In the design of a wound rotor three-phase induction motor. The Slip-ring rotor has _____________ .

Question

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Option A: balanced 3-$\Phi$ winding either connected in Y or $\Delta$

Explanation & Concepts:
A wound rotor induction motor features a rotor winding consisting of 3-phase coils that are physically insulated and connected to external slip rings. These windings are designed to be balanced and can be configured in either Star (Y) or Delta ($\Delta$) connection, allowing for the connection of external resistances to the rotor circuit.

Working Principle: The slip-ring rotor uses insulated windings to allow external resistance insertion during starting. By adding external resistance, the starting torque is increased and starting current is decreased according to the relation $T \propto \frac{sE_2^2 R_2}{R_2^2 + (sX_2)^2}$. This control is impossible in squirrel-cage rotors where the bars are short-circuited by end rings.

Key Formulas: $T_{st} \propto R_2$ — Starting torque is directly proportional to rotor resistance; $s = \frac{N_s - N_r}{N_s}$ — Slip calculation formula

Key Points: Wound rotors are used for high starting torque applications.; External resistance is connected through slip rings and carbon brushes.; The rotor winding must have the same number of poles as the stator winding.; Y-connection is preferred to provide a neutral point for control.

Advantages: High starting torque capability; Reduced starting current; Speed control via rotor resistance

Disadvantages: Higher maintenance due to brushes and slip rings; More expensive than squirrel-cage construction; Lower efficiency during normal operation due to higher rotor resistance

Applications: Cranes and Hoists; Elevators; Conveyor belts

Additional Info: Wound rotor induction motors are also known as Slip-ring induction motors.. Option C refers to Squirrel Cage rotors, which have no external connections.. Option D is incorrect as split-phase windings are used in small single-phase motors.

Answer: A is correct — The wound rotor comprises a balanced three-phase winding that is configured in either Y or $\Delta$ to facilitate connection to external rheostats.

Exam Tip: Always remember that in a wound rotor, adding external resistance decreases the pull-out speed but significantly improves starting performance.

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Option B: $\Phi$ winding

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Option C: rotor bare with end rings

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Option D: Split-$\Phi$ winding

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