Specifications

173
Brushes off neutral plane.
Brushes not seated
properly. Dirty brushes and
commutator High mica,
Rough or eccentric
commutator. Open in
armature
Grounded, open or shorted
field winding
Lack of brush pressure;
brushes sticking in holder
Selective commutation
caused by unequal brush
tension
Open circuit in field
Rheostat
Sparking at brushes6 Set rigging so that brushes
are in proper plane.
Reset.
Clean.
Under cut; Resurface
commutator, Repair/
replace armature.
Repair or replace defective
coil/coils.
Replace tension springs,
clean holders.
Adjust spring tension,
Replace brushes or shunts.
Replace or repair Rheostat.
15.13 Common defects in commutators
A good commutator will have a smooth polished surface dark chocolate brown in colour.
i) Commutator surface pitted and rough : This is mostly the result of excessive sparking,
The surface may be smoothened with fine sand paper on a curved block and polished
with crocus cloth. Never use emery paper or cloth, as the fine emery particles may get
to the commutator.
ii) Ridge formation on commutator : Ridges are formed if the brush holders on different
rocker arms are not properly staggered. Another possible cause in the case of large
armatures is, if the armature does not oscillate to the extent of about 1/8 in. axially on
either side of the core centre. The ridges may be filed off by a smooth file, or by means
of specially shaped commutator grinding stones, whose curvature corresponds to that
of the commutator. If it is a generator, remove all carbon brushes and run it up, If it is a
motor, run up to full speed, switch off the supply and quickly apply the grinding stone to
the surface of the commutator, while the armature is slowing down. Quite a lot of metal
can be removed this way if the work is properly done by a trained hand. After grinding is
over, finish off with smooth sand paper and crocus cloth.
If the commutator surface is very bad, then the armature should be put on a lathe and
skimmed, after checking the trueness of the shaft by a dial indicator. The cuts should
be the lightest possible. The final cut should preferably be by a diamond tipped tool with
fine cuts of the order of .0005 to .001 in. with a feed of 500 threads per in. Finally, mica
should be undercut as usual.