If a synchronous motor fails to pull into synchronism after applying dc field current, the main cause is

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Option D: Low field current

Explanation & Concepts:
The correct answer is D) Low field current. A synchronous motor requires a specific amount of field current to establish sufficient magnetic flux for synchronization with the stator magnetic field.

Working Principle: When the field current is low, the resulting magnetic field is weak, which fails to synchronize the rotor with the rotating magnetic field of the stator. This weak field leads to inadequate torque to pull the rotor into synchronism.

Key Formulas: $T = K \Phi I_a$ — where $T$ is torque, $K$ is a constant, $\Phi$ is magnetic flux, and $I_a$ is armature current.; $\Phi \propto I_f$ — where $\Phi$ is the magnetic flux and $I_f$ is the field current.

Key Points: Synchronous motors operate with a constant speed defined by the supply frequency.; The ability to pull into synchronism depends on the strength of the magnetic field generated by the DC field current.

Advantages: High efficiency; Ability to operate at power factor control

Disadvantages: Requires a starting mechanism; Sensitive to changes in load

Applications: Used in synchronous generators; Used in industrial applications for precise speed control

Additional Info: A typical value for synchronous motor field current may range from 10% to 200% of its rated value depending on the design.. Option A is incorrect because a low short circuit ratio does not directly cause a failure to achieve synchronism. Option B is incorrect since high field current generally increases the strength of the rotor field, aiding synchronization. Option C regarding high core losses is also inaccurate; core losses do impact efficiency but do not directly affect synchronism.

Answer: D is correct — low field current fails to provide enough magnetic force for synchronization.

Exam Tip: Understanding the interplay between field current and torque generation is crucial for many electrical machine questions.

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Option A: Low short circuit ratio

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Option B: High field current

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Option C: High core losses

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