When a dc machine develops some commutation sparking, the user may suspect that the commutating-pole air gap is not set correctly.

“Brush potential curves” are often taken to prove or disprove such suspicions. These are taken by measuring the voltage drops between the brush and commutator surface at four points while the machine is operating at constant speed and load current (see Fig. 8-34).

The voltages at 1, 2, 3, and 4 are taken by touching the pointed lead of a wooden pencil to the commutator surface. The circuit is completed with leads and a low-reading voltmeter is shown.

The voltages are then plotted. A curve such as A of Fig. 8-34 may indicate undercompensation due to a too large commutating-pole gap. Curve C may indicate overcompensation with too much flux density in the commutating-pole air gap.

Curve B is typical of good compensation. Justification for such conclusions is based on the theory that best commutation (coil current reversal) will be linear while the coil passes under the brush.

This is possible only if there are no circulating currents. Undercompensation should cause circulating currents that would crowd the current to the leaving edge of the brush and cause a high voltage at point 4.

Overcompensation would reverse the current too soon and would actually reverse the voltage drop at point 4. Even to an expert, this test is only an indicator that more definitive tests, such as a buck-boost test, are needed.

Many other factors, including brush riding, commutator surface conditions, and sparking, influence the readings. Where machine changes may be required, the manufacturer should be consulted.

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