In series motor which of the following methods can be used for changing the flux per pole?

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Examoogle · Accepted Answer

Any of these

Explanation: In a DC series motor, the speed can be controlled by varying the flux per pole ($\Phi$). Tapped field control, diverter field control, and series-parallel control are all valid methods to achieve this variation in the magnetic flux.

Working Principle: The torque produced is $T_a \propto \Phi I_a$ and speed $N \propto \frac{E_b}{\Phi}$. By reducing the flux through field tapping (fewer turns) or diverters (bypassing current), the flux per pole decreases, which increases the motor speed. Series-parallel control involves changing the configuration of multiple motors to adjust the total voltage or flux per motor.

Key Formulas: $N \propto \frac{V - I_a R_a}{\Phi}$ — Relation between speed, voltage, and flux; $T_a \propto \Phi I_a$ — Torque expression for series motor

Key Points: Flux reduction leads to increased motor speed.; Tapped field control changes the number of effective series turns.; Diverter control uses a variable resistor in parallel with the field winding.; Series-parallel control is common in traction applications.

Advantages: Efficient speed regulation; Wide speed range capability

Disadvantages: Reduced torque at high speeds; Increased complexity with mechanical switches

Applications: Electric traction; Cranes and hoists; Electric vehicles

Additional Info: In series-parallel control, motors are connected in series for low speed and parallel for high speed.. Option D is correct because all listed methods effectively alter the flux passing through the armature circuit.

Answer: D is correct — All the methods listed (tapped field, diverter, and series-parallel) are established techniques for controlling the magnetic flux and thus the speed of a DC series motor.

Exam Tip: Always remember that in series motors, field control is highly effective because flux is directly dependent on the load current.

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