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METHOD FOR THE REPAIR FOR H.T. (STATOR) MOTOR USING
COIL GRAFTING TECHNIQUE
FIELD OF INVENTION
The present invention relates generally to a generalized repair process for the repair of the fused arm of a coil of HT Stator. More particularly the present invention is directed to a method for coil grafting technique for HT (Stator) motors useful for the repair process for the said stators commonly required in case of burnt / damaged HT motor.
BRIEF DESCRIPTION OF THE PRIOR ART
As generally understood, a conventional electric motor utilizes an induced magnetic field to cause driving rotation of its magnetizable rotor. Thus, in a sense, a conventional electric motor is a magnet-type device in that its motive power output is a function of the effectiveness with which the induced magnetic field pulls the rotor through the power stroke of the operating cycle of the motor
To enhance an understanding of the repair for the electric motor during the production process, a specially crafted repair work is required which is fast, economical and user friendly. The breakdown of motors in a production line needs special attention since the production line is held up and is difficult to change the motor since the speed; power, torque etc are equally important parameter for the H.T. motors used for production.
For this the generalized repair process applied in HT motor includes following steps: -
a) Removal of wedged, binding cutting and removal of gutkas.

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b) Lifting of top arms of all the pitch-coils, which are over the defective
coils.
c) Scratching of the insulation and debrazing of connection of the related
series coils.
d) Scratching and removal of insulation of the defective top arm of the
affected coil.
e) Repair of turn conductors and providing inter-turn insulation.
f) Providing sufficient outer insulation required withstanding high voltage.
g) Laying of repaired coil in the slot and binding it in overhang.
h) Repair of lifted pitch-cols with HT insulation required to withstand high
voltage, i) Laying of lifted pitch coils in the slot and binding it in a overhang with
gutka.
j) Wedging of coils in the slot portion with fillers / spacers, k) Brazing of the coil lead joints for seriesing and making jumper
connections.
I) Providing HT insulation to all series joints and jumper connections, m) Final dressing of the winding and heading of stator for moisture
removal, n) Varnishing of the stator winding and painting of the outer body surface.
The above process clearly expresses the amount of work involved for the replacement of single defective coil and the complexity in the problem and handling etc are the major hurdles. In the process of repair certain unavoidable and unwanted weaknesses are injected in the whole motor when the healthy pitch coils are lifted from their slots. While repairing the top arm of most of the coils are damaged because of the lifting of all the pitch coils over the top arm.
LIMITATIONS OF THE REPAIR PROCESS
a) Repair by coil lifting process not possible in VPI type coils.

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b) In moulded type coils repaired type of coils are hand tapped.
c) Possibility of void formation is hand tapped coil.
REASONS FOR THE COMPLEXITY OF THE PROBLEM IN THE REPAIR PROCESS.
a) Coils are made of fixed pre-shaped forms.
b) Two arms of HT coils are not in two parts.
c) Chances of crack in slot portion in moulded coils at the time of lifting.
d) Use of resin, epoxy etc increases the chance of crack in O/H of the
coils.
e) Coils get damages while lifting from the rubbed slots of the stator.
f) Tearing of insulated tape by slot edge at the time of coil laying in slots.
g) Compact slot space in the stator.
SUMMARY OF THE INVENTION
While analyzing a burnt / damaged HT motor we always try to apply some or the other innovative ideas for the betterment of Quality, cost reduction, improvement in productivity and process optimization. Since there cannot be any standard operating procedures for the repair of each and every type of damages, no standard operating manual can be devised. Although there are certain logical ways of proceeding based on theoretical reasoning and proven technology. On the basis of these a generalized repair process has been devised for the repair of the fused arm of a coil of HT Stator-which is the most common type of repair required in the burnt / damaged HT motor.
Therefore, according to the present invention, there is provided a method for the repair for H.T. (stator) motor comprising the steps of - Lifting of the top arm of the faulty coil of the HT Stator; Scraping of outer insulation of the overhang portion of the lifted arm of the coil; Cutting of the top arm (Slot portion); Placing a new half coil in the slot with required adjustment and fittings; Matching the new top arm

