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316 klCIl.-lRDSON'S BLUEBOOK OF PROJECTION
The Commutator
The careful projectionist will pay close attention to the generator commutator. He will study its requirements and if something seems wrong will attend to it immediately. Sparking cannot always be eliminated entirely, especially if the machine is working close to capacity, but it should be reduced as far as possible. The instructions that follow are of great help.
(133) When there is little or no sparking and the surface is smooth, glazed and of a dark brown color it is the best evidence that the commutator is in the finest possible condition. If the condition is good there will probably be a slight squeak from the brushes when the armature is rotated slowly, though its absence does not necessarily indicate that anything is wrong.
Yoke Mark
(134) On all modern types of motor-generator sets the correct setting of the brush yoke is indicated by a mark on the yoke and another on the frame. These should be exactly opposite each other. This setting does not contemplate a continuous overload or underload. If that is the case the brushes may need advancing or retarding.
To Test Brush Pressure
(135) An experienced man can test brush pressure with sufficient accuracy by raising the brush finger or spring. If the commutator has little or no sparking and the proper glaze, it is best not to touch it at all. For the brushes usually found on a commutator used for dynamos of this type a pressure of from one and one-quarter to one and one-half pounds per square inch of brush surface is approximately correct. A small spring balance will measure such a pull quite accurately. This may be hooked to the brush holder finger or spring and the latter raised just out of contact.
(136) If at this point — when the finger is worked up and down by raising and lowering the scale — it is found that there is a wide difference in the scale of reading,