An external resistance is added in the motor circuit during starting condition. The main purpose of this is to

Question

An external resistance is added in the motor circuit during starting condition. The main purpose of this is to __________.

Examoogle · Accepted Answer

The correct answer is Option C: reduce the starting current. The main purpose of adding an external resistance in series with the armature circuit of a DC motor during starting is to limit the high inrush of current. Since the back EMF ($E_b$) is zero at the moment of starting, the armature current would otherwise reach dangerously high levels, potentially damaging the windings.

Step-by-Step Solution & Concepts:
The main purpose of adding an external resistance in series with the armature circuit of a DC motor during starting is to limit the high inrush of current. Since the back EMF ($E_b$) is zero at the moment of starting, the armature current would otherwise reach dangerously high levels, potentially damaging the windings.

Working Principle: According to the DC motor equation $V = E_b + I_a R_a$, the armature current is given by $I_a = \frac{V - E_b}{R_a}$. At the instant of start, the rotor is at rest, meaning the back EMF ($E_b = 0$). To prevent the current $I_a$ from exceeding safe limits, an external starting resistor ($R_{ext}$) is added in series with the armature, effectively modifying the equation to $I_a = \frac{V}{R_a + R_{ext}}$. As the motor gains speed, $E_b$ builds up, and the external resistance is gradually cut out.

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

Key Points: At standstill, $E_b = 0$, leading to a very low impedance path in the armature.; Without external resistance, $I_a$ can be 10 to 20 times the rated full-load current.; External resistance is usually part of a 3-point or 4-point starter.; The resistance is removed in steps as the motor speed increases and back EMF develops.

Advantages: Protects commutator and brushes from sparking; Prevents overheating and burning of armature conductors; Reduces mechanical stress on the shaft during startup

Disadvantages: High power dissipation as heat in the starting resistor; Increases the complexity of the starting circuitry

Applications: DC Shunt Motors; DC Compound Motors

Additional Info: Option A and D are incorrect because resistance does not directly regulate the main flux. Flux is controlled by the field current, not the armature resistance.. Option B is incorrect because increasing the starting current would lead to catastrophic failure of the motor windings.

Answer: C is correct — Adding external resistance in the armature circuit limits the high inrush starting current to a safe, permissible value.

Exam Tip: Remember that in DC motors, current is inversely proportional to the total resistance. If the question asks about field resistance, increasing it decreases flux and increases speed, but for armature resistance, it is strictly for current limiting at start.

Examoogle · Answer

Option A: reduce the main flux

Examoogle · Answer

Option B: increase the starting current

Examoogle · Answer

Option D: increase the main flux

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