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A synchronous machine is
single excited machine
doubly excited machine
Made to run at a speed less than NsтАЛ
Generally a lagging power factor machine
doubly excited machine
A synchronous machine is classified as a doubly excited machine because it requires two separate sources of electrical excitation to function. The stator winding is connected to an AC supply, while the rotor winding is energized by an independent DC source to create the main magnetic field.
A synchronous machine is classified as a doubly excited machine because it requires two separate sources of electrical excitation to function. The stator winding is connected to an AC supply, while the rotor winding is energized by an independent DC source to create the main magnetic field.
NsтАЛ=P120fтАЛ тАФ Synchronous speed formula
EbтАЛ=4.44f╬жTkwтАЛ тАФ Induced EMF in stator winding
In a synchronous machine, the interaction between the stator's rotating magnetic field (produced by AC) and the rotor's stationary (relative to rotor) magnetic field (produced by DC) creates the torque. Because both the stator and the rotor are actively energized by external electrical inputs to facilitate magnetic coupling, it is termed 'doubly excited'. In contrast, induction machines are typically singly excited as the rotor current is induced via electromagnetic induction.
Synchronous machines are doubly excited; DC is supplied to the rotor via slip rings or a brushless exciter.
The machine operates at a constant synchronous speed (NsтАЛ) determined by frequency and pole count.
The excitation level of the rotor can control the power factor of the machine (over-excited, under-excited, or normal).
It acts as a synchronous generator (alternator) or a synchronous motor.
Ability to operate at leading, lagging, or unity power factor.
Constant speed operation regardless of load (within pull-out torque limits).
Requires an auxiliary DC source for rotor excitation.
Not self-starting as a motor (requires starting techniques like damper windings).
Large-scale power generation in thermal and hydro power plants.
Power factor correction using synchronous condensers.
Singly excited machines include transformers, DC series motors, and induction motors, where energy conversion relies on induced currents in the secondary/rotor.
Option C is incorrect because synchronous machines operate exactly at synchronous speed (NsтАЛ).
Option D is incorrect because the power factor can be varied by changing the DC excitation (leading or lagging).
B is correct тАФ A synchronous machine is classified as a doubly excited machine because it requires two independent sources of electrical supply: AC for the stator and DC for the rotor field windings.
Always remember: Doubly excited = Synchronous machines; Singly excited = Induction machines and Transformers. This is a favorite concept for identifying machine types in GATE and ESE.