A DC series motor is that which

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Option A: has its field winding consisting of thick wire and less turns

Explanation & Concepts:
A DC series motor is characterized by having its field winding connected in series with the armature winding. To minimize voltage drop and power loss while carrying the high full-load armature current, the field winding is constructed using thick wire with a relatively small number of turns.

Working Principle: In a series motor, the same current flows through both the armature and field windings ($I_a = I_f = I_s$). Because the flux $\Phi$ is directly proportional to the field current ($I_s$), the torque produced is proportional to the square of the current ($T \propto I_a^2$). The low resistance of the field winding ensures efficient operation under varying load conditions.

Key Formulas: $T_a \propto I_a^2$ — Torque is proportional to the square of armature current at heavy loads; $N \propto \frac{E_b}{\Phi}$ — Speed is inversely proportional to flux

Key Points: Field winding has very few turns of thick wire to handle high current.; Resistance of the field winding is intentionally kept low.; Highly suited for high starting torque applications.; Must never be started at no-load as it can reach dangerous speeds (runaway speed).

Advantages: High starting torque; Compact design due to thick winding

Disadvantages: Unstable speed at light loads; Cannot be operated at no-load

Applications: Traction systems (locomotives); Cranes and hoists; Automobile starters

Additional Info: Option B is incorrect because DC series motors provide very high torque at low speeds.. Option C is incorrect as starting without load causes dangerously high speeds.. Option D is incorrect; shunt motors provide almost constant speed, while series motors have variable speed characteristics.

Answer: A is correct — The field winding of a DC series motor consists of thick wire with fewer turns to minimize resistance and heat dissipation while carrying the full armature current.

Exam Tip: Always remember the 'No-Load' rule for series motors: they are meant for load-connected applications; starting one disconnected can cause catastrophic mechanical failure due to overspeeding.

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Option B: has a poor torque

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Option C: can be started easily without load

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Option D: has almost constant speed

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