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with the remaining portion of the defective coil; Separating of the coil turns at overhang or both part of the coil; Matching of turn to turn in proper sequence; Cutting the coil in step by step with a overlapping to make joints; Selecting a proper insulation scheme to provide inter-turn insulation; Providing sufficient outer layer insulation; Fixing of coil in the slot and wedging and lacing, binding and finishing.
Broadly, it is object of the present invention to devise a repair process for'HT (stator) motors for the repair of fused arm of coil using the coil grafting technique.
Another aspect of the present invention is to provide a process wherein the repair operation is carried out in overhang portion where we can accommodate extra joint thickness.
Yet another aspect of the present invention is the reliability and safety of the repair process is increased since the overhang can provide sufficient inter-turn insulation and outer insulation to withstand high voltage.
In another aspect of the present invention is to make the repair process simple since the repair at the slot portion is not carried out where one cannot provide reliable slot insulation due to space constraints.
In another aspect of the present invention is to provide a process in which no other healthy coils of the stator are distributed and therefore the overall reliability of the complete stator is maintained high.
In another aspect of the present invention the Quantum of job in this process is made very low as compared to old method of repair, which reduces the process time, and the repair cost drastically.
The above brief description, as well as further objects, features, and advantages of the present invention will be more fully appreciated by reference to the

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following detailed description of presently preferred, but nonetheless illustrative embodiments in accordance with the present invention, when taken in conjunction with the accompanying drawings, wherein:
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
FIG. 1 illustrates the process for the replacement of the fused arm by a new arm
(half coil) of the coil and joining in O/H portion.
FIG. 2 illustrates the effect of said process in different parameters using graphical
means.
DETAILED DESCRIPTION
This invention relates to a method for the repair of the fused arm of a coil of HT Stator. More particularly the present invention is discloses a method for coil grafting technique for HT (Stator) motors useful for the repair process for the enhancement of reliability, quality along with cost and manpower reduction.
In contrast to a conventional repair techniques in the operation which involves the replacement of single defective coil by removing flitting of the top arms of all the (pitch) coils over it thereby inducing a weakness to the healthy lifted pitch coils or damaging the coils, a new process is hereby disclosed which overcomes such problems and enhances the life of the repaired motors. The repair by the coil lifting process is not possible in V.P.I type of the coils. In the moulded type of the coils the repaired type of coils are usually hand tapped and the chances of void formation in the hand-tapped coil is maximum. The new disclosed process involves the replacement of the fused arm by a new arm of the coil and joining the same accordingly.

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PROBLEM SUMMARY
(Assuming an average pitch-coil of 1-15)

STATUS BEFORE REPAIR STATUS AFTER REPAIR

FUSED / BURST COIL = 2 Nos FORCIBLE WEAKNESS INJECTED
IN THE COILS = 28 Nos

TOTAL DAMAGED COIL = 2 Nos TOTAL NO. OF REPAIRED COILS =
28 Nos
RELIABILITY AFFECTED IN = 2 Nos RELIABILITY AFFECTED IN COILS =
28 Nos
The impact of the aforesaid type of repair is that the cost is very high

COST HEAD COST (6.6 KV) COST (11 KV)
(COSTVALUE IS IN (COST VALUE IS IN
RS/-) RS/-)
PRIMINR TAPE (A) 10000 15000

V G & GLASS TAPE (B) 2500 5000

WEDGE & VARNISH (C) 3000 3500

NEW H.T. COIL (D) 5000 9000

REPAIR MAN HOUR (E) 6000 6000

COST OF REPAIR (1 COIL) 26500 37500


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The cost for the replacement for each additional defective coil -
Rs. - 5,700 per coil (6.6 KV) Rs. - 9,800 per coil (11 KV)
As per the disclosed process for the repair for a HT (Stator) motor, firstly the top arm of the faulty coil of HT stator is fitted followed by scraping of outer insulation of the overhang portion of the lifted arm of the coil. The scraping of the outer insulation is carried out with the conventional tools and the conventional methods for scraping is adopted. The scraping is done to clear the varnish and adhesives from the coil. The top arm is cut from the slot portion. A new half coil is placed in the slot with required adjustment and fittings. The adjustments and fittings are carried out with respect to the cut portion of the top arm and then matching is carried out with the new half portion with the rest of the coil. The matching is done with great care and skillfully, or otherwise the fault will again arise from the same position. The new grafting technique is a preferred embodiment of the present invention (FIG. 1). For simplicity, the stator core is linearly drawn in figure, but actually the stator core is of cylindrical configuration.
REPAIR DOWN TIME IS MORE

