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SarkariResultOutBMS · 2024

Previous Year Question

In a rotating electromechanical machine, the torque developed depends on
  1. Stator field strength and torque angle.
  2. Stator field, rotor field strengths and torque angle.
  3. Stator field and rotor field strengths.
  4. Rotor field strength and torque angle.

Correct answer: B

Explanation

Electromagnetic torque in a rotating machine is obtained from the change of magnetic coenergy with mechanical position at constant current, or equivalently from interaction of magnetic fields. Therefore it depends on electrical excitation and the position-dependent magnetic coupling/inductance described by option B.

Formula

Te=(∂W′field/∂θ) at constant current

Step-by-step solution

Torque Dependence explanatory diagram

Correct Answer

B — Stator field, rotor field strengths and torque angle.

Concept and Reasoning

Electromagnetic torque in a rotating machine is obtained from the change of magnetic coenergy with mechanical position at constant current, or equivalently from interaction of magnetic fields. Therefore it depends on electrical excitation and the position-dependent magnetic coupling/inductance described by option B.

Calculation / Key Relation

Te=(∂W′field/∂θ) at constant current

Exam Takeaway

Torque comes from position-dependent magnetic coenergy.

Common Mistake

Do not treat torque as depending only on speed; speed is not the fundamental energy-conversion variable in this relation.

Common mistake

Do not treat torque as depending only on speed; speed is not the fundamental energy-conversion variable in this relation.

Exam tip

Torque comes from position-dependent magnetic coenergy.

Quick method

Torque comes from position-dependent magnetic coenergy.