Synchronous speed is defined as the speed at which the

(1) stator magnetic field rotates
(2) rotor rotates on no load
(3) rotor rotates on full load
(4) none of the above

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✅ Correct Answer: (1) stator magnetic field rotates

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📘 Detailed Explanation

🔷 Concept of Synchronous Speed

Synchronous speed (Ns) is the speed at which the rotating magnetic field (RMF) produced by the stator windings rotates.

When a 3-phase AC supply is applied to the stator:

➤ A rotating magnetic field is produced
➤ This field rotates at a constant speed
➤ That speed is called synchronous speed

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🔷 Formula of Synchronous Speed

$$\boxed{{\displaystyle {\mathrm{<span\; style="font-size:\; medium;">N</span>}}_{\mathrm{<span\; style="font-size:\; medium;">s</span>}}<span\; style="font-size:\; medium;">=</span>\frac{\mathrm{<span\; style="font-size:\; medium;">120</span>}\mathrm{<span\; style="font-size:\; medium;">f</span>}}{\mathrm{<span\; style="font-size:\; medium;">P</span>}}}}$$

Where:

➤ Ns = Synchronous speed (rpm)
➤ f = Supply frequency (Hz)
➤ P = Number of poles

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🔷 Important Understanding

✔ The stator magnetic field always rotates at synchronous speed
✔ This speed depends only on frequency and number of poles
✔ It does NOT depend on load

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🔷 Rotor Speed vs Synchronous Speed

🔹 In an Induction Motor:

➤ Rotor speed (Nr) is always less than Ns
➤ Slip must exist for torque production
➤ Therefore, Nr < Ns

🔹 In a Synchronous Motor:

➤ Rotor rotates exactly at Ns
➤ No slip exists

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❌ Why Other Options Are Incorrect

❌ (2) Rotor rotates on no load

➤ At no load, rotor speed is very close to Ns
➤ But it is never exactly equal to Ns
➤ Therefore incorrect

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❌ (3) Rotor rotates on full load

➤ Under full load, rotor speed decreases further
➤ Slip increases
➤ So Nr is even less than Ns
➤ Hence incorrect

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❌ (4) None of the above

➤ Option (1) correctly defines synchronous speed
➤ So this option is incorrect

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🔎 Example for Better Clarity

If:

➤ Frequency = 50 Hz
➤ Poles = 4

Then:

$${\mathrm{<span\; style="font-size:\; medium;">N</span>}}_{\mathrm{<span\; style="font-size:\; medium;">s</span>}}<span\; style="font-size:\; medium;">=</span>\frac{\mathrm{<span\; style="font-size:\; medium;">120</span>}<span\; style="font-size:\; medium;">\times </span>\mathrm{<span\; style="font-size:\; medium;">50</span>}}{\mathrm{<span\; style="font-size:\; medium;">4</span>}}<span\; style="font-size:\; medium;">=</span>\mathrm{<span\; style="font-size:\; medium;">1500</span>}\text{<span style="font-size: medium;"> rpm</span>}$$

So, stator magnetic field rotates at 1500 rpm

But rotor speed may be:

➤ 1480 rpm (light load)
➤ 1450 rpm (full load)

But never exactly 1500 rpm (in induction motor).

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✨ One-Line Exam Statement

Synchronous speed is the speed at which the stator magnetic field rotates.

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🎯 Final Conclusion

Synchronous speed is defined as the rotational speed of the stator’s rotating magnetic field and is given by $N_s={\displaystyle \frac{120f}{P}}$.

✔ Correct Answer: (1) stator magnetic field rotates