FIELD OF THE INVENTION
The present invention relates to a method for coil assembling /winding of small core bore
HT motors adapted to withstand high inter-turn and high supply voltage. More particularly,
the present invention is directed to a reliable method for coil winding of electrical
conductors of selective section in the small core bore such as of 400mm diameter
spanning over a length such as of 1100mm of coil involving two half-coils using 'half-coil
grafting techniques' with selective two tier protection for inter-turn insulation suitable for
withstanding high inter-turn voltage in adjacent conductors. Advantageously also, the
method of the invention is directed to favour replacement of damaged coils for the HT
motor to overcome the complexity of maintaining the desired precise curve in the
overhang portion without any distortion of arc of the knuckle. The method of coil winding
for motors and the like of the invention is further directed to take care of the required
insulation wall adapted to small core bore stator slots and to provide for adequate heat
dissipation from slot without heat accumulation. Such insulation is adapted to ensure safe
operation against any external fault or turbulence. Importantly, the method of motor coil
assembling is adapted to achieve high reliability and enhanced service life. Moreover, the
method of coil replacement of HT motor with small-bore design according to the invention
is thus simple and needing less time, complexity and costs while ensuring high reliability
and long service life and thus favoring wide industrial application.
BACKGROUND ART
It is known in the related process and systems thereof in steel plants and other like
industries wherein bulk production and supply of Nitrogen and Argon gases are required at
different stages of steel production/other processes. The 3000 RPM HT (11KV) motors play
a vital role in driving compressors for supplying Nitrogen and Argon continuously, which is
essential requirement for the normal operation of the steel melting shop and continuous
casting shop. Non-availability or short supply of these gases brings down the production
drastically and as a consequence performance and productivity is severely affected. The
conventional HT Motors used for the compressor units for the purpose of supply of
Nitrogen and/or Argon gases conform to very old design configuration, usually having no
standard spares available off-the-shelve. New procurement/replacement involve huge
cost. Thus prior art process suffered from break down or damage of coils of such high

speed HT motors cause havoc leading to stoppage of production of steel and considerable
down time is expended before the system could be restored to normalcy, if at all feasible
technically/economically. Attending to the problem of service/maintenance in case of
burning/damaged coils for the HT motors are found to be further compounded due to the
complexity of construction of the stator coils having conductor wound through slots of
narrow core bore wherein the inter-turn insulation is a critical factor as the adjacent inter-
turn voltage is of high order. Serious constraints comprising inaccessibility due to lack of
space in the stator slots, complicated coil shape having long overhang precise portion with
precise curve and knuckle angle at the ends, found to be difficult to repair/restore in case
of any burn/flash or damage of coils even by highly skilled technical hands/winders having
long experience.
The nature of major breakdown of such high speed HT motor used for Nitrogen
compressor in steel melting shop, posing serious threat to continuous production plants
and having no viable repair/maintenance solution, comprised:
a. Flashing of coils at the ends on connection side so that the conductor of the coils
are melted and damaged the pressure plate as well as slot end. The coils comprise-
top & bottom in a single affected slot- are spread among 13+13- 1 (common)= 25
numbers of slots in conventional stator configuration.
b. Insulation of several other coils in the vicinity of the burnt coils are also damaged
inter-turn insulation of some more coils are found to be affected at the over-hang
portion, which is revealed/ascertained only after removing the flashed coils.
c. One of the flashed coils found to be directly connected to the supply lead of the
stator, which remains at relatively maximum stress when the motor is in running
condition.
The problems felt in overhauling/repairing/attending to the burnt/damaged stator
winding of the nature as given above, are found to be of the following magnitude:
a. All major HT Motor manufacturer including the original manufacturer and supplier
M/s Crompton Greaves admitted that the kind of HT Motor which had undergone
flashing of coils is of much old design and now obsolete, which used insulation
system conventional mica based bitumen tape and having no standard spares
readily available.

