FIELD OF INVENTION
The present invention relates to a method of improving electrical insulation of a brush
holder mounting arm of a DC auxiliary machine.
BACKGROUND OF THE INVENTION
Indian Railways are running a number of passenger and good trains to move
passengers and goods traffic across the country. Diesel electric and AC locomotives are
used to run these trains. DC auxiliary machines are extensively used in the diesel
electric locomotive for auxiliary purpose such as cooling of dynamic braking resisters,
excitation supply to main power pack alternator and for providing power for battery
charging etc. These locomotives are desired to give uninterrupted service for many
years at various operating conditions at different geographical location of India from
north to south and east to west of the country.
The DC auxiliary machines are used for diesel electric locomotive application and are
subjected to diesel oil fume, diesel engine exhaust fume, carbon dust, atmospheric dust
and high moisture content etc. in humid Indian operating conditions. In spite of all
above abuses of operating conditions the insulated brush-holder mounting arms should
always show high insulation resistance. The insulated brush holder mounting arm is one
of the most vital component of a DC auxiliary machine of diesel electric locomotive
which provides adequate insulation between brush gear assembly, which is at higher
potential with respect to magnet frame, because magnet frame is at ground potential.
Earlier many cases of low insulation resistance problem of DC auxiliary machines have
been faced specially during rainy season, which have frequently created operational
problem and eventual failure of auxiliary machines in diesel electric locomotives. After
detailed investigation of the low insulation resistance (IR) problem, it has been found
that failures in DC auxiliary machines have occurred mainly due to low insulation
resistance of brush holder mounting arms.
To eliminate the low IR problem of DC traction machines, a novel design brush holder
mounting arm with high electrical insulation characteristics has been developed by the
inventors, which ensures high insulation resistance of auxiliary machine in all sorts of
operating conditions.
OBJECTS OF INVENTION
Therefore it is the object of invention to propose a method of improving electrical
insulation of a brush holder mounting arm of DC auxiliary machines for traction
application such that the failure on account of low insulation resistance (IR) of brush
holder mounting arm is eliminated.
Another object of the present invention is to propose a method of improving electrical
insulation of a brush holder mounting arm of DC auxiliary machines which enables to
reduce the diesel electric locomotive down time due to low IR at site even during
extreme operating conditions of high humidity monsoon period.
Yet another object of the invention is to propose a method of improving electrical
insulation of a brush holder mounting arm of DC auxiliary machines which enables to
reduce the site failures thereby reducing the cost of maintenance and to increase
serviceability and overall efficiency of diesel electric locomotives.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig. A - shows the sketch of the brush gear assembly of DC auxiliary machines.
Fig.l - shows the sketch of prior art FRP moulded brush holder mounting arm.
Fig.2 - shows the sketch of novel design epoxy moulded brush holder mounting arm of
the present invention.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
Insulated brush holder mounting arms (Fig. A item No.-ll) of a DC auxiliary machines
are meant for secured mounting of brush holder assembly (Fig-A item No.-13) and
brush gear assembly (Fig. A) to the magnet frame (Fig. A item No.-12). It is worth
mentioning here that, DC auxiliary machines are regularly manufactured by BHEL,
which are meant for auxiliary power supply in case of auxiliary generators as well as for
cooling of dynamic braking resister grids in case of auxiliary motors. These machines
play vital role in operation of diesel electric locomotive. The auxiliary machines are
normally DC series/shunt machines with certain number of brush holder mounting arms
per machine. Each insulated brush holder mounting arm carries the load of certain
number of brush holder assemblies and also brush gear assemblies consisting cable
leads, carbon brushes (Fig-A item No.-14) with copper flexible pig tails, copper
connectors with connection bolts and clamps etc. Hence these insulated brush holder
arms should be mechanically robust to carry the load of brush gear assembly as well as
electrically strong enough to provide high insulation resistance at adverse operating
conditions.
The invention is a single piece bright steel bar design, which provides excellent
mechanical strength and the sandwich of multi layer insulation provides requisite
electrical strength. More details of prior art and invention are covered separately in
subsequent details.
