These days DC motors are widely used in

Question

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electric traction

Explanation: DC series motors are widely used in electric traction because they possess a high starting torque and their speed is inversely proportional to the torque, making them ideal for accelerating heavy loads like trains.

Working Principle: The DC series motor exhibits a torque-speed characteristic where $T \propto I_a^2$ at low speeds (before saturation) and $N \propto \frac{1}{I_a}$. This allows the motor to provide massive starting torque for heavy trains while naturally slowing down as the load increases, effectively handling the high inertia requirements of traction systems.

Key Formulas: $T_a \propto \Phi I_a$ — Basic torque equation for DC machines; $T_a \propto I_a^2$ — Torque for DC series motors (below magnetic saturation)

Key Points: DC series motors have a characteristic of very high starting torque.; The speed of a DC series motor varies inversely with the load.; Electric traction requires high initial acceleration and the ability to handle varying loads.; Series motors can handle heavy surge currents effectively.

Advantages: High starting torque; Self-relieving property with increasing load; Simple control with series-parallel switching

Disadvantages: Risk of 'runaway' at light loads or no-load conditions; Requires maintenance due to the commutator and brushes

Applications: Electric locomotives; Heavy-duty cranes; Hoists and conveyors

Additional Info: Never operate a DC series motor at no-load, as the speed becomes dangerously high ($N \propto \frac{1}{\Phi}$ and $\Phi \rightarrow 0$ at no-load).. Option A (pumping sets) usually uses Shunt or Induction motors for constant speed operation.. Option D (machine shops) typically uses DC shunt motors for constant speed requirements.

Answer: C is correct — DC series motors are specifically chosen for electric traction due to their high starting torque capability and variable speed characteristics suitable for heavy-load acceleration.

Exam Tip: In competitive exams, always remember: for constant speed tasks use DC Shunt motors, and for high starting torque/traction tasks use DC Series motors.

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