At the instant of starting, the per unit slip of a 3-phase induction motor is

1️⃣ 0.05

2️⃣ 0.1
3️⃣ 1.0
4️⃣ 0.5

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✅ Answer (Detailed Solution Below)

✔️ Option 3: 1.0

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📖 Detailed Solution

🔷 Concept of Slip in Induction Motor

In a 3-phase induction motor:

✔️ The stator magnetic field rotates at synchronous speed (Ns)
✔️ The rotor rotates at speed (N)
✔️ The difference between Ns and N is called slip speed

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🔷 Slip Formula

Fractional slip is defined as:

$$s=\frac{N_s-N}{N_s}$$

Percentage slip:

$$\mathrm{\%}s=\left(\frac{N_s-N}{N_s}\right)\times 100$$

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🔷 Condition at Starting

At the instant of starting:

✔ Rotor speed N = 0
✔ Synchronous speed Ns = Maximum

Substitute in slip formula:

$$s=\frac{N_s-0}{N_s}$$$$s=\frac{N_s}{N_s}$$$$s=1$$

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🎯 Per Unit Slip at Start = 1.0 (or 100%)

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🔎 Important Understanding

✔ At start → Slip = 1
✔ At normal running → Slip ≈ 0.01 to 0.05 (1–5%)
✔ At synchronous speed → Slip = 0

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❌ Why Other Options Are Incorrect

❌ 1️⃣ 0.05

This corresponds to 5% slip during normal operation, not at start.

❌ 2️⃣ 0.1

This corresponds to 10% slip, not starting condition.

❌ 4️⃣ 0.5

This would mean rotor speed is half of synchronous speed — not starting.

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⭐ Important Exam Points

✔ Slip is maximum at starting
✔ Slip decreases as rotor accelerates
✔ Slip must exist for torque production
✔ At Ns → Slip = 0 → Torque = 0

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📌 Key One-Line Exam Statement ⭐

At the instant of starting, the rotor speed is zero, therefore the per unit slip of an induction motor is 1.0.

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📝 Final Conclusion

Since rotor speed is zero at starting,

$$\boxed{{\displaystyle s=1}}$$

👉 Correct Answer: Option 3 (1.0)