In the design of a wound rotor three-phase induction motor. The number of turns on rotor winding can be calcul

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In the design of a wound rotor three-phase induction motor. The number of turns on rotor winding can be calculated by assuming proper

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The correct answer is Option B: voltage between the slip rings. In a wound rotor induction motor, the rotor winding design is primarily governed by the target open-circuit voltage at the slip rings when the rotor is stationary. This voltage must be compatible with the external starting rheostats or secondary control equipment used for speed and torque regulation.

Step-by-Step Solution & Concepts:
In a wound rotor induction motor, the rotor winding design is primarily governed by the target open-circuit voltage at the slip rings when the rotor is stationary. This voltage must be compatible with the external starting rheostats or secondary control equipment used for speed and torque regulation.

Working Principle: The stator behaves like the primary of a transformer, inducing an EMF in the rotor winding. According to the transformer EMF equation, $E_2 = 4.44 f_2 N_2 \Phi_m K_w$. The number of turns $N_2$ is selected such that the induced rotor voltage $E_2$ matches the required rating for the connected external resistance starters, ensuring safe operation and effective starting torque control.

Key Formulas: $E_2 = 4.44 f_2 N_2 \Phi_m K_w$ — Fundamental EMF equation for the rotor winding; $E_2 = s E_{2(standstill)}$ — Induced EMF at slip 's'

Key Points: The rotor winding is designed to have a manageable slip ring voltage for insulation and safety reasons.; Higher rotor voltages require higher insulation levels for the slip rings and brushes.; Lower rotor voltages result in very high rotor currents, requiring thicker conductors and larger slip rings.

Advantages: External resistance allows for improved starting torque.; Allows for speed control through secondary resistance.; Reduces starting current drawn from the supply.

Disadvantages: Higher maintenance due to brushes and slip rings.; Increased cost compared to squirrel cage motors.; Lower efficiency due to power loss in external resistors.

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

Additional Info: The open-circuit voltage at the slip rings is typically kept within a reasonable range (usually 100V-600V) depending on motor rating.. Option A is incorrect because slot number is a mechanical/geometric design choice, not the primary electrical constraint for winding turn calculation.. Option C is incorrect as the current is a result of the design parameters and load, not a primary independent variable for turns calculation.

Answer: B is correct — The number of turns in the rotor winding is determined primarily to ensure the induced voltage at the slip rings matches the ratings of the external starting equipment.

Exam Tip: In transformer and motor design questions, always prioritize the induced EMF formula ($E \propto N$) as the primary constraint for winding turn calculations.

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Option A: numbers of rotor slot

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Option C: rotor current

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Option D: All of these

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