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(3.1)

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The open-circuit characteristics of a seperately-excited DC generator are given in a table.

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Plot the open-circuit characteristics with the values in the table.

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Graph:

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Draw the coordinate plane.

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Plot the points from the table.

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Connect the points to a smooth curve.

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The graph shows the open-circuit characteristics of a seperately-excited DC generator.

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Step 2 :

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(3.1.1)

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Find the open-circuit voltage if the field circuit resistance is 8 ohms.

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Draw a 8 ohms resistance line on open-circuit characteristics curve.

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Consider the points (0, 0) and (1, 8) with a slope of 8.

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Now draw a line connecting (0, 0) and (1, 8) on open-circuit characteristics curve.

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Observe the graph:

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The 8 ohm resistance line intersects the open-circuit characteristics curve at (2, 16).

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Therefore the open-circuit voltage when the field circuit resistance 8 ohms is 16 V.

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Step 3 :

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(3.1.2)

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Find the open-circuit voltage if the field circuit resistance is 8 ohms.

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Draw a resistance line on open-circuit characteristics curve such that it is tangent line to the curve at origin.

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Observe the graph:

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The line resistance represents critical resistance line.

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Critical resistance is the slope of the tangent line.

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Slope of the tangent line is .

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Therefore the critical resistance is 24 ohms.

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Solution :

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The critical resistance is 24 ohms.

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