Mostly the Slip-ring rotor has _____________ winding.

Question

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Option C: Y connected 3-$\Phi$ winding

Explanation & Concepts:
A slip-ring induction motor, also known as a wound-rotor induction motor, features a 3-phase winding on the rotor that is star (Y) connected. The three ends of the winding are brought out to slip rings mounted on the shaft, allowing external resistances to be connected in series with the rotor circuit.

Working Principle: The Y-connection is preferred for the rotor winding to provide a common neutral point, which simplifies the external connection of starting resistances. By inserting variable external resistance into the rotor circuit through the slip rings, the motor's starting torque is increased, and the starting current is reduced. This arrangement is particularly useful for applications requiring high starting torque and smooth speed control.

Key Formulas: $T_{st} \propto \frac{R_2}{R_2^2 + X_2^2}$ — Starting torque relationship where $R_2$ is total rotor resistance; $I_{st} = \frac{E_2}{\sqrt{R_{2(ext)}^2 + X_2^2}}$ — Starting current equation with external resistance

Key Points: The rotor winding is physically identical to the stator winding but designed for a lower voltage and higher current.; External resistance is only used during the starting phase; once the motor reaches operating speed, the rings are shorted.; Star connection is standard because it facilitates the insertion of balanced resistance across all three phases.

Advantages: High starting torque capability; Reduced starting current draw from the supply; Speed control through external resistance

Disadvantages: Higher maintenance due to brushes and slip rings; Lower efficiency during normal operation if external resistance is not completely bypassed; Higher initial cost compared to squirrel cage induction motors

Applications: Cranes and hoists; Elevators; Compressors requiring high starting torque

Additional Info: Option B refers to squirrel-cage rotors which use bars and end rings, not windings.. Delta connection is theoretically possible but practically avoided in rotors as it makes the introduction of external resistance complex and unbalanced.

Answer: C is correct — The rotor of a slip-ring induction motor is always wound with a 3-phase, star (Y) connected winding to allow for external resistance connection.

Exam Tip: Remember that while squirrel-cage motors are robust, slip-ring motors are the go-to choice for high-inertia loads where starting current must be strictly controlled.

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Option A: $\Delta$ connected 3-$\Phi$ winding

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Option B: rotor bare with end rings

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Option D: Any of these

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