When the speed of D.C motor is increased

Question

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Option A: Back E.M.F increases and current drawn decreases

Explanation & Concepts:
In a DC motor, the back EMF ($E_b$) is directly proportional to the speed ($N$). As the speed increases, the induced back EMF increases, which opposes the supply voltage ($V$), resulting in a decrease in the armature current ($I_a$).

Working Principle: The DC motor operates based on the interaction between the armature current and the magnetic field. The back EMF is defined by $E_b = \frac{\Phi Z N P}{60 A}$. Since $V = E_b + I_a R_a$, increasing the speed raises $E_b$, which forces the net voltage $(V - E_b)$ to decrease, thereby reducing the armature current $I_a = \frac{V - E_b}{R_a}$.

Key Formulas: $E_b = \frac{\Phi Z N P}{60 A}$ — Back EMF equation; $I_a = \frac{V - E_b}{R_a}$ — Armature current equation

Key Points: Back EMF acts as a self-regulating mechanism in DC motors.; If speed increases, $E_b$ approaches the supply voltage $V$.; A decrease in $I_a$ leads to a decrease in the electromagnetic torque ($T_e \propto \Phi I_a$).; This behavior ensures stability in speed-load characteristics.

Advantages: Self-regulating nature protects the armature from excessive current at high speeds.; Helps in maintaining speed stability.

Disadvantages: High starting current occurs because $E_b$ is zero at standstill.; Rapid speed fluctuations can lead to temporary instability in current demand.

Applications: DC shunt motor speed control.; Traction systems using series motors.

Additional Info: The armature resistance $R_a$ is typically very low, making the current sensitive to small changes in $(V - E_b)$.. Option B is the exact opposite of physical behavior. Option C and D violate Kirchhoff's voltage law for the armature circuit.

Answer: A is correct — As speed increases, the induced back EMF increases, which reduces the effective voltage driving the armature current, thereby decreasing the current drawn.

Exam Tip: Always remember that $E_b$ always opposes the supply voltage; this is the fundamental reason for the DC motor's inherent speed-current regulation.

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Option B: Back E.M.F decreases and current drawn increases

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Option C: Back E.M.F and current drawn both increases

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Option D: Back E.M.F and current drawn both decreases

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