In the design of a wound rotor three-phase induction motor. Which of the following is the limiting factor whil

Question

In the design of a wound rotor three-phase induction motor. Which of the following is the limiting factor while defining the rotor numbers of turns?

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Option D: Maximum Rotor voltage between Slip rings

Explanation & Concepts:
In a wound rotor induction motor, the rotor winding is connected to external resistors via slip rings. The number of turns per phase in the rotor is primarily limited by the maximum allowable voltage between the slip rings during starting or under open-circuit conditions to prevent insulation failure and arc-over.

Working Principle: The induced EMF in the rotor at standstill is given by $E_2 = 4.44 f_2 N_2 \Phi_m$. Since the magnetic flux $\Phi_m$ is determined by the stator design and the rotor frequency $f_2$ at standstill is equal to the supply frequency $f_1$, the only variable controlling the open-circuit rotor voltage $E_2$ is the number of turns $N_2$. Exceeding a safe voltage limit would necessitate excessive insulation and risk breakdown of the slip ring assembly.

Key Formulas: $E_2 = 4.44 f_1 N_2 \Phi_m$ — Standstill EMF induced in the rotor phase; $V_{ring} = \sqrt{3} E_2$ — Voltage between slip rings in a star-connected rotor

Key Points: The rotor voltage at standstill is the secondary induced EMF.; High rotor turns increase $E_2$, which could stress the insulation of the slip ring and brush gear.; Insulation class and slip ring spacing dictate the voltage ceiling.; Rotor slots are constrained by manufacturing and magnetic loading, not just turn count.

Advantages: Allows speed control by adding external resistance.; Provides high starting torque with low starting current.

Disadvantages: Higher maintenance due to brushes and slip rings.; Increased cost compared to squirrel cage induction motors.

Applications: Cranes and hoists; Compressors requiring high starting torque

Additional Info: Rotor frequency at standstill is identical to the supply frequency.. Option B (rotor frequency) is not a design limit as it is fixed by the supply at standstill.. Option C (rotor impedance) is a design outcome, not a constraint on the number of turns.

Answer: D is correct — The maximum voltage induced at the slip rings at standstill must be limited to prevent insulation breakdown and electrical arcing.

Exam Tip: Always remember that in electrical machine design, voltage limits are primarily determined by insulation capabilities, whereas current limits are determined by thermal considerations (I^2R losses).

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Option A: numbers of rotor slot

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Option B: rotor frequency

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Option C: rotor impedance

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