b. As the bitumen taped coil motors have mica as its base, mica gets damaged and
the insulation system become highly unreliable and the coil reparability becomes
very difficult even by the use of other insulation system.
c. Supply voltage per coil is quite high as number of coils per phase is very less (e.g.
14 nos.) which in turn generates high inter-turn coil voltage in each coil.
d. Partial repair of burnt portion of coil is not possible and also not reliable due to the
duty /rating, compact core of very small bore and long length, having high inter-
turn coil voltage of the damaged coil of HT Motor.
e. It is estimated that complete overhauling and revamping of the flashed stator coil
involving external vendor using their facilities is around sixty lakhs. Due to its very
old design identical motor having same design parameters- electrical & other- is not
available with the motor manufacturers. They were even not ready to manufacture
such type of special motor made of primitive technology. As against the alternative
proposal for new complete system along with this H.T. motor will cost more than 10
crores with no fixed time frame.
f. The insulations required to be of very special grade to take care of high inter-turn
voltage and space constraints in the slot. The insulation scheme of the replacement
coil must provide for sufficient insulation to withstand high inter-turn voltage as
well as voltage to ground.
g. Coils had to be repaired manually in absence of molding or other
equipments/facilities needing high skill level.
h. The coils must be of adequate size and shape to fit with the space available in the
overhang and it must be sealed against the moisture at overhang potion.
There has therefore been a persistent need in the existing art for developing means for
reliable method coil assembling/ winding of small core bore HT motors adapted to
withstand high inter-turn and high supply voltage which would in turn be directed to
favour repairing the flashed/damaged coils of the stator winding of the high speed HT
Motor using in house facilities and technology to thereby reduce the down time for
maintenance. There is further the need for ensuring longer reliable service of the HT motor
such as those used to drive compressor for Nitrogen and Argon gases in steel melting shop
or the like purpose which would meet the requirement of desired inter-turn insulation and
also the inter-turn voltage, even under the constraints of narrow core bore slots of stator
and inaccessibility or providing desired insulation quality/thickness to sustain desired inter-
turn voltage with the limitation of narrow passage for the conductor within the slot.

OBJECTS OF THE INVENTION
It is thus the basic aspect of the present invention to provide a reliable method for coil
assembling/winding of small core bore HT motors adapted to withstand high inter-turn and
high supply voltage which would further benefit the repairing of the flashed/damaged coils
of the stator winding of the high speed HT Motor using in house facilities and technology to
thereby reduce the down time for maintenance.
Another object of the present invention is directed to meet the need for ensuring longer
reliable service of the HT motor such as those used to drive compressor for Nitrogen and
Argon gases in steel melting shop or the like purpose which would meet the requirement
of desired inter-turn insulation and also the inter-turn voltage, even under the constraints
of narrow core bore slots of stator and inaccessibility or providing desired insulation
quality/thickness to sustain desired inter-turn voltage with the limitation of narrow
passage for the conductor within the slot.
A further object of the present invention is directed to provide a reliable method for stator
winding of high-speed 11KV HT motor used to drive large capacity gas compressors for
supply of Nitrogen/Argon in steel melting shop adapted to provide inter-turn insulation
having two tiers of protection to withstand high inter-turn voltage as well as outer
insulation for 11KV along with limitations of small core bore of slot in stator while also
taking care of heat dissipation and slot factor avoiding heat accumulation for achieving
desired high speed reliable HT drive and enhanced life of the stator winding.
A further object of the present invention is directed to a method of stator winding of 11KV
high speed HT motor adapted to take care of the complicated shape of coil winding
comprising long overhang portion with precision curve and knuckle angle at the end of
turn without any distortion of windings.
Another object of the present invention is directed to a method of stator winding of 11KV
high speed HT motor for gas compressors or like application wherein the number of
conductor turns are selectively maintained in twenty eight turns arranged in two half coils
arranged in a manner to take care of space constraints due to narrow core bore slots of
stator and inter-turn insulation adapted to withstand high inter-turn voltage.

