The speed of synchronous motor

Question

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Option C: always remain constant

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

Working Principle: The stator windings, when energized by a 3-phase supply, produce a rotating magnetic field (RMF) that rotates at the synchronous speed $N_s = \frac{120f}{P}$. The rotor is excited by a DC source creating fixed magnetic poles. Due to magnetic locking between the stator RMF and the rotor poles, the rotor is compelled to run at exactly $N_s$. Even if load varies, the motor maintains this average speed by shifting its rotor angle (load angle), failing only if the torque exceeds the pull-out torque limit.

Key Formulas: $N_s = \frac{120f}{P}$ — Synchronous speed equation; $\omega_s = \frac{2\pi f}{P/2}$ — Angular synchronous velocity

Key Points: The rotor speed is strictly synchronized with the stator supply frequency.; Variations in mechanical load result in changes to the torque angle (load angle) $\delta$, not the speed.; If the load exceeds the maximum pull-out torque, the motor loses synchronism and stalls (pole slipping).; It operates at a constant speed from no-load to full-load conditions.

Advantages: Constant speed regardless of load variations; Capability of power factor correction (over-excited operation)

Disadvantages: Not self-starting (requires auxiliary starting method); Requires DC excitation for the rotor

Applications: Power factor correction units (Synchronous Condensers); Constant speed industrial drives; Large capacity centrifugal pumps and compressors

Additional Info: The synchronous motor is categorized as a 'constant speed motor'.. Options A and B suggest variable speed behavior typical of DC shunt or induction motors, which is incorrect for synchronous machines.

Answer: C is correct — The synchronous motor maintains a constant speed equal to the synchronous speed regardless of the load applied, until the pull-out torque is exceeded.

Exam Tip: In competitive exams, remember that synchronous motors are constant-speed devices, whereas induction motors exhibit 'slip' which causes speed to drop as load increases.

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Option A: increases as the load increases

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Option B: decreases as the load decreases

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Option D: none of the above

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