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Examoogle • User • info@examoogle.com • EE-2024-8821
Chapter 1 of 12 • Page 1 of 248🔒 Protected PDF • Watermarked
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The starting resistance of D.C motor is generally
High
Infinite
Low
None of the above
Low
Quick Summary: 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.
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.
Ia=RaV−Eb — Armature current equation
Eb=60APΦNZ — Back EMF equation
At starting, the back EMF Eb is zero, causing the armature current to be Ia=RaV−Eb=RaV. Since Ra is inherently very low, the initial current Ia 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.
A DC motor has low internal armature resistance (Ra).
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.
Minimizes voltage dip on the supply line.
Protects armature windings from thermal damage.
Prevents mechanical stress on the shaft during startup.
Requires additional space for starter assembly.
Adds complexity to the control circuit.
DC Shunt Motor Starters
DC Series Motor Starters
The armature resistance Ra 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 Ra for high torque-to-current ratios.
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.
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.