Specifications

174
iii) Commutator is eccentric or having high bars and low bars : Excessive blackening or
burning of a few commutator bars only shows presence of high or low bars, mostly
caused by the bolts holding the commutator V-ring becoming slack. The bolts should
be tightened up fully, after the commutator has run in for some time and is fully warm.
After this is done, the commutator should be skimmed on a lathe. If only a portion of the
commutator surface shows signs of severe sparking, it is a clear sign of the commutator
being eccentric. No commutator should show a variation of more than 0.0254mm (.001
in.) when checked by a dial indicator, when mounted on its bearings on the motor.
iv) Protecting mica between bars : All commutators should have the mica between the
bars undercut to a depth of 0.8mm (1/32 in.) approximately. After grinding or turning of
the commutator, undercutting should invariably be done either manually or by a machine.
It is essential to round off the sharp edges of the bars, and remove all bars, as also to
thoroughly clean the mica slots and blow off all dust. Undercutting should also be done
periodically.
v) Flashover of commutator: Flashover across commutator is frequently caused by heavy
accumulation of dust over the commutator surface or due to low-resistance leakage
path either in the commutator risers, or between the bars or at the sides of the
commutator. Such a flashover constitutes a dead short across the mains and before
the fault is isolated by the circuit-breaker, the heat generated may be so much as to
cause appreciable damage to the commutator surface and the surrounding area. The
dust and dirt should be cleaned by washing and brushing with petrol; all carbonized
matter, foreign particles and deposits should be scraped off. The inter-segment gaps
should be kept clean; otherwise they may get filled up with dirt or carbon causing sparking
and further carbonization.
15.14 De-greasing:
By
‘
de-greasing
’
is meant the removal of dirty grease and oily matter by means of a chemical
solvent, very similar in action to the well-known
‘
dry cleaning
’
of clothes. The solvent for de-
greasing is marketed under various trade names but they are all one form or the other of stabilized
tri-chloro-ethylene, which has a boiling point of 188
0
F. The de-greasing plant consists of a tank
half-filled with the de-greasing liquid, which is kept boiling by means of steam pipes immersed in
the liquid. The part to be degreased, whether a dirty armature or a field coil, is lowered into the
tank and supported above the liquid level in a suitable manner and the lid put on. The dirty
windings will then be exposed to the hot vapours, which will condense over the windings and
very soon all the dirty grease runs out and drops down into the tank. Within a very short time of
10 to 15 minutes, the windings can be removed out in a perfectly clean condition. It emerges dry,
requiring no further drying. The solvent and vapour have no adverse effect on the insulation of
the windings. The process is very quick and also economical in operation. Suitable provision is
made to prevent the vapour escaping out into the atmosphere; it is condensed and recovered
and could be used over and over again. All the muck and dirt which collect at the bottom of the