Synchronous motor always runs at

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Option A: the synchronous speed

Explanation & Concepts:
A synchronous motor is a constant speed motor because its rotor rotates at the same speed as the rotating magnetic field produced by the stator. This speed is known as the synchronous speed ($N_s$), which is determined strictly by the supply frequency and the number of poles of the motor.

Working Principle: The stator windings are supplied with a three-phase AC, creating a rotating magnetic field in the air gap at synchronous speed $N_s = \frac{120f}{P}$. The rotor, which is excited by a DC source, acts as a permanent magnet or electromagnet. The rotor poles lock onto the rotating magnetic field poles due to magnetic attraction, causing the rotor to be dragged along at the exact speed of the stator field.

Key Formulas: $N_s = \frac{120f}{P}$ — Synchronous speed formula where $f$ is frequency and $P$ is the number of poles.; $\omega_s = \frac{2\pi N_s}{60}$ — Synchronous angular velocity in rad/sec.

Key Points: Synchronous speed is independent of the load applied to the shaft.; If the load exceeds the pull-out torque, the motor loses synchronism and stops (stalls).; The rotor speed is fixed for a given frequency and pole configuration.

Advantages: Constant speed operation regardless of load; Ability to improve power factor by varying DC excitation

Disadvantages: Not self-starting (requires auxiliary starting methods); Requires both AC and DC supply sources

Applications: Power factor correction (Synchronous Condensers); High-power industrial drives like rolling mills and pumps

Additional Info: The synchronous speed is given by $N_s = \frac{120f}{P}$ RPM.. Option B refers to Induction motors (Asynchronous), where rotor speed $N_r. Option C is physically impossible for a standard synchronous motor under steady-state operation.

Answer: A is correct — A synchronous motor operates at a fixed speed, known as synchronous speed, determined by the supply frequency and the number of magnetic poles.

Exam Tip: Always remember that in induction motors, speed depends on slip ($s > 0$), whereas in synchronous motors, slip is exactly zero ($s = 0$) at steady state.

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Option B: less than synchronous speed

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Option C: more than synchronous speed

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Option D: none of above

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