In which of the following motors the stator and rotor magnetic field rotate at the same speed?

Question

Examoogle · Accepted Answer

Synchronous motor

Explanation: In a synchronous motor, the rotor magnetic field locks with the rotating stator magnetic field. Because they rotate at the same synchronous speed, the motor operates at a constant speed regardless of the load, provided the load does not exceed the pull-out torque.

Working Principle: The stator is supplied with a 3-phase AC supply which produces a rotating magnetic field (RMF) at a synchronous speed $N_s = \frac{120f}{P}$. The rotor is fed by a DC supply, creating fixed magnetic poles. Due to magnetic interlocking between the stator's RMF and the rotor's poles, the rotor is pulled at the same speed as the RMF.

Key Formulas: $N_s = \frac{120f}{P}$ — Relation between speed, frequency, and poles; $N_r = N_s$ — Condition for synchronous operation

Key Points: Synchronous motors operate at exactly the synchronous speed $N_s$.; They require a DC excitation source for the rotor field.; The rotor poles lock onto the stator magnetic field poles.; They are constant speed machines regardless of load.

Advantages: High efficiency at high power ratings; Power factor can be adjusted by varying DC excitation

Disadvantages: Not self-starting (requires damper windings or starting methods); Higher initial cost and complex maintenance

Applications: Power factor correction (Synchronous Condensers); High power industrial drives (e.g., pumps, compressors)

Additional Info: In induction motors, the rotor always runs at a speed $N_r. Universal motors operate on both AC and DC and function essentially as series-wound DC motors, featuring variable speed characteristics.

Answer: B is correct — In a synchronous motor, the rotor speed is identical to the synchronous speed of the stator's rotating magnetic field.

Exam Tip: Remember that the term 'synchronous' explicitly implies that the rotor follows the frequency of the supply perfectly, meaning the speed is rigidly tied to the grid frequency.

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