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SarkariResultOutBMS · 2024

Previous Year Question

Consider following statements: Assertion: DC series motors are suitable for electric locomotives. Reason: DC series motors provide high starting torque. Of these statements:
  1. Both Assertion and Reason are true and Reason is the correct explanation of Assertion.
  2. Both Assertion and Reason are true, but Reason is not the correct explanation of Assertion.
  3. Assertion is true but Reason is false.
  4. Assertion is false but Reason is true.

Correct answer: A

Explanation

A DC series motor develops very high starting torque because, before saturation, flux is approximately proportional to armature current and torque T∝ΦIa≈Ia². This makes it suitable for traction applications such as electric locomotives. Both assertion and reason are true, and the reason correctly explains the assertion: option A.

Formula

T∝ΦIa; unsaturated series motor Φ∝Ia → T∝Ia²

Step-by-step solution

High Starting Torque explanatory diagram

Correct Answer

A — Both Assertion and Reason are true and Reason is the correct explanation of Assertion.

Concept and Reasoning

A DC series motor develops very high starting torque because, before saturation, flux is approximately proportional to armature current and torque T∝ΦIa≈Ia². This makes it suitable for traction applications such as electric locomotives. Both assertion and reason are true, and the reason correctly explains the assertion: option A.

Calculation / Key Relation

T∝ΦIa

unsaturated series motor Φ∝Ia → T∝Ia²

Exam Takeaway

Series motor + traction → remember high starting torque.

Common Mistake

Do not operate a DC series motor at no load because its speed can rise dangerously.

Common mistake

Do not operate a DC series motor at no load because its speed can rise dangerously.

Exam tip

Series motor + traction → remember high starting torque.

Quick method

Series motor + traction → remember high starting torque.