Discussion
The critical field resistance of a shunt generator is the value at which
Answer: C
Correct answer: the field resistance line becomes tangent to the magnetisation curve
The field resistance line is a straight line through the origin whose slope is the field circuit resistance. As the resistance is increased the line steepens, and at the critical value it just touches the initial straight part of the magnetisation curve. Below that value the line cuts the curve and the machine builds up; above it there is no intersection except the origin, and the machine fails to excite. There is a corresponding critical speed, since lowering the speed depresses the magnetisation curve to meet a given resistance line.
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