A further object of the present invention is directed to a method of stator winding of 11KV
high speed HT motor wherein insulation of conductors in the slot section is moulded to
achieve desired insulation wall thickness.
A still further object of the present invention is directed to a method of stator winding of
11KV high speed HT motor wherein thickness of insulation is selected preferably less than
40% of slot area such that heat dissipation and slot factor matches and there occur no
unwanted heat accumulation in the slot.
A still further object of the present invention is directed to a method of stator winding of
11KV high-speed HT motor wherein 'half coil grafting techniques' with precise joining with
brazing free of any bulging and step-cutting for joining spread in the in the overhang with
minimum swelling used to accommodate the coil turns in narrow core bore slots of stator
avoiding distortion and any damage to the precision curve and knuckle angle at the end of
turns.
A still further object of the present invention is directed to a method of stator winding of
11KV high-speed HT motor wherein inter-turn insulation provided in the jointed overhang
portion which is similar to the inter-turn slot insulation and the outer insulation provided
which can sustain the high voltage, surge voltage and the normal voltage.
A still further object of the present invention is directed to a method of stator winding of
11KV high-speed HT motor wherein end windings are securely braced, thus ensuring the
prevention of movement during starting and service to achieve desired reliability.
A still further object of the present invention is directed to a method of stator winding of
11KV high-speed HT motor wherein the position of the starting coil and the last coil is
interchanged by changing the star point to supply point and vice versa to assure reliability
and enhance its working life.
A still further object of the present invention is directed to a method of stator winding of
11KV high speed HT motor wherein the down-time for stator winding or repair of the
flashed/damaged coil using in-house resource to minimize the time and cost of the narrow
core bore compact HT motor with desired reliability in service.

SUMMARY OF THE INVENTION
Thus according to the basic aspect of the present invention there is provided a method
for coil assembling/ winding of small core bore HT motors adapted to withstand high inter-
turn and high supply voltage comprising:
providing two half coils adapted to take care of relatively high inter-turn voltage ;
providing inter-turn insulation of desired protection for added safety against any external
fault/turbulences;
providing insulation of selective thickness such that the heat dissipation and slot factor of
less than 40% matches and there is no unwanted heat accumulation in the slot;
providing a precise top joint with half coil grafting such that there is no bulging of the coil
with minimum swelling at the overhang joints;
providing inter-turn insulation in the jointed overhang portion followed by outer insulation
adapted to sustain high voltage, surge voltage and normal voltage;
finally putting the coils inside the slots, joined and insulated.
Another aspect of the present invention is directed to said method for stator winding of
small core bore HT motors comprising developing coil drawing cum winding data- sheet
based on the size of the conductor, current density and other related technical parameters.
A further aspect of the present invention is directed to said method for stator winding of
small core bore HT motors wherein the step of fabricating the two half coils comprise:
28 turns having conductor-turn arrangement 14 in a column stacked in two rows wherein
the 1st turn is adjacent to 15th, 2nd adjacent to 16th and finally 14th adjacent to 28th having
high inter-turn voltage in adjacent conductors.

A still further aspect of the present invention is directed to a method for stator winding of
small core bore HT motors wherein based on the space constraint the slot and the
conductor size the inter-turn insulation is provided to have two tier of protection for added
safety against external faults or turbulences.
According to yet another aspect of the present invention directed to said method for stator
winding of small core bore HT motors wherein the jointing of turns is spread in the
overhang by step-cutting in the over hang of half coil such that there is minimum swelling
at the overhang joints.
A still further aspect of the present invention is directed to a method of replacing flashed
coils and rendering a motor stable, safe and reliable comprising:
i) taking out the damaged coils from the motor; and
ii) following the method of for coil assembling/ winding as of the present invention.
According to yet another aspect of the present invention directed to said method wherein
the position of starting coil and the last coils are interchanged by changing the star point
to supply point and vice-versa for better reliability and enhanced working life.
A still further aspect of the present invention is directed to a method wherein the motor
comprises a two-pole motor having high inter-turn voltage preferably a motor adapted to
supply nitrogen and argon continuously in steel melting shop and continuous casting shops
in steel plants.
An yet further aspect of the present invention is directed to a small core bore HT motor
adapted to withstand high inter-turn and high supply voltage adapted to withstand high
inter-turn and high supply voltage comprising:
two half coils adapted to take care of relatively high inter-turn voltage ;
inter-turn insulation of desired protection for added safety against any external
fault/turbulences;
insulation of selective thickness such that the heat dissipation and slot factor of less than
40% matches and there is no unwanted heat accumulation in the slot;

