In Synchronous motor, The back emf in the stator is depends on

Question

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Option B: DC excitation

Explanation & Concepts:
The correct answer is B (DC excitation). In a synchronous motor, the back emf generated in the stator is primarily dependent on the level of DC excitation provided to the rotor. This excitation influences the magnetic field strength, which in turn affects the back emf.

Working Principle: The back emf in a synchronous motor is produced due to the rotation of the rotor within the stator's magnetic field. The strength of this magnetic field, generated by the DC excitation, is critical because it determines the magnitude of the emf induced in the stator windings according to Faraday's law of electromagnetic induction.

Key Formulas: $E_b = k \Phi N \omega$ — where $E_b$ is the back emf, $k$ is a constant, $\Phi$ is the magnetic flux, $N$ is the number of turns, and $\omega$ is the angular velocity.; $\Phi \propto I_{DC}$ — the magnetic flux is proportional to the DC current in the rotor.

Key Points: The back emf in synchronous motors is influenced by the rotor's DC excitation level.; Higher excitation increases the back emf, which aids in maintaining synchronous speed.

Advantages: Synchronous motors provide high efficiency and good power factor.; They maintain constant speed under varying load conditions.

Disadvantages: Complex starting procedures are necessary due to the synchronous nature.; They require a separate DC source for excitation.

Applications: Power factor correction in industrial applications.; Used in synchronous condensers and in high-load applications.

Additional Info: DC excitation in synchronous motors can be varied to control the output power and reactive power.. Option A is incorrect as the speed of the rotor does not directly affect back emf; rather it is constant in synchronous operation.

Answer: B is correct — Back emf is dependent solely on DC excitation provided to the rotor.

Exam Tip: Remember that the performance of synchronous motors is largely influenced by their excitation method, which can be a significant exam topic.

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Option A: Speed of the rotor

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Option C: Load on the motor

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Option D: both Speed of the rotor & DC excitation

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