Field of the Invention
The present invention relates to effective life of repaired H.T. motors and, in particular, to a method of enhancing the effective life of H. T. motors after repairing/servicing which would be simple, fast and importantly also cost-effective. The method for such enhancing of effective life of repaired H. T. motors is directed to improve the quality of repair to get economical service life of the H. T. motors. Importantly, the method of the invention favours reduction in repair cost for the H. T. motors and improvement in optimized performance and effective life cycle of the motor. The method of the invention is reliable and favour reduction of damage of stator coil from 30% to 5%. More importantly, by way of the above method of repairing / servicing of H.T. motors the machine down time is reduced saving cost due to increased life and also saving effective man hours due to substantial reduction of the down time for such repairing and servicing.
Background Art
It is known to the art that when a burnt or failed H. T. motor after several years of performance is repaired, its service life is not up to the desired mark. In particular, it is found that the failure of such repaired motors were due to several factors which include failure related to supply, machine problems, system failures and/ or lack of proper manufacturing facilities or skilled expertise to oversee or control the manufacturing.
More specifically, it is identified that the causes of motor failure due to supply can be due to the surge voltage, single phasing, voltage fluctuation and frequent start/stop operations. Such complexities of supply thus do have a bearing on the motor life after repairing and can cause further failure of the motor.
It is also found that the machine specification such as rubbed slots , vibrations and entry of foreign elements also contribute towards early failure of repaired/serviced H. T. motors.
As regards the problems of motor failure after servicing / repairing resulting from manufacturing problems the same include poor workmanship, compact slot space and insufficient insulation.
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It is, therefore, apparent from the above state of the art that for variety of reason the problem of frequent and immediate failure of serviced / repaired H. T. motors have been a continuing concern in the art since the repairing cost was high and found to be not adequately realized due to the short and not so effective desired post servicing / repairing life cycle of the H.T. motors. It has thus been a continuing need in the art to identify the reasons for such post repair failure of the H.T. motors and also to improve the quality of repair to get the economical service life by repair quality improvement of H.T. motors.
Objects of the Invention
It is thus basic the object of the present invention to identify the reasons for failure of repaired H.T. motors and provide for means for improving the quality of repair of such damaged H.T. motors to get economical service life by improving the quality and reliability of the repair of the motor.
Another object of the present invention is directed to identify the reasons for failure of the repaired H.T. motor and provide for improvement in repaired quality and in the process get economical service life of motor with optimized performance.
A further object of the present invention is directed to enhance the life of repaired H.T. motors which would substantially extend the effective life of the repaired motor and thereby enable its effective utilization saving unwanted downtime or frequent post-repairing machine failure.
A further object of the present invention is directed to a method of repairing H.T. motors which would be cost effective and yet favour extending the effective life of the motor after repairing and thereby improve the reliability of the coils after repairing.
A further object of the invention is directed to enhancing the life of H.T. motors by simple and effective repairing process whereby the damage to stator coil can be reduced from about 30% to about 5%.
A further object of the present invention is directed to enhancing the life of H.T. motors by simple and effective repairing process whereby the machine down time can be reduced from about 115 days from about 20 days thereby saving unwanted production loss and also improving the man hour utilization in processes involving such H.T. motors.
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Summary of the Invention
Thus according to the basic aspect of the present invention there is provided a method of repairing H.T. motors with enhanced repaired life of the H.T. motors comprising:
(i) inspecting the route/path of supply /star lead cable/strip and feasible
possibility of rerouting; (ii) removing all the three flexible supply cable /formed jumper strip/leads
as applicable; (iii) removing all the three leads of flexible star connection cable/ formed
jumper strip and re-jointing in proper (same ) sequence with three
phases of the supply side; and (iv) jointing of the flexible supply cable /formed jumper strip in proper
(same ) sequence with three phases of the star connection.
Importantly, the above method repairing H.T. motors with enhanced repaired life the motors comprises providing required insulation at joints, applicable rerouting and proper lacing avoiding any crossing. Also, it is important to confirm the proper state of all the coils used in repairing.
In particular, the above method of repairing with enhanced life has been achieved by way of detailed studies to ascertain the root cause of failure of such repaired H. T. motors carried out under the present invention which have revealed that the majority of failed coils of the motor were found to be nearer the supply lead (9 o'clock position). It is also noted that about 77% of the failure was due to earth fault and Ph-Ph faults. 80% of the cases of failures were identified in the first coil of the coil groups (supply lead).
It is thus possible by way of the above disclosed method of repairing of H.T. motors to enhance the life of the H.T. motors by way of a simple yet effective interchanging of the connection of supply leads with star point lead involving almost negligible cost of repairing. The invention, therefore, achieves the required improvement in the quality of repair of H.T. motors to thereby get economical service life from such repaired H.T. motors.