a precise top joint with half coil grafting such that there is no bulging of the coil with
minimum swelling at the overhang joints;
providing inter-turn insulation in the jointed overhang portion followed by outer insulation
adapted to sustain high voltage, surge voltage and normal voltage;
said coils arranged inside the slots joined and insulated.
The present invention and its objects and advantages are described in greater details with
reference to the accompanying non limiting illustrative figures.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1: is the illustration of the burnt stator of HT Motor for Nitrogen Compressor
motor, showing the flashed portion of stator coil and the complicated position of turns in
the coil with precision curve and knuckle angle at the overhang potion.
Figure 2: is the illustration of the tabular presentation of the winding data sheet
corresponding to the 11 KV 350.0 KW HT motor of the Nitrogen compressor for carrying
out the coil winding.
Figure 3: is the illustration of the coil drawing for the HT motor corresponding to the data
sheet of Figure 2.
Figure 4: is the illustration of the arrangement of stator winding in cross sectional view
having, selective size of conductor arranged in preferred sequence of 7 conductors in one
stack x4 stacks comprising 28 turns in numbered sequence in the stator slots of 18mmx
36mm size, adapted for high inter-turn insulation to with stand high inter-turn voltage and
also extra insulation adapted for high supply line voltage(HKV).
Figure 5 a-g: are the illustrations of different stages of the 'Half coil Grafting' technique
according to the present invention adopted for the coil winding of burnt stator of the HT
motor.
Figure 6: is the illustration of the changing of the Star point leads with Supply point leads
and vice-versa by interchanging the position of the starting coil and the last coil the front

and bank location of the stator winding, to achieve desired reliability and enhanced
working life.
Figure 7 a-d: is the illustration of the stages of coil lifting and grafting in progress up to
the final repaired/wound stage of the burnt/flashed 3500KW stator of 11KV, 3000 RPM HT
Motor.
DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE
ACCOMPANYING DRAWINGS
The present invention relates to a method for stator winding of 3000 RPM, 3500 KW, 11KV
HT Motor for Nitrogen or Argon gas compressor for use in steel plants in steel melting,
including repairing flashed/damaged winding in a reliable manner ensuring long life in
service. The method of the invention provide for novel techniques for the stator winding
even with constraints of limited space for the narrow core bore slot of stator and the
precision curve and knuckle of the overhang portion of the coil attended with the
requirements of high inter-turn and extra-turn insulation to withstand the high inter-turn
as well as the high supply voltage. The major technical characteristics deployed to meet
the above functional requirements or to over come the stated limitations in the traditional
configuration of such large capacity high speed synchronous HT motors of much older
design while undertaking in situ repairing of any flashed coil or defining the stator winding
parameters comprised:
a. Providing 'molded type inter-turn insulation' of two tier protection to take care of
space constraint due to small core bore size on one hand and on the other hand
ensure desired reliability of the repaired coil during service life;
b. 'Half-coil grafting technique' used to ensure quality of finished winding maintaining
the precision of the curve and knuckle angle of the turns of the coils at the
overhang;
c. Interchanging of 'Star point leads' with 'Supply point leads' and vice-versa
implemented to ensure reliability and enhanced life of the repaired /rewound stator
of HT Motor.

The above aspects and the other steps involved in the method of stator winding of the HT
Motor according to the present invention are described in further details with reference to
illustrative figures as follows:
Reference is first invited to the accompanying Figure 1 that illustrates the condition of the
flashed/burnt stator of HT Motor for Nitrogen Compressor drive in steel melting plant,
showing the flashed portion of stator coil and the complicated position of turns in the coil
with precision curve and knuckle angle at the overhang potion, making the task of winding
critical and difficult to achieve remaining with the space limitation of the narrow core bore
of the stator. The problem of winding/repairing of stator winding is further compounded to
carry out said winding work in situ position and to provide inter-turn insulation with two-
tier protection in a long span of 1100 mm of coil through a small bore of 400mm diameter
with complicated coil shape in long overhang portion.
Reference is next invited to the accompanying Figure 2 that illustrates the tabular
presentation of the winding data sheet corresponding to the 11 KV 3500 KW HT motor of
the Nitrogen compressor. The winding data sheet has been carefully developed by
calculation/computing related variable factors considering the constraint and also to
achieve desired performance and reliability. The important parameters like the size/cross-
section of the conductor, current density, coil and core bore dimensions, lead length, coil
groups, end/extension dimensions, coil weight, coil type number of vents etc. Based on
the parameters so computed guided by the basic design standards and matching with the
stator configuration, coil drawing is developed which needs to be treated in conjunction
with the said data sheet. The accompanying Figure 3 is the illustration of the coil drawing
for the HT motor corresponding to the data sheet of Figure 2. The reference numerals used
in said Figure 3 corresponds to respective part details with dimension and characterizing
feature in the data sheet of Figure 2, so as to facilitate the construction of the stator
winding of said 11 KV, HT Motor.
Attention is now invited to the accompanying Figure 4 that illustrates the conductor
arrangement in the slots of stator wherein each turn of coil selectively numbered and
sequentially disposed shown in cross-sectional view, in order to take care of adequate
insulation to with stand inter-turn voltage and also the input high supply line voltage,
remaining with the space constraint of the small core bore diameter/size of slots in stator.
It is clearly apparent from the accompanying Figure 4, that the coil comprise winding with

