The starting resistance of D.C motor is generally

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Examoogle · Accepted Answer

Low

Explanation: The starting resistance of a DC motor is kept very low (typically that of the armature winding alone) because the back EMF is zero at the moment of starting. External resistance is added only temporarily via a starter to limit the high inrush current, but the internal resistance of the motor itself is designed to be low to ensure high efficiency during operation.

Working Principle: At starting, the back EMF $E_b$ is zero, causing the armature current to be $I_a = \frac{V - E_b}{R_a} = \frac{V}{R_a}$. Since $R_a$ is inherently very low, the initial current $I_a$ is excessively high, which can damage the windings. A DC motor starter inserts external variable resistance in series with the armature to keep the starting current within safe limits (usually 1.5 to 2 times the rated current). As the motor gains speed, the back EMF increases, and the external resistance is gradually cut out.

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

Key Points: A DC motor has low internal armature resistance ($R_a$).; High starting current is due to the absence of back EMF at standstill.; External starting resistance is used to limit current, not because the motor's own resistance is high.; Without a starter, the high current may lead to severe sparking at brushes and overheating.

Advantages: Minimizes voltage dip on the supply line.; Protects armature windings from thermal damage.; Prevents mechanical stress on the shaft during startup.

Disadvantages: Requires additional space for starter assembly.; Adds complexity to the control circuit.

Applications: DC Shunt Motor Starters; DC Series Motor Starters

Additional Info: The armature resistance $R_a$ is intentionally kept low to maintain high electrical efficiency ($eta$) when the motor is running at rated speed.. Option B (Infinite) is physically impossible as it would result in zero current flow.. Option A (High) is incorrect because motors are designed with low $R_a$ for high torque-to-current ratios.

Answer: C is correct — The armature resistance of a DC motor is inherently low to maintain efficiency, requiring external resistance only during start-up to limit inrush current.

Exam Tip: Always remember that in DC machines, the starting resistance is added externally; it is NOT an intrinsic property of the machine, which is always designed for low resistance.

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