What is the advantage of the slip ring rotor?

Question

Examoogle · Accepted Answer

Option D: All of these

Explanation & Concepts:
A slip ring (wound rotor) induction motor allows for the insertion of external resistance into the rotor circuit via slip rings and brushes. This capability directly improves starting torque, enables efficient speed control, and facilitates regenerative braking.

Working Principle: The torque of an induction motor is proportional to the rotor resistance ($T \propto \frac{R_2}{s}$). By adding external resistance $R_{ext}$ to the rotor, the starting torque increases as the rotor circuit resistance approaches the rotor reactance. Furthermore, changing this resistance modifies the slip at which maximum torque occurs, allowing for speed control. During regenerative braking, the rotor can be configured to feed energy back into the supply or dissipate it, enabling control over the braking process.

Key Formulas: $T_{st} = \frac{3}{\omega_s} \cdot \frac{V_1^2 R_2}{R_2^2 + X_{20}^2}$ — Starting torque relationship showing dependency on $R_2$; $s_m = \frac{R_2}{X_2}$ — Slip at maximum torque where $R_2$ directly influences speed

Key Points: External resistance can be added to the rotor circuit, unlike squirrel cage rotors.; Starting current is reduced while starting torque is increased.; Suitable for high-inertia loads and frequent starting applications.; Slip ring motors are also known as Wound Rotor Induction Motors (WRIM).

Advantages: High starting torque (up to full load torque); Reduced starting current compared to squirrel cage motors; Effective speed control via rotor resistance; Ability to support regenerative braking

Disadvantages: High maintenance due to brushes and slip rings; Higher cost and complex construction; Lower efficiency during speed control via resistance

Applications: Cranes and Hoists; Elevators; Compressors; Conveyor belts

Additional Info: Regenerative braking in slip ring motors is achieved by utilizing the slip energy recovery system (SERS), which converts rotor power back to the grid.. Option B is correct because the slip rings allow access to the rotor circuit, enabling electrical control during the braking phase.

Answer: D is correct — Slip ring induction motors provide superior control over torque, speed, and braking by allowing external modifications to the rotor circuit resistance.

Exam Tip: Always remember that while adding external resistance increases starting torque, it decreases full-load efficiency due to $I^2R$ losses in the external resistors.

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Option A: High starting torque

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Option B: power recovery is possible during breaking

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Option C: Speed control from rotor side

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