The direction of rotation of D.C series motor can be changed by

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Interchanging field terminal

Explanation: The direction of rotation of a DC series motor depends on the direction of current flow through both the armature and the field windings simultaneously. Reversing the supply terminals does not change the direction of rotation because the current in both the field and armature reverses at the same time, maintaining the polarity relationship required for torque in the same direction. Working Principle: The torque $T$ in a DC motor is proportional to the product of flux $\Phi$ and armature current $I_a$ ($T \propto \Phi I_a$). In a series motor, $\Phi \propto I_a$. If supply polarity is reversed, both $I_a$ and $\Phi$ reverse direction, so their product remains positive ($(-I_a) imes (-\Phi) = +T$). By interchanging only the field terminal connections, the relative polarity between the field and the armature is altered, resulting in a reversal of the electromagnetic torque. Key Formulas: $T_e = k_a \cdot \Phi \cdot I_a$ — Electromagnetic torque equation; $\Phi \propto I_f$ — Flux relationship in series motors Key Points: Changing the supply polarity reverses both field and armature current, keeping the direction of rotation the same.; Reversing the physical connections of either the field or the armature winding (but not both) reverses the rotation.; DC series motors are commonly used in traction where bidirectional operation is controlled by contactors reversing the field.; The net torque direction is governed by the relative orientation of the magnetic field and armature current. Advantages: Simple control circuitry; Effective torque reversal for traction applications Disadvantages: Requires mechanical access to field terminals; Risk of arcing during terminal reversal under load Applications: Electric traction systems; Cranes and hoists Additional Info: In a shunt motor, reversing the supply also keeps the rotation same, but reversing the field or armature changes it.. Option B is incorrect as it results in the motor maintaining its original rotation due to the simultaneous reversal of both flux and armature current. Answer: A is correct — The direction of rotation of a DC series motor can only be changed by reversing the current in either the field winding or the armature winding, not both. Exam Tip: Always remember that for any DC machine, reversing both the armature and field terminals simultaneously results in the same direction of rotation; you must reverse only one to achieve reversal.

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