In flux control method the speed of DC motor is obtained _____

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Option A: Above Base speed

Explanation & Concepts:
In DC motors, the flux control method involves decreasing the field flux to increase the motor speed. Because the induced back EMF $E_b$ is directly proportional to flux $\Phi$ and speed $N$, weakening the field flux $\Phi$ necessitates an increase in speed $N$ to maintain the terminal voltage balance, allowing the motor to operate above its rated base speed.

Working Principle: The speed of a DC motor is given by $N = K \cdot \frac{V - I_a R_a}{\Phi}$. By inserting an external resistance in the shunt field circuit, the field current $I_f$ is reduced, which in turn reduces the flux $\Phi$. Since $N \propto \frac{1}{\Phi}$, reducing the flux causes the motor speed to rise above the base speed (the speed at rated voltage and rated flux).

Key Formulas: $N = \frac{V - I_a R_a}{K_\phi \Phi}$ — Fundamental speed equation for a DC motor; $\Phi \downarrow \Rightarrow N \uparrow$ — Relationship showing that flux reduction leads to speed increase

Key Points: Flux control is a constant power drive method.; This method is highly efficient as there is very little power loss in the field rheostat.; It is the standard method for obtaining speeds higher than the nameplate rated speed.; The range of speed control is limited by the mechanical commutation limits of the motor.

Advantages: High efficiency as power loss in field circuit is negligible; Simple and easy to implement

Disadvantages: Speed cannot be controlled below base speed; Weak field can lead to unstable operation and sparking at high speeds

Applications: Machine tools; High-speed industrial drives; Fans and blowers

Additional Info: Base speed is defined as the speed reached when rated voltage and rated flux are applied at full load.. Option B refers to the Armature Voltage Control method, which is used for speeds below base speed.

Answer: A is correct — The flux control method allows for speed regulation above the base speed by reducing the magnetic flux.

Exam Tip: Always remember that armature control is for below base speed (constant torque) and flux control is for above base speed (constant power).

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Option B: Below base speed

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Option C: Both above and below base speed

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Option D: Remain constant

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