The working of synchronous motor is similar to

Question

Examoogle · Accepted Answer

transmission of mechanical power by shaft

Explanation: A synchronous motor acts like a rigid mechanical shaft coupling between the stator's rotating magnetic field and the rotor. Just as two gears or a stiff shaft transmit power with a fixed speed relationship, the rotor of a synchronous motor is magnetically locked to the stator's flux, rotating at exactly the synchronous speed $N_s$.

Working Principle: The synchronous motor operates based on the principle of magnetic interlocking. The stator creates a rotating magnetic field (RMF) at synchronous speed $N_s = \frac{120f}{P}$. The rotor, excited by DC, acts as a permanent magnet and locks onto the RMF; if a mechanical load is applied, the rotor angle (torque angle) shifts while maintaining the same average speed, similar to the tension in a loaded physical drive shaft.

Key Formulas: $N_s = \frac{120f}{P}$ — Synchronous speed formula; $\delta$ — Torque angle representing the 'twist' between stator and rotor

Key Points: Synchronous motors run at constant speed regardless of load variations, within the pull-out torque limit.; The rotor speed is fixed to the frequency of the supply grid.; Analogous to a mechanical shaft where the torque angle $\delta$ represents the elastic displacement under load.

Advantages: Constant speed operation; Power factor improvement capability

Disadvantages: Not self-starting (requires damper windings or pony motor); Requires DC excitation

Applications: Power factor correction; High-power industrial drives (pumps, compressors)

Additional Info: The 'shaft analogy' holds because if the load exceeds the pull-out torque, the magnetic lock breaks (similar to a shaft shearing).. Option A is incorrect as gear trains allow speed ratios; Option C involves mutual induction without constant synchronous speed locking; Option D is a prime mover, not a power-transmission analogy.

Answer: B is correct — The synchronous motor operates via magnetic locking, providing a rigid link between the rotating stator field and the rotor, effectively behaving like a direct mechanical shaft drive.

Exam Tip: Always remember that the torque angle $\delta$ in a synchronous machine acts as the 'spring constant' in the shaft analogy; as $\delta$ increases, the machine produces more torque until the stability limit is reached.

Related Questions

Discussion (0)