1.
INTRODUCTION
Flashovers are caused.
They do not just happen.
Something seems puzzling and mysterious only if it is not
understood. With the gain of knowledge,
the mystry disappears. If any one can find out
what flashovers are and how are they caused, what to do
to prevent them, that makes sense.
A generator flashover, seen for the first time, is
truly awesome. The blast of fire, the smoke and noise are enough to make
one jump as if it was a stroke of lightening. The traction motor flashover is also caused
in the same way.
The commutator is the stage on which the
flashover appears. Fig.
1 shows how the commutator is built up of copper segments
separated from each other by thickness of mica. Each
pair of segments has an armature
coil connected between them. Electricity enters by way of one set of
brushes, through the copper segments and
into the winding. When it reaches the segment under the other set of brushes,
it leaves. The mica
insulation separates the copper
segments and keeps the
electricity flowing through the armature coil. If this insulation breaks down, electricity will short cut across the surface
of the commutator. Almost instantly, the
current jumps from one brush
holder to other brush holder with explosive force forming an
arc. This is known as flashover.
2. THE TROUBLE
The
voltage between the segments of a machine is quite low and
the thicker mica has an insulation capacity
many times greater for the purpose. What then causes such relatively wide spaces to breakdown and permit the
machine to flashover ? (Fig. 2 indicates
the distribution of voltage.)
Across the top of the mica, there is an air space. If dirt does
collect at these spaces and packs between
the segments, the current
begings to leak through it. The space is made wide so
that it will take longer to fill
with dirt and be harden to
bridge. If the space
is not cleaned in
time, insulation breaks down and flash
over may result. These insulating
space may also be bridged by copper fins or
copper dust left
over from stonning and resurfacing
the commutator. Dirt and foreign
materials are not the only cause of flashover.
Air, being a good
insulator is broken down
into conductive gas by the action
of intense heat. The change of air to a
conductive gas is known as ionisation.
It can be caused by flame or
spark, by high voltage or by certain kinds of radiation.
Under
certain operation conditions,
motor or generator brushes will
spark. The affect of this is not
always serious. What happens
depends upon how intense the sparking is
and how long it lasts.
Under some abnormal condition the spark
at the brush may
be so vicious and hot that it blasts
a cloud of conductive gas and fiery particles across
the commutator surface. These
bridge the spaces between
segments and electricity short cuts
across the commutator surface. Every thing is then set for a flashover. The
intense spark that sets off a flash over may occur when a
brush bounches off the commutartor while the machine is carrying
a load. It
may also occur when there is a
sudden extreme change in load,
for greater than the machine can handle. The insulating spaces between
the segments may be bridged by hot
conducting gases generated by the intense heat resulting from :-
i) Dirt between segments which burns when current
flows through it.
ii) Loss of contact of brushes from commutator which draws a hot spark.
iii) Intense sparking at the brushes caused by sudden extreme load
changing.
2.1 DIRT
Dirt
and foreign particles in the insulating space between commutator segments caused the majority of the
flashovers. When enough dirts
collects to bridge the space
between segments, current begins to leak across (Fig. 5A) .
The dirt heats and fuses into a better path. Current flow incfreases, specially as the oeprating voltage increases. The spot grows , and
finally begins to glow (Fig.5B).
As the commutartor turns, these glowing spot looks like a continuous
ring of fire. Finally the spot gets
white hot. Then
it errupts conductive gases
and incandescent particles
(Fig.5C). As the commutator turns
(Fig.6A), these form a fiery trail
behind the spot. These breaks down the
insulating air space between segments
that may not be glowing and sets the
stage for next act.
The
current short cuts (Fig.6B) from
the hot spot, across the segments
bridged by the firery gases,
back to the
brush holder in a sizzling
vicious spark. The intense heat and
energy in these spark blast conductive gases acrosss
the commutator circuit (Fig.6C) with explosive violence. The gas cloud
races ahead of the
glowing spot and breaks down the
air resistance across the rest of
the commutator from brush to brush, then
full power of the
machine jumps across
(Fig.6D) in the
final flashover.
2.2 LOSS OF CONTACT
Dirt may be the most frequent, but it is not the only cause of
flashovers. Sometimes loss of brush
contact will be to blame. These may be
expected at high speed with
a rough commutator surface or
weak brush holder springs. It may
also occur when brushes
are jammed in the holders by muck
or dirt so that they cannot follow
the commutator surface
quickly enough. Servere mechanical shock may jar the brush
off the commutator.
If
brush breaks contact with the commutator, it
draws an electric arc (Fig.7 ).
If these are severe enough, it will
spray conductive gases over the
commutator. If the fiery gas bridges
enough segments, the collective voltage will cause the current to arc back to
the brush (Fig.6B). The blast of
conductive gas from these arc back may reach across the surface of the
commutator to the next
brush (Fig.6C). The full power of the machine
then flashes over these short cut path (Fig.6D). Again,
instead of doing useful
work, the energy
will be expanded
in the terrifically hot, destructive blast of
flashover.
