Short circuit characteristic plots between

Question

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Option C: Armature and field current

Explanation & Concepts:
The Short Circuit Characteristic (SCC) of an alternator is defined as the curve representing the relationship between armature current ($I_a$) and field current ($I_f$) when the alternator terminals are short-circuited.

Working Principle: In the SCC test, the alternator is driven at a constant rated speed with a short-circuit across its armature terminals. The field current is increased in steps, and the corresponding armature current is recorded. Because the armature reaction is purely demagnetizing (due to the short-circuit current lagging the induced emf by nearly $90^\circ$), the graph is essentially a straight line passing through the origin.

Key Formulas: $I_a \propto I_f$ — Linear relationship under unsaturated conditions; $Z_s = \frac{E_{oc}}{I_{sc}}$ — Synchronous impedance calculation using SCC data

Key Points: The SCC is essentially a straight line because the iron core is unsaturated due to the demagnetizing effect of the armature reaction.; The test is performed at rated frequency by keeping the prime mover speed constant.; SCC is used in conjunction with the Open Circuit Characteristic (OCC) to determine the synchronous impedance of the machine.

Advantages: Used to determine the synchronous reactance ($X_s$); Helps in calculating the voltage regulation of the alternator

Disadvantages: Requires high current capability in the armature windings during testing; Does not account for saturation effects directly

Applications: Synchronous machine performance analysis; Determining Potier Reactance

Additional Info: The SCC is one of the two fundamental curves (the other being OCC) required for the EMF method of regulation.. Option A refers to the Open Circuit Characteristic (OCC), not the Short Circuit Characteristic.

Answer: C is correct — The Short Circuit Characteristic is specifically defined as the plot of armature current versus field current at a constant rated speed.

Exam Tip: Always remember: OCC uses voltage vs field current (saturation dependent), whereas SCC uses current vs field current (saturation independent).

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Option A: Voltage and field current

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Option B: Voltage and impedance

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Option D: None of above

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