When the value of slip of an induction motor approaches zero, the effective resistance

1️⃣ Is very low and motor is under no-load

2️⃣ Of the rotor circuit is very high and the motor is under no-load
3️⃣ Is zero
4️⃣ Of the rotor circuit is infinity and the motor is equivalent to short-circuited two-winding transformer

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✅ Correct Answer

✔ 2️⃣ Of the rotor circuit is very high and the motor is under no-load

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📘 Detailed Explanation

An induction motor develops torque because of slip between stator magnetic field and rotor.

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➤ Step 1: Slip Formula

Slip is given by:

$$\mathrm{<span\; style="font-size:\; medium;">s</span>}<span\; style="font-size:\; medium;">=</span>\frac{{\mathrm{<span\; style="font-size:\; medium;">N</span>}}_{\mathrm{<span\; style="font-size:\; medium;">s</span>}}<span\; style="font-size:\; medium;">-</span>{\mathrm{<span\; style="font-size:\; medium;">N</span>}}_{\mathrm{<span\; style="font-size:\; medium;">r</span>}}}{{\mathrm{<span\; style="font-size:\; medium;">N</span>}}_{\mathrm{<span\; style="font-size:\; medium;">s</span>}}}$$

Where
Ns ➝ Synchronous speed
Nr ➝ Rotor speed

When rotor speed approaches synchronous speed:

Nr ⟶ Ns
⟹ s ⟶ 0

This condition occurs during no-load operation.

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➤ Step 2: Effective Rotor Resistance

In the induction motor equivalent circuit:

$${\mathrm{<span\; style="font-size:\; medium;">R</span>}}_{\mathrm{<span\; style="font-size:\; medium;">2</span>}<span\; style="font-size:\; medium;">,</span>\mathrm{<span\; style="font-size:\; medium;">e</span>}\mathrm{<span\; style="font-size:\; medium;">f</span>}\mathrm{<span\; style="font-size:\; medium;">f</span>}}<span\; style="font-size:\; medium;">=</span>\frac{{\mathrm{<span\; style="font-size:\; medium;">R</span>}}_{\mathrm{<span\; style="font-size:\; medium;">2</span>}}}{\mathrm{<span\; style="font-size:\; medium;">s</span>}}$$

Where
R₂ ➝ Rotor resistance
s ➝ Slip

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➤ Step 3: When Slip Approaches Zero

If:

s ⟶ 0

Then:

R₂ / s ⟶ ∞

⟹ Effective rotor resistance becomes very high
⟹ Rotor current becomes very small
⟹ Induced EMF becomes very small
⟹ Developed torque becomes almost zero

Therefore motor runs under no-load condition.

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➤ Physical Meaning

At No Load:

✔ Rotor speed ≈ Ns
✔ Slip ≈ 0
✔ Rotor current ≈ 0
✔ Effective resistance ≈ Very high
✔ Only magnetizing current flows

Motor behaves almost like an open secondary of a transformer.

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✘ Why Other Options Are Incorrect

✘ 1️⃣ Very low resistance

When s ⟶ 0, R₂/s becomes very large, not small.

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✘ 3️⃣ Resistance is zero

Resistance never becomes zero.
It actually approaches very high value.

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✘ 4️⃣ Equivalent to short-circuited transformer

At s = 1 (starting), rotor behaves like short-circuited transformer.
At s ⟶ 0, rotor behaves almost like open circuit, not short circuit.

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★ Important Concept Table

Slip (s) = 1
⟹ R₂/s = R₂
⟹ Starting condition

0 < s < 1
⟹ Moderate resistance
⟹ Normal load

s ⟶ 0
⟹ R₂/s ⟶ ∞
⟹ No-load condition

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✍ Exam One-Line Concept

When slip approaches zero, the effective rotor resistance becomes very high and the induction motor operates under no-load condition.

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✅ Final Conclusion

Slip ⟶ 0
⟹ Rotor speed ≈ Synchronous speed
⟹ Relative speed ≈ 0
⟹ Rotor current ≈ 0
⟹ Torque ≈ 0
⟹ Effective rotor resistance ≈ Very high

✔ Correct Answer: 2️⃣ Of the rotor circuit is very high and the motor is under no-load.