Which of the following motors are preferred for traction work?

Question

Examoogle · Accepted Answer

series motor

Explanation: DC series motors are preferred for traction applications because they possess high starting torque, which is essential for accelerating heavy loads from a standstill. Their torque-speed characteristics align perfectly with the requirements of electric locomotives and trams.

Working Principle: In a DC series motor, the armature and field windings are connected in series, meaning the same current flows through both. Since the torque $T$ is proportional to the product of flux $\Phi$ and armature current $I_a$, and flux $\Phi$ is proportional to $I_a$ (in the linear region), the torque becomes proportional to $I_a^2$. This quadratic relationship generates high torque at low speeds during start-up, while the speed naturally increases as the load decreases, providing efficient operation across varying conditions.

Key Formulas: $T_a \propto \Phi I_a$ — Basic torque equation of a DC motor; $\Phi \propto I_a$ — Flux relation in a series motor (before saturation); $T_a \propto I_a^2$ — Combined torque characteristic

Key Points: High starting torque allows the motor to pull heavy trains from rest.; The speed of the DC series motor is inversely proportional to the load.; It is robust and simple to control using series-parallel combinations.; Not suitable for constant speed applications as it may run at dangerously high speeds at no-load.

Advantages: Exceptionally high starting torque; Automatic adjustment of speed with load variations; Simple control circuit for starting and speed regulation

Disadvantages: Cannot be operated at no-load (risk of 'runaway' speed); Maintenance requirements due to brushes and commutator

Applications: Electric locomotives; Trams and trolley buses; Cranes and hoists

Additional Info: Universal motors are mainly used in domestic appliances like mixers and drills.. Three-phase induction motors are used in constant speed applications but require complex frequency-control drives for traction.. Synchronous motors are rarely used in traction due to the constant speed constraint.

Answer: B is correct — D.C. series motors are uniquely suited for traction due to their characteristic high starting torque and load-dependent speed regulation.

Exam Tip: Always remember: 'Series = High Torque/Variable Speed' and 'Shunt = Constant Torque/Constant Speed'.

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