Choppers and commutation; Morgan chopper Assertion (A): A chopper always requires forced commutation. Reason (R): A Morgan chopper uses a saturable reactor for commutation.
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ABoth A and R are correct and R is correct explanation of A
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BBoth A and R are correct but R is not the correct explanation of A
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CA is correct but R is wrong
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DA is wrong but R is correct
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E—
Answer
Correct Answer: Both A and R are correct but R is not the correct explanation of A
Explanation
Introduction / Context:DC choppers using thyristors operate on DC without natural current zero crossings. Therefore, they need a method to force devices to turn off. The Morgan chopper is a classic commutation scheme employing a saturable reactor.
Given Data / Assumptions:
- Device: thyristor-based chopper on a DC source.
- No natural current zero in DC, unlike AC line commutation.
- Morgan commutation known as a magnetic (saturable reactor) assisted method.
Concept / Approach:
Since the load and source are DC, the thyristor must be reverse-biased or its current forced to zero by an auxiliary network (LC or magnetic means) to achieve turn-off. This is the essence of forced commutation. The Morgan chopper is one specific method, but the general need for forced commutation exists regardless of whether a saturable reactor is used.
Step-by-Step Solution:
Evaluate Assertion: For SCR-based DC choppers, forced commutation is required → True.Evaluate Reason: Morgan chopper uses a saturable reactor for commutation → True.Causality check: While R is true, it does not establish the general necessity stated in A; it merely gives one implementation example → Not a correct explanation.Verification / Alternative check:
Other classes of forced commutation (Class C, D, E, F) use different networks (capacitors/inductors/pulse commutation), underscoring that forced commutation is a general need, not limited to Morgan's method.
Why Other Options Are Wrong:
Options suggesting A is wrong contradict basic SCR behavior on DC. Claiming R explains A would imply Morgan's method alone justifies the need, which is not the case.
Common Pitfalls:
Confusing “one example exists” with “general explanation”; also forgetting that with transistor switches (MOSFET/IGBT), turn-off is controlled, but the question is about SCR-based choppers.
Final Answer:
Both A and R are correct but R is not the correct explanation of A