A DC motor has flux = constant and armature current = 20 A.1.Torque is directly proportional to armature current under constant flux condition.2.If current doubles, torque also doubles.3.Torque is independent of flux.4.Torque depends on product of flux and current.Which of the above statements is/are correct?

Correct Answer: A - 1, 2 and 4 only

Quick Concept Explanation

The electromagnetic torque of a DC motor is proportional to the product of air-gap flux and armature current. If flux is held constant, torque varies directly with armature current, so doubling current ideally doubles torque. Key relation: T=kΦI_a.

Exam focus: Constant flux converts the general torque equation into T∝I_a. Common trap: Do not generalize constant-flux behavior into 'torque is independent of flux'.

Statement-wise Verification

• Statement 1 - Correct.
  Torque is directly proportional to armature current under constant flux condition.
  The electromagnetic torque of a DC motor is proportional to the product of air-gap flux and armature current.
• Statement 2 - Correct.
  If current doubles, torque also doubles.
  If flux is held constant, torque varies directly with armature current, so doubling current ideally doubles torque.
• Statement 3 - Incorrect.
  Torque is independent of flux.
  The correct principle is: Constant flux converts the general torque equation into T∝I_a.
• Statement 4 - Correct.
  Torque depends on product of flux and current.
  The electromagnetic torque of a DC motor is proportional to the product of air-gap flux and armature current.

Core Concept

The electromagnetic torque of a DC motor is proportional to the product of air-gap flux and armature current. If flux is held constant, torque varies directly with armature current, so doubling current ideally doubles torque. The flux term cannot be omitted in the general torque equation.

Formula / Key Relationship

T=kΦI_a.

Why the Other Options Are Wrong

Option B is incorrect because it includes incorrect statement(s) 3 and omits correct statement(s) 2, 4. Option C is incorrect because it includes incorrect statement(s) 3 and omits correct statement(s) 1, 4. Option D is incorrect because it includes incorrect statement(s) 3.

Exam Shortcut / Approach

Constant flux converts the general torque equation into T∝I_a.

Common Mistake

Do not generalize constant-flux behavior into 'torque is independent of flux'.

Quick Revision

DC Motor Torque and Armature Current is linked with Torque Equation, DC Motors and Electrical Machines. Constant flux converts the general torque equation into T∝I_a.