ACTIVITIES MAN x HR
WEDGE AND BINDING REMOVAL 2x08 = 16

CONNECTION DEBRAZING 2 x 04 = 08

COIL LIFTING AND PLACING 2x16 = 32

REPLACEMENT OF NEW COIL 2 x 04 = 08

LIFT COIL REPAIR & PLACEMENT 2 x 24 = 48

BINDING & WEDGE PLACEMENT 2x08 = 16

CONNECTION BRAZING & TAPPING 2x08 = 16

TESTING CARNISHING ETC 2x08 = 16

TOTAL TIME 2 x 80 = 160

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MAN HOURS CONSUMED AND COST OF REPAIR

CAUSES OF CODE No. OF PERCENTAGE CUMULATIVE
DIFFICULTY IN COILS PERCENTAGE
REPAIR OF STATOR

COILS ARE NOT IN A Z2 50 50
TWO PARTS

SLOTS ARE RUBBED B 26 20 70

MOULDED TYPE OF C 23 15 85
INSULATION

USE OF EPOXY ETC D 12 10 95
IN COILS

OTHERS E 08 05 100

TOTAL 141 100 100

The matching step is followed by separating the coil turns at overhang of both part of the coil. The separating of the coil turns is again followed by matching of coils by turn to turn in a proper sequence. The matching is done with great care keeping the turn orientation in mind. The coil is then cut step by step avoiding the swelling of the overhang position. The cutting is so done that a few millimeter overlapping is kept for making the joints. A person skilled in art can easily evaluate the reasons for the overlapping of the coils i.e. to avoid the swelling of the overhang portions.
The coils are then insulated by using varnish etc using a proper insulation scheme to provide inter-turn insulation keeping in mind the required mechanical as well as electrical stress and space constraints. A sufficient outer layer insulation is provided with utmost care and skill to withstand high Voltage and to have sufficient mechanical strength and total reliability. The coils are then fixed in the slot with wedging and lacing, binding and finishing.

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Therefore the salient features* of the invention are: -

TOTAL No. OF COILS PITCH RELIABILITY OLD RELIABILITY NEW
IN THE STATOR COIL METHOD METHOD

108 1 -17 85% 99.0%

72 1-16 77% 98.6%

60 1-15 75% 98.3%

- This is for having only one fused top arm of a coil out of total coils
COST FACTOR

COST HEAD OLD METHOD NEW METHOD
(Thousand Rs) 6.6 (KV) 11 6.6 (KV) 11
(KV) /KV)
HALF COIL 2.5 4.5 5.0 9.0

OTHER MATERIALS 1.0 2.0 15.5 22.4

MAN-HRS ETC 1.5 1.5 6.0 6.0

TOTAL COST 5.0 8.0 26.5 37.5
At RDCIS, SAIL, Ranchi, the disclosed method for the repair of HT (Stator motor has been implemented including the half coil grafting technique in around 32 jobs as an experimental basis and the results were found very satisfactory. As calculated the total projected saving utilizing the said process will be around
Rs. 7.42 Lakhs per year from material cost, Rs. 70 Lakhs per year from outsourced repair work and around 2,596 man-hours savings in the repair downtime.

The above-described embodiments of the invention are intended to be examples of the present invention. Numerous modifications changes and improvements within the scope of the invention will occur to the reader. Those of skill in the art may effect alterations and modifications thereto without departing from the scope of the invention, which is defined solely by the claims appended hereto.

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WE CLAIM: -
1. A method for the repair for H.T. (stator) motor comprising the steps of :-
lifting of the top arm of the faulty coil of the HT Stator;
scraping of outer insulation of the overhang portion of the lifted arm of the coil;
cutting of the top arm (Slot portion);
placing a new half coil in the slot with required adjustment and fittings;
matching the new top arm with the remaining portion of the defective coil;
separating of the coil turns at overhang or both part of the coil;
matching of turn to turn in proper sequence;
cutting the coil in step by step with a overlapping to make joints;
selecting a proper insulation scheme to provide inter-turn insulation;
providing sufficient outer layer insulation;
fixing of coil in the slot and wedging and lacing, binding and finishing.
2. The method as claimed in claim 1, wherein the other healthy coils are not
disturbed excepting the damaged / burnt coil.
3. The method as claimed in claim 1, wherein the overlapping of the coils is
within few millimeters.

4. The method as claimed in claim 1, wherein the insulation is carried out
with respect to the required mechanical stress, electric stress and the space
constraints.
5. The method for the repair for H.T. (stator) motor, substantially as herein
described with particular reference to the accompanying drawings,.
Dated this 23rd day of June, 2006.

To,
The Controller of Patents,
The Patent Office, Kolkata