What is/are the advantages of Rotor Resistors Starter compared to other IM starters?

Question

Examoogle · Accepted Answer

Option D: All of these

Explanation & Concepts:
A Rotor Resistance Starter is used specifically for Slip Ring (Wound Rotor) Induction Motors by adding external resistance to the rotor circuit during starting. This increases the rotor resistance ($R_2$), which shifts the maximum torque toward starting conditions and reduces the high starting current. Consequently, the power factor at start improves, allowing the motor to accelerate quickly under heavy loads.

Working Principle: The starting torque of a 3-phase induction motor is directly proportional to the rotor resistance ($T_{st} \propto \frac{R_2}{R_2^2 + X_2^2}$). By inserting external resistance into the rotor circuit via slip rings, the rotor impedance angle decreases, improving the starting power factor ($cos\phi_2 = \frac{R_2}{\sqrt{R_2^2 + X_2^2}}$). As the motor accelerates, the external resistance is gradually cut out until the slip rings are short-circuited for normal running.

Key Formulas: $T_{st} \propto \frac{E_2^2 R_2}{R_2^2 + X_2^2}$ — Starting torque equation showing direct dependence on rotor resistance $R_2$; $\cos\phi_2 = \frac{R_2}{\sqrt{R_2^2 + X_2^2}}$ — Rotor power factor at starting; $s_m = \frac{R_2}{X_2}$ — Slip at which maximum torque occurs

Key Points: External rotor resistance reduces the high inrush starting current while simultaneously increasing starting torque.; Higher starting torque allows the motor to start smoothly under heavy load conditions.; The starting power factor improves significantly because the resistive component of rotor impedance becomes dominant.

Advantages: High starting torque with low starting current.; Improved starting power factor ($cos\phi_2$).; Smooth and quick acceleration under heavy load conditions.; Can be used for speed control by adjusting external resistance.

Disadvantages: Can only be used with Slip Ring (Wound Rotor) Induction Motors, not Squirrel Cage motors.; Higher initial cost and maintenance due to slip rings and brushes.; Power loss ($I^2R$) in external resistors during the starting period.

Applications: Lifts and elevators; Cranes and hoists; Conveyors and crushers requiring high starting torque under load

Additional Info: Option A is correct because adding $R_2$ increases starting torque and improves power factor ($cos\phi_2 = R_2/Z_2$).. Option B is correct because high starting torque allows the motor to be started easily on heavy loads.. Option C is correct because the motor accelerates quickly to rated speed due to high acceleration torque.

Answer: D is correct — Rotor resistor starters simultaneously increase starting torque and power factor, allowing quick acceleration even under heavy load conditions.

Exam Tip: Remember: Rotor resistance starting is exclusive to Wound Rotor (Slip Ring) motors; Squirrel Cage induction motors cannot use rotor resistance starters because their rotor bars are permanently short-circuited.

Examoogle · Answer

Option A: Starting PF & torque is increased

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Option B: Can be started on heavy load

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Option C: Motor acceleration quickly

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