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SarkariResultOutAdditional Assistant Engineer (Electrical) · 2026

Previous Year Question

In flux control (field weakening), how does reducing the field current affect the motor's speed and torque?
  1. Speed and torque both increase
  2. Speed and torque both decrease
  3. Speed increases, torque decreases
  4. Speed decreases, torque increases

Correct answer: C

Explanation

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

Formula

N ∝ (V-I_aR_a)/Φ ; T ∝ ΦIa

Step-by-step solution

Field Weakening explanatory diagram

Correct Answer

C — Speed increases, torque decreases

Explanation

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

Key Concept

Field Weakening

Formula

N ∝ (V-I_aR_a)/Φ ; T ∝ ΦIa

Exam Point

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

Exam tip

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

Quick method

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