single bare conductor of section 5.0mmxl.5mm which is uniformly insulated with a
thickness of 0.4mm to thereby putting the overall dimension of each single conductor to
5.4mmx 1.9mm. Conductors are arranged in stacks of 7 numbers of insulated conductors
in a group in a column. An extra additional insulation of 1.0mm to withstand high inter-
turn voltage is provided in a group of 7 conductors in a column. Further, 28 numbers of
insulated conductors stacked in two rows with extra insulation to withstand extra inter-
turn voltage. Outer insulation of 1.5mm provided to withstand High Voltage with respect to
earth and coil. Thus the stack of 28 numbers of insulated conductors stacked in two rows
inside the stator core is covered with insulation having two-tier protection viz. (i) inter-
turn insulation of each single conductor and extra insulation of group of 7 conductors
arranged in a vertical column, and (ii) Outer layer of insulation to withstand high voltage
with respect to body. The inner inter-turn insulation and outer extra insulation layers are
selected from nomex tape, kaptane tape, resin based kaptane mica tape respectively, for
improved safety and operational reliability and extended life expectancy of the fresh stator
winding for HT Motors.
Reference is now invited to the accompanying Figure S (a) to (g), wherein the different
stages of the Half-coil grafting techniques applied for the coil winding has been shown. The
Figures clearly illustrates the method of implementation of the formation of half turn of
each coil for joining by brazing to form a complete loop, arranging the conductors in
columns of 7 and providing the precise curve and knuckle angle at the overhang portions
at two ends of each coil.
Reference is now invited to the accompanying Figure 6, wherein the interchange of the
Star point leads and the Supply point leads and vice-versa has been implemented, in order
to ensure achieving the high reliability and enhanced operating life of the stator winding
obtained following the method according to the invention.
Reference is now invited to the Figure 7 (a) to (d), that illustrate the different stages of
the winding method for the HT stator according to the present invention involving lifting
and grafting progressively up to the final repaired/wound stage of the burnt/flashed
3500KW stator of 11KV, 3000 RPM HT Motor. The accompanying Figure 7 shows the
stages involved in the repair/stator winding of the stator of the HT Motor such as the coil
lifting for repair/removing damaged coil, coil grafting using Half-coil grafting technique of
the method of the invention, grafted coil brazed/joined up to its knuckle end and to finally

