A motor takes a large current at starting because

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Option C: Back e.m.f. is low

Explanation & Concepts:
A DC motor draws a large starting current because the back EMF ($E_b$) is zero at the instant of starting when the armature speed is zero. According to the motor current equation, the current is inversely proportional to the armature resistance, which is typically very low, leading to a high surge of current.

Working Principle: The fundamental equation for a DC motor is $V = E_b + I_aR_a$. At the moment of starting, the speed $N = 0$, so the back EMF $E_b = \frac{\Phi Z N P}{60A} = 0$. Consequently, the starting current $I_a$ is determined solely by $I_a = \frac{V - 0}{R_a}$. Since $R_a$ is deliberately kept very small to minimize losses, the resulting starting current is typically 5 to 10 times the full-load current.

Key Formulas: $E_b = \frac{\Phi Z N P}{60A}$ — 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 a DC motor.; A DC motor requires a starter to limit the initial surge current and prevent winding damage.; The low value of $R_a$ is essential for high efficiency during normal operation but dangerous during starting.; Starting current is independent of field flux at the instant of standstill.

Advantages: Back EMF provides inherent speed regulation; High starting torque due to initial high current

Disadvantages: High starting current can cause voltage dips in the supply network; Potential for commutator sparking and insulation overheating if not limited

Applications: DC motor starters (e.g., 3-point or 4-point starters); Industrial motor protection circuits

Additional Info: To limit the starting current, external resistance is inserted in series with the armature during starting and gradually removed as the motor gains speed.. Option B is incorrect because armature resistance is kept low to maintain high efficiency; high $R_a$ would actually reduce current but decrease efficiency significantly.

Answer: C is correct — Since back EMF is directly proportional to speed, it is zero at the start, causing a massive rush of current into the low-resistance armature.

Exam Tip: Remember that the back EMF in a DC motor acts like a voltage source opposing the supply, similar to how it works in a DC generator but in reverse polarity relative to current flow.

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Option A: Shunt field is producing the weak field

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Option B: The armature resistance is high

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Option D: None of the above

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