Correct AnswerA. Magnetic behaviour of the air gap of the DC machine
Quick Explanation
The air-gap line is the straight unsaturated reference line on a DC machine magnetization curve. It represents the magnetic behaviour when the reluctance is dominated by the air gap and iron saturation is neglected.
Formula / Key Relation
In the unsaturated region:
Φ ∝ If
E0 ∝ Φω
Air-gap line: linear E0–If reference.
Detailed Explanation
Concept & ReasoningOn the open-circuit characteristic of a DC generator, the initial portion is nearly linear because the iron is unsaturated. Extending this linear portion gives the air-gap line. It indicates the field current that would be required if the magnetic circuit behaved linearly, essentially representing the air-gap dominated mmf requirement. As saturation develops, the actual OCC bends below the air-gap line because additional field current produces progressively smaller flux increase.How to Solve It in the ExamAir-gap line = unsaturated straight-line reference of the OCC.Important Exam PointUse the air-gap line to visualize saturation and synchronous-reactance methods.Related Revision PathThis question sits…
Concept & Reasoning
On the open-circuit characteristic of a DC generator, the initial portion is nearly linear because the iron is unsaturated. Extending this linear portion gives the air-gap line. It indicates the field current that would be required if the magnetic circuit behaved linearly, essentially representing the air-gap dominated mmf requirement. As saturation develops, the actual OCC bends below the air-gap line because additional field current produces progressively smaller flux increase.
How to Solve It in the Exam
Air-gap line = unsaturated straight-line reference of the OCC.
Important Exam Point
Use the air-gap line to visualize saturation and synchronous-reactance methods.
Related Revision Path
This question sits in the revision path Electrical Machines → DC Machines → Open-Circuit Characteristic .
Quick Trick
Air-gap line = unsaturated straight-line reference of the OCC.
Common Mistake
Do not interpret it as an induction-motor-specific characteristic merely because the term 'air gap' is used.
Exam Tip
Use the air-gap line to visualize saturation and synchronous-reactance methods.
SEO + Knowledge GraphRelated PYQs & Exam ContextExam path, knowledge path and deeper practice links.
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