In accordance with an aspect of the invention in the method of repairing H.T. motors with enhanced repaired life the motors the said step of confirming the proper state of the coils comprises (a) visual and physical inspection,(b) balanced inductance and
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resistance of all the phases,(c) controlled H.V. tests of all the coils of the stator and inter-turn testing of the complete stator winding.
Moreover, the method of repairing H.T. motors with enhanced repaired life the motors further comprises complete H.T. stator testing.
In accordance with another aspect of the present invention in the above method of repairing H.T. motors with enhanced repaired life the motors the complete H.T. stator testing comprises (a) balance of resistance, inductance and sequence of all the three phases,(b) controlled H.V. test as per the specifications of the stator and (c ) inter-turn test and power magger and related testings.
In accordance with yet another aspect of the invention in the above method of repairing H.T. motors with enhanced repaired life of the motors further comprises providing for the changed direction of rotation of the drive due to the change in the star point with the supply leads and vise-versa..
Importantly, the above method of repairing H.T. motors with enhanced repaired life the motors includes rerouting the supply jumper cable and the star point cable internally to match the change in position of the supply leads with the star leads and vice-versa for effective connection with Bus bar. In case of strip bars as jumpers routed through framed rings such that the position of supply point from the stator (motor) side is maintained unchanged by connection interchange at the jumper rings only.
The method of the invention is thus based on extensive studies of burnt report which lead to the identification of core cause of the failure which include a) majority of failed coils where identified to be nearer the supply led (9 o'clock position), b) 77% of the failure was found to be due to earth fault and Ph-Ph fault and more importantly 80% of the cases of failure were found to be in the first coil of the coil groups (supply lead).
It is only by way of such extensive studies of the cause of motor failure and its insufficient life cycle post repairing that has provided for the identification of the selective solution to avoid such unwanted repeat failures of the serviced or repaired H.T. motors. As discussed above the present invention thus provided by way of a selective simple interchanging of star point leads with the supply leads and vice versa to avoid the problems of repeated and close failure of the repaired H.T. motors.
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The details of the invention, its object and advantages are explained hereunder in greater detail in relation to the following non- limiting accompanying figures wherein:
Figure 1: Illustrates the results of the study on the extent of damage to the coils in case of motors of different drives due to motor failure;
Figure 2: Illustrate the break down voltage of one phase of 11KV stator;
Figure 3: Illustrate the interchanging of connection of star point leads with the supply leads and vice versa in accordance with the present method of repairing of motors, and
Figure 4: Illustrates the precautions required in arrangement due to the change in direction of rotation of drive due to the change of star point with supply leads.
In order to ascertain the repaired life of H.T. motors, the life of revamped and repaired motors where studied and its impact in terms of the revamping cost and expenses involved were ascertained. For the purpose variety of repaired life of H.T. stators (motor) were studied and the life of the revamped motors and also the life of repaired motors were identified as detailed hereunder in Table 1.

As would be apparent from the details of Table 1 above the average life of a repaired H.T. motor was found to be only about six years. The impact of the revamping cost was also studied and the details are noted in the following Table 2:
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It would be apparent from Table 2 above that the average annual revamping cost for a 20 H.T. status was to the tune of Rs. 120 lakhs out of which Rs.48 lakhs remained unrealized due to 40% lesser life span ( 6 years compared to 10 years ) of the repaired motors. There has thus been indirect impact on the cost in order to restore the motor for its desired operation, which was to the tune of about Rs.800 lakhs for restoring the job.
Moreover, the machine downtime required for such servicing / repairing also contributed to the complexities and losses in the use of such repaired H.T. motors. Some trial repaired jobs were studied and the down time for repair of the motors were noted as hereunder in Table 3:

It would be apparent from Table 3 above that there is substantial damage in case of motor failure and in case of failed motor on an average 30% of the coils used get damaged. This was apparent from the further studies on the extent number of coil
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damaged in case of motors of different drives which is further illustrated in detailed in accompanying figure 1.
Also, the nature of failure (electrical) of the various stators were studied for a period of six months and the details obtained are reproduced in Table 4:

It would be apparent from Table 4 above that out of the 60 nos. of failures noted, 48 failures are caused by earth or Ph-Ph fault and out of this 32 times first coil of the coil group was found affected as also represented in following Table 5.

Moreover, breakdown voltage of one phase of 11 KVstator was studied and the details are represented in accompanying figure 2.
Based on the above extensive studies, it was identified that an alternate solution for avoiding frequent breakdown of repaired H.T. motor could be achieved by simply selectively interchanging the connection of the star point leads with the supply lead or vice versa. Such interchanging of connections is clearly represented in detail in the accompanying figure 3.
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In particular, in accordance with the representation in Figure 3 the selectively interchanging the connection of the star point leads with the supply lead or vice versa and for effective repairing of HT. Motors with enhanced life the process of repairing in accordance with a preferred aspect of the invention should include the following:
For the successful implementation of the process it is extremely important to ascertain the healthiness of all the coils. This can be achieved by following, a) visual and physical inspection of the coils by an expert, b) balancing of the inductance and resistance of all the phases, c) controlled H.V. test of all the coils of the stators and d) inter-turn testing of the complete stator winding.
