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Figure: 5.37 Mode I operation of CSI
Mode II: The circuit for this mode is shown in Fig. 5.38. Diodes, D2 & D4, are
already conducting, but at
= tt 1 , diodes, D1 & D3, get forward biased, and start conducting. Thus, at the end of time t1, all four
diodes, D1
–
D4 conduct. As a result, the commutating capacitors now get connected in parallel with the
load (L).
Figure: 5.38 Mode II operation of CSI
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Load Commutated CSI
Two commutating
capacitors, along with four diodes, are used in the circuit for
commutation from one
pair of thyristors to the second pair. Earlier, also in VSI, if the
load is capacitive, it was shown that forced
commutation may not be needed. The operation of a single-phase CSI with capacitive load (Fig. 5.39) is
discussed here. It may be noted that the capacitor, C is assumed to be in parallel with resistive load (R).
The capacitor, C is used for storing the charge, or voltage, to be used to force-commutate the conducting
thyristor pair as will be shown. As was the case in the last lesson, a constant current source, or a voltage
source
with large inductance, is used as the input to the circuit.
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