commutation ◦Process of turning off a conducting thyristor.

Current CommutationVoltage Commutation

NATURAL COMMUTATION

FORCED COMMUTATION

~

T

+

v ov sR

• Occurs in AC circuits

t

t

t

t

S u p p ly v o lta g e v s S in u s o id a l

Vo l ta g e a c r o s s S C R

L o a d v o lta g e v o

Tu rn o ffo c c u r s h e re

0

0

2

2

3

3

tc

G a te P u ls e

AC VOLTAGE CONTROLLERS.

PHASE CONTROLLED RECTIFIERS. CYCLO CONVERTERS.

Applied to dc circuits Commutation achieved by reverse biasing

the SCR or by reducing the SCR current below holding current value.

Commutating elements such as inductance and capacitance are used for commutation purpose.

Self commutation. Resonant pulse commutation. Complementary commutation. Impulse commutation. External pulse commutation. Load Commutation. Line Commutation.

Elements such as inductors and capacitors are involved.

Forced commutation is applied to DC circuits.

It is applied to choppers & inverters.

Self Commutation Or

Load Commutation Or

Class A Commutation

(Commutation By Resonating The Load)

Under damped condition using L,C in series with load results in oscillating currents .

This results in turning off an SCR when the current through load is zero.

V

R L V (0)c

C

T i

Load

+ -

In load side commutation the discharging and recharging of capacitor takes place through the load. Hence to test the commutation circuit the load has to be connected. Examples of load side commutation are Resonant Pulse Commutation and Impulse Commutation.

2

For effective commutation

the circuit should be under damped.

1That is

2

With R = 0, and the capacitor initially uncharged

that is 0 0

sin

Note:

C

R

L LC

V

V ti t

L LC

C u r r e n t i

C a p a c i to r v o l t a g e

G a te p u l s e

Vo l ta g e a c r o s s S C R

0 / 2t

t

t

t

V

V

2 V

Conduction time of SCR

V S V A U X

L

C

T 1 T 3T 2

R L 2V A U X

+