Illus

TESTING ARMATURE FOR "SHORT"

A strong flash should be seen as contact is broken. No flash or a very weak flash indicates an open circuit.

Repeat this test between all segments, turning armature so that each test is made on the line of contact between armature core and "growler" V. If an open circuit is found check for loose or broken wires at commutator connections. If none are found that can be repaired, armature must be replaced with a new one.

ILLUS. 92 TESTING ARMATURE FOR "OPEN"

Turning Down Commutator

If commutator is found worn and irregular, it should be turned down in a lathe and smoothed with No. 00 sandpaper. When turning, mount armature shaft on its bearing seats; do not mount on shaft centers.

Undercutting Commutator

After commutator has been turned down, the mica insulation between segments must be undercut to a depth of approximately .025". Unless mica is properly undercut, brushes will not seat firmly against commutator segments and generator output will not be normal. Also, there will be excessive arcing at brushes.

Undercutting is usually done with a special undercutting machine. However, if such a machine is not available, it can be done as shown in Dlus. 93.

STARTING GROOVE IN MICA UNDERCUTTING MICA WITH PIECE

WITH 3 CORNERED FILE OF HACKSAW BLADE

SEGMENTS ^ / MICA

STARTING GROOVE IN MICA UNDERCUTTING MICA WITH PIECE

WITH 3 CORNERED FILE OF HACKSAW BLADE

SEGMENTS ^ / MICA

WRONG WAY RIGHT WAY

MICA MUST NOT BE LEFT MICA MUST BE CUT AWAY

WITH A THIN EDGE NEXT CLEAN BETWEEN SEGMENTS

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