Which action causes the failure of voltage buildup in a self-excited DC generator?
Increasing the field resistance beyond the critical resistance
A self-excited DC generator can build up voltage only when a definite set of conditions is satisfied, and removing or violating any one of them prevents the build-up.
The requirements are: some residual magnetism in the poles to start with, the field connected so that its current strengthens that residual flux, rotation at the proper speed, and a field-circuit resistance below a limit called the critical resistance.
Build-up happens where the machine's open-circuit magnetisation curve intersects the straight field-resistance line at a high voltage; the slope of that line is set by the field resistance.
If the field resistance is increased beyond the critical value, the resistance line becomes steeper than the initial part of the magnetisation curve, so the two meet only close to the origin and the voltage cannot rise.
Therefore increasing the field resistance past the critical resistance is the action that causes voltage build-up to fail.
By contrast, maintaining residual magnetism, connecting the load only after the voltage has built up, and running at rated speed all support build-up rather than block it.
Hence, the answer is Increasing the field resistance beyond the critical resistance.
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