The present invention of brush holder mounting arm with high electrical insulation
characteristics shall eliminate the low insulation resistance (IR) problem faced in DC
auxiliary machines used in diesel electric locomotive and also ensure reliable operation
of DC auxiliary machines, which will eventually increase the service life of the DC
auxiliary machines as well as shall improve locomotive availability/reliability.
As shown in Fig. A, brush gear assembly of DC auxiliary machines consists of a brush
holder mounting arms (11), a brush holder assemblies (13), brush stud clamp (16) and
carbon brushes (14), which are assembled on commutator (15) of DC auxiliary machine
and further anchored to the magnet frame (12).
In case of prior art, various cases of low insulation resistance were faced and number of
cases were very high specifically in rainy season due to penetration of moisture in the
insulation moulding of brush holder mounting arm. There was also problem of
breakage/crack developed in moulded insulation of earlier design brush holder
mounting arm.
As shown in Fig.l, the prior art FRP moulded brush holder mounting arm consists of a
steel stud (21) and glass cloth insulation (22).
To overcome the above problems of prior art, it was conceived to develop a new design
of brush holder mounting arm with improved electrical insulation characteristics even at
95% RH (relative humidity) in monsoon period and mechanically stronger insulation
moulding.
The new brush holder mounting arm consists of a steel stud (1) which forms the base
for the brush holder mounting arm. The steel stud (1) is insulated with multi-layer
insulation consisting of mica glass folium (2), glass cloth (3) and additional sealing
epoxy washer to ensure proper isolation of steel stud (1) because the steel stud is
tightened with magnet frame of DC machine, which is at ground potential whereas
brush gear lead and brush holder body is at supply potential.
Further the base of the steel stud (1) is knurled, i.e. special machining (Fig.2 Item No.
5) is introduced for better insulation grip between the steel stud and insulation to
ensure a mechanically stronger mould.
Adoption of above novel design of brush holder mounting arm has resulted improved
isolation characteristics at all type of stringent operating conditions and has increased
the reliability of machine. The invention is able to give trouble free service without
much maintenance involved and also enhances the customer satisfaction.
The details of invention vis a vis prior art is explained in the tabular form as under.
COMPARISON OF PRIOR ART AND NEW PRODUCT INVENTION
Novel design brush holder mounting arm details are shown in Fig.2. The prior art brush
holder mounting arm is also shown in Fig.l. The comparison of constructional features
of the prior art and the invention details are as per table below:
WE CLAIM
1. A method of improving electrical insulation of a brush holder mounting arm of a
DC auxiliary machine comprising:
knurling (5) the stud (1) to provide better insulation grip between the stud (1)
and insulation mould;
moulding in a special die and curing at 100 to 120 degree C temperature for 5
hours to avoid micro porosity of mould;
treating the final surface with epoxy based anti-tracking varnish to ensure
perfect sealing of surface to prevent moisture penetration in extreme weather
and adverse operating conditions;
providing higher temperature class epoxy based varnish to consolidate mica
glass folium (2) and woven glass cloth (3);
characterized in that, the said stud is insulated with multilayer sandwiched
insulation of mica glass folium (2) and woven glass cloth (3) with epoxy resin as
binder to provide mechanically strong mould and to improve insulation
characteristics of brush holder mounting arm.
2. A method as claimed in claim 1, wherein the stud (1) is made of steel.
3. A method as claimed in claim 1, wherein the stud (1) is tightened with magnet
frame (12) of DC machine.
4. A method as claimed in claim 1, wherein end sealing is made with epoxy glass
washer (4).

A steel stud (1) is insulated with multi layer sandwiched insulation which consists of
mica glass folium (2) and woven glass cloth (3). The steel stud is specially machined for
knurling on the surface to ensure proper grip/bondage of insulation mould and steel
support and to make the mould mechanically strong. Moulding is carried out in a special
die and is cured at 100 to 120 degree C temperature for 5 hours to avoid micro porosity
of mould. Epoxy resin is used as binder to make the mould mechanically stronger. Final
surface is further treated with epoxy based anti-tracking varnish to ensure perfect
sealing of surface to prevent moisture penetration in extreme weather and adverse
operating conditions.