Torque developed by a DC motor depends upon

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all of above

Explanation: The electromagnetic torque developed in a DC motor is directly proportional to the magnetic flux per pole, the number of armature conductors, and the armature current. Since these factors collectively define the motor's internal torque equation, the torque depends on all of the listed parameters.

Working Principle: According to the Lorentz force law, a current-carrying conductor placed in a magnetic field experiences a force $F = BIl$. When these conductors are placed on a rotating armature, the summation of these forces at a distance from the axis of rotation produces the developed electromagnetic torque. The total torque is defined by the motor torque equation $T_a = \frac{P \Phi Z I_a}{2 \pi A}$.

Key Formulas: $T_a = \frac{P \Phi Z I_a}{2 \pi A}$ — Fundamental electromagnetic torque equation for a DC motor; $F = B I L$ — Force on a single conductor in a magnetic field

Key Points: Magnetic field flux ($\Phi$) is provided by the field poles.; The number of armature conductors ($Z$) is a constructional parameter.; The physical length of the conductors ($L$) within the magnetic field determines the force magnitude.; Torque is also directly proportional to armature current ($I_a$).

Advantages: Predictable relationship between flux and torque; High starting torque capability

Disadvantages: Saturation of magnetic material limits flux; Commutation limits at high armature currents

Applications: Electric traction systems; Industrial drives requiring variable speed

Additional Info: The constant $K_a = \frac{P Z}{2 \pi A}$ aggregates constructional features.. Option B is correct because the length of the conductor interacting with the magnetic flux determines the force per turn.

Answer: D is correct — The torque developed in a DC motor follows the equation $T_a = \frac{P \Phi Z I_a}{2 \pi A}$, which integrates magnetic flux, number of conductors, and length of conductors.

Exam Tip: Always remember that armature current is the 'load-responsive' variable, while flux and constructional features (Z, A, P) are 'machine-design' variables in torque calculations.

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