The DC shunt motor is running with a certain load. The effect of adding an external resistance in field circui

Question

The DC shunt motor is running with a certain load. The effect of adding an external resistance in field circuit is to

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Option A: Increase the Motor Speed

Explanation & Concepts:
Adding external resistance in series with the shunt field circuit increases the total field resistance, which decreases the field current. Consequently, the magnetic flux produced by the field winding decreases, leading to an increase in the motor speed to maintain the back EMF balance.

Working Principle: In a DC shunt motor, the back EMF is given by $E_b = \frac{P\Phi NZ}{60A}$. Since $E_b \approx V$ (constant), the speed $N$ is inversely proportional to the flux $\Phi$ ($N \propto \frac{V}{\Phi}$). By adding external resistance, the field current $I_f = \frac{V}{R_{sh} + R_{ext}}$ reduces, causing $\Phi$ to drop. As flux decreases, the motor speed must increase to satisfy the EMF equation.

Key Formulas: $N = K \cdot \frac{V - I_a R_a}{\Phi}$ — General speed equation for a DC motor; $I_f = \frac{V}{R_{sh} + R_{ext}}$ — Field current relationship with external resistance

Key Points: Field flux control is a constant power method of speed control.; This method is used for controlling speeds above the rated speed (base speed).; Decreasing flux leads to higher speeds but results in reduced torque capability.; Excessive resistance can lead to magnetic field instability if the field current is too low.

Advantages: High efficiency as there is no power loss in the armature circuit; Simple and economical implementation

Disadvantages: Speed control is only possible above the rated speed; Reduced torque capacity at high speeds

Applications: Variable speed drives in machine tools; Applications requiring high-speed operation above base speed

Additional Info: This technique is known as Field Flux Weakening Control.. Option B is incorrect because adding resistance decreases flux, which forces the motor to speed up, not stop.. Option C and D are incorrect as they describe characteristics opposite to the physical laws of DC machines.

Answer: A is correct — Adding external resistance to the field circuit reduces the flux, causing the motor to increase its speed to maintain back EMF equilibrium.

Exam Tip: Always remember that for a DC shunt motor, speed is inversely proportional to flux ($N \propto 1/\Phi$). If flux is weakened, speed increases; if flux is strengthened, speed decreases.

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Option B: Stop the Motor

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Option C: Reduce the motor Speed

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Option D: Reduce the armature speed

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