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Chapter 1 of 12 тАв Page 1 of 248ЁЯФТ Protected PDF тАв Watermarked
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The working of synchronous motor is similar to
gear train arrangement
transmission of mechanical power by shaft
distribution transformer
turbine
transmission of mechanical power by shaft
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 NsтАЛ.
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 NsтАЛ.
NsтАЛ=P120fтАЛ тАФ Synchronous speed formula
╬┤ тАФ Torque angle representing the 'twist' between stator and rotor
The synchronous motor operates based on the principle of magnetic interlocking. The stator creates a rotating magnetic field (RMF) at synchronous speed NsтАЛ=P120fтАЛ. 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.
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 ╬┤ represents the elastic displacement under load.
Constant speed operation
Power factor improvement capability
Not self-starting (requires damper windings or pony motor)
Requires DC excitation
Power factor correction
High-power industrial drives (pumps, compressors)
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.
B is correct тАФ 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.
Always remember that the torque angle ╬┤ in a synchronous machine acts as the 'spring constant' in the shaft analogy; as ╬┤ increases, the machine produces more torque until the stability limit is reached.