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When the SCR2 is triggered, SCR is turned ON and simultaneously a negative polarity is applied across
the SCR1. So this reverse voltage across the SCR1 immediately causes to turn OFF the SCR1. Now the
capacitor starts charging with a reverse polarity through the path of E+
–
R1- C+
–
C- SCR2
–
E-. And
again, if the SCR 1 is triggered, discharging current of the capacitor turns OFF the SCR2.
This commutation is mainly used in single phase inverters with a centre tapped transformers. The Mc
Murray Bedford inverter is the best example of this commutation circuit. This is a very reliable method of
commutation and it is also useful even at frequencies below 1000Hz.
Class D Commutation
This is also called as auxiliary commutation because it uses an auxiliary
SCR to switch the charged
capacitor. In this, the main SCR is commutated by the auxiliary SCR. The main SCR with load resistance
forms the power circuit while the diode D, inductor L and SCR2 forms the commutation circuit.
Figure: 1. 19. Class D Commutation circuit and waveforms
When the supply voltage E
is applied, both SCRs are in OFF state and hence the capacitor voltage is zero.
In order to charge the capacitor, SCR2 must be triggered first. So the capacitor charges through the path
E+
–
C+
–
C-
–
SCR2- R- E-.
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When the capacitor is fully charged the SCR2 becomes turned OFF because no current flow through the
SCR2 when capacitor is charged fully. If the SCR1 is triggered, the current flows
in two directions; one is
the load current path E+
–
SCR1- R- E- and another one is commutation current path C+
–
SCR1- L- D-
C.
As soon as the capacitor completely discharges, its polarities will be reversed but due to the presence of
diode the reverse discharge is not possible. When the SCR2 is triggered capacitor starts discharging
through C+
–
SCR2- SCR1- C-. When this discharging current is more than the load current the SCR1
becomes turned OFF.
Again, the capacitor starts charging through the SCR2 to a supply voltage E and then the SCR2 is turned
OFF. Therefore, both SCRs are turned OFF and the above cyclic process is repeated. This commutation
method is mainly used in inverters and also used in the Jones chopper circuit.
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