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Chapter 1 of 12 тАв Page 1 of 248ЁЯФТ Protected PDF тАв Watermarked
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In which of the following motors the stator and rotor magnetic field rotate at the same speed?
Universal motor
Synchronous motor
Induction motor
Reluctance motor
Synchronous motor
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.
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.
NsтАЛ=P120fтАЛ тАФ Relation between speed, frequency, and poles
NrтАЛ=NsтАЛ тАФ Condition for synchronous operation
The stator is supplied with a 3-phase AC supply which produces a rotating magnetic field (RMF) at a synchronous speed NsтАЛ=P120fтАЛ. 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.
Synchronous motors operate at exactly the synchronous speed NsтАЛ.
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.
High efficiency at high power ratings
Power factor can be adjusted by varying DC excitation
Not self-starting (requires damper windings or starting methods)
Higher initial cost and complex maintenance
Power factor correction (Synchronous Condensers)
High power industrial drives (e.g., pumps, compressors)
| Feature | Induction Motor | Synchronous Motor |
|---|---|---|
Speed Type | Variable (Slip-dependent) | Constant (Synchronous) |
In induction motors, the rotor always runs at a speed NrтАЛ<NsтАЛ to produce torque, a difference known as 'slip'.
Universal motors operate on both AC and DC and function essentially as series-wound DC motors, featuring variable speed characteristics.
B is correct тАФ In a synchronous motor, the rotor speed is identical to the synchronous speed of the stator's rotating magnetic field.
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.