Open circuit characteristic plots between

Question

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Option A: Voltage and field current

Explanation & Concepts:
The Open Circuit Characteristic (OCC) of an alternator is defined as the curve representing the relationship between the terminal voltage (generated EMF) and the field current, while the speed is kept constant and the armature circuit remains open. It is also known as the magnetic characteristic or saturation curve of the machine.

Working Principle: As the field current ($I_f$) increases, the flux in the magnetic circuit increases, leading to a rise in induced EMF ($E_0$). Initially, this relationship is linear (air-gap line), but as the magnetic core reaches saturation, the curve bends due to non-linearity in the B-H curve of the ferromagnetic material. The terminal voltage is given by $E_0 = \sqrt{2} \pi f N_ph \Phi K_w$.

Key Formulas: $E_0 = 4.44 f \Phi N_{ph} K_w$ — EMF equation of an alternator; $\Phi \propto I_f$ — Flux relationship in linear region

Key Points: The OCC is plotted with the alternator running at synchronous speed with the armature terminals open.; The linear portion of the curve is called the air-gap line.; Saturation occurs because of the magnetic properties of the iron core in the stator and rotor.; It is used to determine the synchronous impedance when combined with Short Circuit Characteristics (SCC).

Advantages: Used to predetermine the voltage regulation of an alternator.; Provides essential data regarding the magnetic circuit's excitation requirements.

Disadvantages: Does not account for armature reaction directly under load conditions.; Requires an external DC source for field excitation.

Applications: Synchronous generator testing; Analysis of machine excitation characteristics

Additional Info: The OCC test is essentially a magnetization test.. Option B (Voltage and impedance) relates to load characteristics; Option C (Armature and field current) describes the Short Circuit Characteristic (SCC).

Answer: A is correct — Open Circuit Characteristic (OCC) is the plot of generated voltage versus field current at constant rated speed with an open armature.

Exam Tip: Always remember that OCC relates EMF (or $V_{oc}$) to field current, while SCC relates $I_a$ to $I_f$ at zero terminal voltage.

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Option B: Voltage and impedance

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Option C: Armature and field current

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Option D: None of above

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