2.3 SUDDEEN EXTREME LOAD CHANGING (The surprise attack)
Flashovers,
that occur when the commutator is in perfect mechanical and electrical condition are most complexing.
These are caused by sudden and
extreme change in load, too great for machine to handle. Fig.
8A shows that, in a machine,
current divides as it enters the
winding. It remits and leaves through the
out going brushes. Current flows
in one direction when the coil is on
one side of the brush and in the opposite
direction when it gets to the
other side. So the current must
reverse in the split second it takes for the coil to pass
under the brush, which is known as
commutation.
If
the current does not reverse in time the coil will come out
from under the brush with the current still flowing in old
direction. The meeting
point with the current in
other part,
which
is known as neutral point will no
longer will be in the brush.
This shifting of neutral point
corwds the current to one edge of the
brush. Then it breaks out over the
surface of the commutator in a spark to reach in a shifted point (Fig.8B).
The
greater the current, the harder it is to get it all
completely reversed as the coil zips
under the brush. Machines have interpoles or commutating poles
, to speed up this current reversal and keeps the neutral point under the
bursh. These are smaller poles
located between the main
poles in
the machine frame. They help commutation only. The magentism of these poles builds up a voltage in the armature coil as they
pass through the zone covered
through the brush contact. This voltage
speeds up the current reversal to get it done before the coil leaves the brush contact.
These poles are designed to do a good
commutating job up to, and even beyond full load. When ,
however, a very overpowering
current flows through the winding, the magnmetism in the
iron cannot build up quickly
enough. This means there is not enough voltage to
reverse the current in time
and sparking results.
Moreover, as after
saturation of the
pole pieces no
more magnetism can be expected , hence, there is a limit to the help the
pole can
give in reversing the current in the
coil. When the current
gets so heavy that this help is
not enough then
this sparking is the ultimate result.
When
the machine is operating at full voltage, the jolt of sudden extreme overload causes vacious sparking at
the brushes. Conductive
gas bridges segments (Fig.8C).
Current starts leak over
the commutator surface
(Fig,6B). The blast of fiery
gas completes the short circuit
between the brush holder (Fig.6 C&D).
Every
day motors and generators
demonstrate their ability Still the flashover occur if anything
goes wrong. For instance, a contactor fails to operate momentarily, short
circuiting generator.
A
sudden surge of current occurs during high speed wheel slip.
Taking a cross over at high speed may cause a brush
of motor to bounce and flash a
motor over. It is just like
short circuiting of the
generator because the current
is no longer flowing through the motor
winding, but short cutting across the commutator. So
the current drawn from the generator
reaches unreasonably high value.
It knocks the generator off balance. The heavy sparking and flashover is the
knock out blow.
3. THE ULTIMATE EFFECT
The
space surrounding the commutator
is filled with flame and conductive
gases. These reach between brush holder
and also over the frame part of the
machine. Current can now flow from the brush
holder to the frame and through the frame back to opposite bursh holder.
Flashover
current can also
strike from the
commutator circuit through the fiery gases to the steel commutator cap.
From here it finds its way to ground through shell, armature shaft and bearing.
This is the cause of electric pitting of roller bearings and races.
When the confined space around the
commutator is filled with ionised air and flame, the current can strike in many directions
with destructive force (Fig.9). String bands are burnt,
brush holders are flashover,
bearings are damaged and if grease and dirt are present they may be set
on fire. However, the current strikes the ground and it is
detected by the ground relay.
4. DETECTION AND REMEDY
Detection
of these types
of defects can only
be done visually. Insulation
resistance between the segments cannot
be taken with the
help of a meter as they are
connected to the windings.
Megger readings and
high pot tests are of no good because they check what is called
resistance to ground.
Inspecting the defects visually, they
can be rectified by cleaning,
undercutting mica so that they look white or
grey, air curing the machine or by blowing the commutator surface with compressed air.
In case of
improper or inadequate
brush pressure, the brush
gear can also be
attended. Polishing,
grinding or machining may also be required if
the commutator surface is rough,
having the defects of high bar etc. In
some of the cases short circuited or open circuited
winding may also cause flash over and can be detected by
bar to bar milli-volt drop test or taking the micro ohm readings.
5. SUMMARY
Flashover of DC
machines is a chronic disease. It is the prime cause of pre-mature failures of
most of the DC machines. Moreover, it remains a mystery to the user that when
the machine will fail and how an expert rectified the fault. This unit
describes the causes of the flashover due to dirt deposition, loss of contact
of carbon brushes and sudden extreme load changing, which are very common in
case of traction machines.
Stage wise
development of defects and ultimate effect on the machine has been elaborately
described to help the maintainer to understand these defects and take remedial
measures. Checking to judge the healthiness of the machine has also been
described.
6. SELF ASSESSMENT EXERCISES
1.
Describe how does the dirt deposition
on the commutator surface lead to flashover.
2.
Describe the reason of
flashover due to loss of contact between the carbon brush and commutator.
3.
Describe the process of
detection and remedy of a machine suffered from flashover.
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