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Field Weakening

In flux control (field weakening), how does reducing the field current affect the motor's speed and torque?

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Answer & Explanation

Correct AnswerC. Speed increases, torque decreases

Quick Explanation

DC motor speed is approximately inversely proportional to flux, so weakening the field raises speed. Since electromagnetic torque T ∝ ΦI_a, reducing flux reduces torque available for a given armature current.

Formula / Key Relation

N ∝ (V-I_aR_a)/Φ ; T ∝ ΦI_a

Detailed Explanation

Correct Answer

C — Speed increases, torque decreases

Explanation

Key Concept

Field Weakening

Formula

N ∝ (V-I_aR_a)/Φ ; T ∝ ΦI_a

Exam Point

Field weakening = above-base speed, reduced torque capability.

Quick Trick

Field weakening = above-base speed, reduced torque capability.

Exam Tip

Field weakening = above-base speed, reduced torque capability.

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01 ADDITIONAL ASSISTANT ENGINEER.pdf2026Q. 107

Field Weakening

Quick Explanation

Formula / Key Relation

Detailed Explanation

Correct Answer

Explanation

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Formula

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