In Which condition the Rotor voltage between Slip rings is maximum?

Question

Examoogle · Accepted Answer

Option B: $S=1$ (Stand Still)

Explanation & Concepts:
The rotor induced EMF in an induction motor is directly proportional to the slip (S). At standstill (S=1), the relative speed between the stator rotating magnetic field and the stationary rotor conductors is maximum, resulting in maximum flux linkage rate of change and thus maximum induced voltage across the slip rings.

Working Principle: According to Faraday's law of electromagnetic induction, $E_2 = S 	imes E_20$, where $E_20$ is the induced EMF at standstill. Since $S=1$ at standstill, the induced EMF is at its standstill value ($E_20$). As the motor speeds up towards synchronous speed ($S 	o 0$), the relative speed decreases, causing the induced voltage in the rotor windings to decrease proportionally.

Key Formulas: $E_r = S \cdot E_20$ — rotor induced EMF formula; $S = \frac{N_s - N_r}{N_s}$ — slip calculation

Key Points: At standstill, the rotor is stationary ($N_r = 0$), hence slip $S = 1$.; The frequency of the induced rotor voltage is also maximum at standstill ($f_r = S \cdot f = f$).; As the motor accelerates, $S$ decreases, reducing both rotor frequency and induced voltage.

Advantages: High starting torque due to maximum induced EMF.; Allows for external rotor resistance insertion in slip-ring induction motors.

Disadvantages: High inrush current at standstill ($S=1$).; Thermal stress on windings due to high induced voltage and current.

Applications: Slip-ring induction motor starters.; Variable speed drives using rotor impedance control.

Additional Info: S=0 refers to the rotor running at synchronous speed, where relative motion is zero, making induced EMF zero.. For $0

Answer: B is correct — At standstill ($S=1$), the relative speed between the stator field and rotor is at its maximum, leading to maximum induced voltage.

Exam Tip: Remember that rotor frequency $f_r = S \cdot f$ follows the same logic as rotor voltage; both are maximum at startup.

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Option A: $S=0$ (Synchronous Speed)

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Option C: $0

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Option D: All of these

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