achieve complete winding of the stator according to the winding data sheet computed
based on operating parameters and stator dimensions as well as inter-turn insulation and
high inter-turn voltage.
The method of stator winding of the 11KV HT Stator according to the present invention for
implementing the reliable performance of the winding and ensure extended operating life
thus comprise the steps of:
a. At first the damaged/flashed coils are lifted and taken out carefully using special
tools developed/fabricated for the purpose so that the adjacent coil turns do not get
damaged;
b. A winding data sheet has been prepared by computing relevant parameters that
included details of the size of the conductor, current density and other technical
parameters were considered and calculated accordingly.
c. Two half coils are fabricated considering the relatively high inter-turn voltage to
with stand. This has a total 28 turns having 14 conductor turn in a column stacked
in two rows, so arranged that 1st turn comes adjacent to 15th, 2nd comes adjacent
to 16th and so on and finally 14th coming adjacent to the 28th turn, such
arrangement leads to high inter-turn voltage in adjacent conductors. Also,
considering the space constraints in the slots of 18mm width and the individual
single conductor size for winding, an innovative method has been adopted to
provide inter-turn insulation having two tier of protection, one layer each for
individual conductor and each stack of 7 conductors as well as one additional outer
layer of insulation for the 28 turns insulated conductors stacked in two rows to
protect against high inter-turn voltage as well as high input supply voltage. This
insulation scheme so arranged ensure added safety against any external fault or
turbulence. The insulated winding stack, having 14 conductors stacked in each
column comprising group of 7 is tested and two such rows comprising total of 28
coils, cured in a furnace to assure integrity of slot insulation and its reliability.
d. The thickness and scheme of insulation of the winding is selected in such a manner
that heat dissipation and slot factor, maintained at less than 40%, is matched and
there occur no unwanted heat accumulation in the slot.
e. The inside and outside insulation materials for the winding, are selected from
Kaptane tape, Nomex tape, Mica based Silicon resin tape, and glass tape.

f. A precise top joint with half coil is accomplished with due care which after joining
by brazing, there remains no bulging of the coil. Jointing of turns is spread in the
overhang by step cutting of each half coil so that there occur minimum swelling at
the overhang joints. The method of such coil winding and joining is termed 'Half-
coil grafting' which is novel.
g. Inter-turn insulation provided in the joined overhang portion is similar to the inter-
turn slot insulation. Finally, outer insulation is provided which is able to sustain the
high voltage, surge voltage as well as the normal voltage.
h. The wound coil comprising 28 turns in all, is then put inside the slot, jointed and
insulated properly,
i. End windings are securely braced to ensure prevention of movement during
starting and service,
j. The position of the starting coil and the last coil in interchanged to assure
performance reliability and enhance its working life. This is implemented by
changing the Star point to Supply point and vice versa,
k. Total stator winding is sealed and painted by a sealant. Anti-corrosive coating is
also given and finally enameled.
I. Finally, the stator winding was subjected to high voltage test and inter-turn voltage
by surge comparison. The stator successfully sustained all the tests parameters
required to ascertain the expected performance reliability and life,
m. The motor with stator winding so developed has been meggered at 5KV and run on
no-load with expected performance, before putting into normal service as drive for
Nitrogen compressor.
It is thus possible by way of the present invention to provide for a method for coil
assembling /winding of small core bore HT motors adapted to withstand high inter-turn
and high supply voltage. The invention would further favour a reliable method for coil
winding of electrical conductors of selective section in the small core bore such as of
400mm diameter spanning over a length such as of 1100mm of coil involving two half-
coils using 'half-coil grafting techniques' with selective two tier protection for inter-turn
insulation suitable for withstanding high inter-turn voltage in adjacent conductors. The
method according to the invention would further enable repairing of burnt/flashed coil in a
safe, faster, reliable and cost effective manner ensuring the enhanced life of winding. The
method of the invention facilitate stator winding even with constraints of limited space in
the slots and small core bore. Importantly, the method enable providing two tier

protection for the inter-turn insulation for the successive conductors adapted to withstand
high inter-turn and outer insulation layer to withstand high input supply voltage or safe
guarding against any external fault or turbulence, ensuring achieving the precise curve
and bend in the knuckle portion in the overhang of the coil on either end and also joining
of the two ends of 'Half Coil Grafting' method adopted for winding without any bulging or
minimum swelling for reliable performance in service. The method of the invention may be
adopted as a general method for repair for HT motors for a number of applications using
the three important techniques used for the present invention with some alteration of
parameters e.g. the half coil grafting, inter-turn insulation adapted for two tier protection
and interchanging the Star point leads and Supply point leads to assure reliability and
enhanced life of the stator winding. The method further involve pin pointing the nature of
fault and extent of damage and related computation for preparing a winding data sheet
and executing the winding following the innovative steps as stated above, ensuring
reliability ad life in service. The method of the invention employs simple tools and gadgets
to construct the stator winding of desired characteristic parameters to thereby effecting
substantial cost saving and thus finding prospects of wide industrial applications including
in Steel plants for Steel Melting Shop for driving compressor for supply of Nitrogen or
Argon in steel making.

