The synchronous machine is also called a doubly excited machine because

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Option D: Both its rotor and stator are excited

Explanation & Concepts:
The synchronous machine is termed a doubly excited machine because both its rotor and stator experience excitation. This dual excitation allows for control over its magnetic field and improves performance characteristics.

Working Principle: In a synchronous machine, the rotor is excited by direct current supplied to the rotor winding, creating a magnetic field. Simultaneously, the stator winding is supplied with alternating current, producing a rotating magnetic field. The interaction between these two magnetic fields facilitates torque production and enables synchronous operation.

Key Points: The rotor and stator require different types of excitation.; The ability to control the magnetic fields aids in the machine's performance.

Advantages: High efficiency in energy conversion.; Ability to operate at a constant speed under varying loads.

Disadvantages: Complex construction and regulation.; Higher initial cost compared to other machines.

Applications: Used in power plants for energy generation.; Employed in synchronous converters for energy conversion.

Additional Info: Standard synchronous machines operate at fixed speeds defined by the supply frequency.. Option A is incorrect because over-excitation is a condition rather than a reason for the term 'doubly excited'.

Answer: D is correct — both the rotor and the stator require excitation in synchronous machines, which is why they are called doubly excited machines.

Exam Tip: In competitive exams, focus on the differences between types of electrical machines to better understand their unique features.

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Option A: It can be over-excited

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Option B: It needs twice the normal exciting current

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Option C: It has two sets of rotor poles

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