Discussion
Consider the following statements :
The diodes in a voltage source inverter (McMurray Inverter) should be able to :
(1) Withstand a large voltage in the reverse direction
(2) Carry the commutating current excess of load current
(3) Provide the required reverse-bias to the outgoing thyristor
(4) Feedback the reactive current to the source.
Of these statements :
Answer: C
Statements (2), (3) and (4) are correct, so the answer is (2), (3) and (4) are correct.
In the McMurray inverter each thyristor has a diode in inverse parallel. During commutation the LC circuit drives a current pulse larger than the load current through the outgoing thyristor; the excess flows through its antiparallel diode (2). The forward drop of that conducting diode is what reverse biases the outgoing thyristor so that it can turn off (3). With an inductive load the same diodes return the reactive (load) current to the DC source (4).
Statement (1) is wrong: because each diode is connected in antiparallel with a thyristor, the reverse voltage across it can never exceed the thyristor's on-state drop, so it never has to withstand a large reverse voltage. That rules out every option containing (1).
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