We Claim:
1. A method for coil assembling/ winding of small core bore HT motors adapted to
withstand high inter-turn and high supply voltage comprising:
providing two half coils adapted to take care of relatively high inter-turn voltage ;
providing inter-turn insulation of desired protection for added safety against any external
fault/turbulences;
providing insulation of selective thickness such that the heat dissipation and slot factor of
less than 40% matches and there is no unwanted heat accumulation in the slot;
providing a precise top joint with half coil grafting such that there is no bulging of the coil
with minimum swelling at the overhang joints;
providing inter-turn insulation in the jointed overhang portion followed by outer insulation
adapted to sustain high voltage, surge voltage and normal voltage;
finally putting the coils inside the slots Joined and insulated.
2. A method for stator winding of small core bore HT motors as claimed in claim 1
comprising developing coil drawing cum winding data- sheet based on the size of the
conductor, current density and other related technical parameters.
3. A method for stator winding of small core bore HT motors as claimed in anyone of
claims 1 or 2 wherein the step of fabricating the two half coils comprise:
28 turns having conductor-turn arrangement 14 in a column stacked in two rows wherein
the 1st turn is adjacent to 15th, 2nd adjacent to 16th and finally 14th adjacent to 28th having
high inter-turn voltage in adjacent conductors.
4. A method for stator winding of small core bore HT motors as claimed in anyone of
claims 1 to 3 wherein based on the space constraint the slot and the conductor size the
inter-turn insulation is provided to have two tier of protection for added safety against
external faults or turbulences.

5. A method for stator winding of small core bore HT motors as claimed in anyone of
claims 1 to 4 wherein the jointing of turns is spread in the overhang by step-cutting in the
over hang of half coil such that there is minimum swelling at the overhang joints.
6. A method of replacing flashed coils and rendering a motor stable ,safe and reliable
comprising:
i) taking out the damaged coils from the motor; and
ii) following the method of coil assembling/ winding as claimed in anyone of claims 1 to 5.
7. A method as claimed in claim 6 wherein the position of starting coil and the last coils
are interchanged by changing the star point to supply point and vice-versa for better
reliability and enhanced working life.
8. A method as claimed in anyone of claims 6 or 7 wherein the motor comprises a two-
pole motor having high inter-turn voltage preferably a motor adapted to supply nitrogen
and argon continuously in steel melting shop and continuous casting shops in steel plants.
9. A small core bore HT motor adapted to withstand high inter-turn and high supply
voltage adapted to withstand high inter-turn and high supply voltage comprising:
two half coils adapted to take care of relatively high inter-turn voltage ;
inter-turn insulation of desired protection for added safety against any external
fault/turbulences;
insulation of selective thickness such that the heat dissipation and slot factor of less than
40% matches and there is no unwanted heat accumulation in the slot;
a precise top joint with half coil grafting such that there is no bulging of the coil with
minimum swelling at the overhang joints;
providing inter-turn insulation in the jointed overhang portion followed by outer insulation
adapted to sustain high voltage, surge voltage and normal voltage;

said coils arranged inside the slots, joined and insulated.
10. A method for coil assembling/winding of small core bore HT motors adapted to
withstand high inter-turn and high supply voltage and a motor having such coil
assembly/winding substantially as hereindescribed and illustrated with reference to the
accompanying figures.

The present invention relates to a method for stator winding of high speed (3000 RPM)
High Tension (HT) 11KV Motor, having selective two-tier insulation for the inter-turn coils
adapted for high inter-turn voltage and high supply voltage. A method for
installing/replacing damaged coils in situ for such HT Motors adopted technique of
electrical conductors of selective section in the small core bore, having said selectively
configured conductor-turn arrangement in a column stacked in two rows, using 'half-coil
grafting techniques' with selective two tier protection for inter-turn insulation suitable for
withstanding high inter-turn voltage in adjacent conductors. The method facilitate
winding/assembly of coils maintaining precise curve and knuckle in the overhang portion.
The winding of the conductor, and the thickness of insulation wall adapted to small core
bore stator slots to achieve adequate heat dissipation from slot without accumulation,
ensuring safe operation against any external fault or turbulence. Importantly, the method
involves changing of 'Star point leads' with 'Supply point leads' and vice versa to achieve high reliability and enhanced service life.