ELECTRICAL EQUIPMENT—Q.7
2.
Fit the commutator end bracket over the
commutator and release the brushes.
3.
Hold back each of the brush springs and move
the brush by pulling gently on its flexible
connector. If the movement is sluggish, remove
the brush from its holder and ease the sides by
lightly polishing on a smooth file. Always refit
the brushes in their original positions. If the
brushes are badly worn, new brushes must be
fitted and bedded to the commutator. The
minimum permissible length of brush is 9/32 in
(7,14 mm), i.e. when the spring arm reaches the
brush box.
4.
Measure the brush spring pressures, using a
spring balance held radially to the commutator.
The tension on the springs should be 30 ozf
(0,85 kgf), maximum, when exerted on a new
brush and 13 ozf (0,37 kgf), minimum, on a
brush worn to 9/32 in (7,14 mm). Both
pressures should be measured and defective
springs replaced.
(d)
Commutator
1.
A commutator in good condition will be smooth
and free from pits and burned spots. Two types
of commutator, the moulded and the fabricated,
will be found in service. Moulded commutators
can be recognized by the exposed end being
quite smooth, unlike that of the fabricated
commutator from which a metal roll-over and an
insulating cone protrude (see Fig. Q.3).
2.
A moulded commutator can be re-skimmed
during service but care must be exercised to
ensure that the finished diameter is not less
Fig. Q.3.
Comparison of Fabricated and Moulded Commutators.
Fig. Q.4.
Undercutting Insulators.
than 1.450 in (36,83 mm). The process of re-
skimming
consists
of
rough
turning
(if
necessary)
followed
by
diamond
turning.
Whether or not rough turning is carried out
depends upon the severity and unevenness of
the wear which has taken place. A moulded
commutator requires no undercutting in
service, the production undercut being of
sufficient depth to obviate any further need for
this. The insulation slots, however, muss be
kept clear of copper and carbon residue.
3.
To remedy a worn fabricated commutator,
undercut the insulators between the segments to
a depth of 1,32 in (0,79 mm) using a hacksaw
blade ground to the thickness of the insulator,
then mount the armature, with or without the
drive end bracket, in a lathe, rotate at high
speed and take a light cut with a very sharp tool.
Do not remove more metal than is necessary.
Polish the commutator with very fine glass
paper.
(e)
Armature
Indication of an open-circuited armature winding
will be given by burnt commutator segments. If
armature testing facilities are not available, an
armature can be checked by substitution.
