The starting resistance of D.C motor is generally

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Examoogle · Accepted Answer

Low

Explanation: {"type":"technical","methodBadge":"Concept & Principle","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.","workingPrinciple":"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.","diagramSvg":"Starting Current Formula$I_{start} = \frac{V}{R_{armature} + R_{starter}}$Note: At starting, $E_b = 0$.$R_{armature}$ is kept low for efficiency.","keyFormulas":["$I_a = \frac{V - E_b}{R_a}$ — Armature current equation","$E_b = \frac{P \Phi N Z}{60 A}$ — Back EMF equation"],"keyPoints":["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"],"comparisonTable":[],"additionalInfo":["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.","correctedOptions":["High","Infinite","Low","None of the above"],"coreConcepts":["Back EMF ($E_b$)","Armature Resistance ($R_a$)","Inrush Current","DC Motor Starters"],"crossTopicTip":"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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