Importantly, in accordance with the preferred aspect the different process steps directed to effective repairing of the H.T. motor include:
i. Proper inspection and study of the route / path of supply / star lead cable / strip and feasible possibility of re-routine, ii) removing all the three flexible supply cable/formed jumper strip/leads as the case may be, MI) removing all the three leads of the flexible star connection cable / formed jumper strip and re-jointing them in proper (same) sequence with three phases of supply side, iv) jointing of flexible supply cable/form jumper strip in proper (same) sequence with three phases of star connection, v) providing sufficient insulation at joints, suitable re-routine and proper lacing avoiding any crossing.
Finally, the complete H.T. stator is required to be tested completely by I) balance of resistance, inductance and sequence of all the three phases, ii) controlled H. V. test as per specification of the stator and iii) inter-turn test, power magger etc.
Importantly, due to the changing of star point with supply leads and possible changing direction of rotation of the drive is required to be taken care of as further illustrated by way of accompanying Figure 4.
As shown in Figure 4, due to interchanging of position of supply lead with star point lead and vice versa there was possibility of problem while making connection with the bus bar which has also been taken care of by re-routing the supply jumper cable as well as the star point cable internally. Advantageously, in case of strip bars as jumpers which are routed through framed rings as shown in the figure such that the position of supply point from stator (motor) side can be kept unchanged by connection interchange only at the - jumper rings.
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It has thus been possible by way of the above process of interchanging of position of supply leads with star lead and vice versa to successfully enhance the effective life of variety of H.T. motors such as 9.5 M.W., 11 K.V. Synchronous motor of air separation unit of oxygen plant, 5.0 MW, 11 KV, vertical synchronous motor used in water management, 3.3 MW. 6.6KV. Synchronous motor of exhausters in center plants, 2.0 MW. 6.6 KV. Asynchronous boiler feed pump motor of PPP, 1600 KW, 6.6 KV, synchronous motor of exhauster of blast furnace and 630 KW, 6.6 KV synchronous motor of aspirator of center plants.
Advantageously, the method of enhancing the life of H.T. motor of the invention, which is simple and cost effective, is found to favour reliability of coils with negligible repair cost. The process adapted to improve average life of motor and reduced the damage of stator coil from 30% to 5%. The machine down time is reduced from about 115 days to only about 20 days with a possible saving of about 1.16 crore per year due to increased life of motor. Moreover, increased life of the repaired motor also favours saving of costly man hours and it is estimated that the enhanced life of repaired motor can save about 8000 man hours per year due decreased down time.
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We Claim:
1. A method of repairing H.T. motors with enhanced repaired life of the H.T. motors
comprising:
(i) inspecting the route/path of supply /star lead cable/strip and feasible possibility of rerouting;
(n) removing all the three flexible supply cable /formed jumper strip/leads
as applicable; (Mi) removing all the three leads of flexible star connection cable/ formed
jumper strip and re-jointing in proper (same ) sequence with three
phases of the supply side; and (iv) jointing of the flexible supply cable /formed jumper strip in proper
(same ) sequence with three phases of the star connection.
2. A method of repairing H.T. motors with enhanced repaired life of the motors as
claimed in claim 1 comprising providing required insulation at joints, applicable
rerouting and proper lacing avoiding any crossing.
3. A method of repairing H.T. motors with enhanced repaired life the motors as
claimed in anyone of claims 1 or 2 comprising confirming the proper state of all
the coils used in repairing.
4. A method of repairing H.T. motors with enhanced repaired life of the motors as
claimed in claim 3 wherein said step of confirming the proper state of the coils
comprises (a) visual and physical inspection,(b) balanced inductance and
resistance of all the phases,(c) controlled H.V. tests of all the coils of the stator
and inter-turn testing of the complete stator winding.
5. A method of repairing H.T. motors with enhanced repaired life of the motors as
claimed in anyone of claims 1 to 4 comprising-complete H.T. stator testing .
6. A method of repairing H.T. motors with enhanced repaired life of the motors as
claimed in claim 5 wherein the complete H.T. stator testing comprises (a) balance
of resistance, inductance and sequence of all the three phases,(b) controlled H.V.
test as per the specifications of the stator and (c ) inter-turn test and power
magger and related testings.
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7. A method of repairing H.T. motors with enhanced repaired life of the motors as
claimed in anyone of claims 1 to 6 comprising providing for the changed direction
of rotation of the drive due to the change in the star point with the supply leads
and vise-versa..
8. A method of repairing H.T. motors with enhanced repaired life of the motors as
claimed in claim 7 rerouting the supply jumper cable and the star point cable
internally to match the change in position of the supply leads with the star leads
and vice-versa for effective connection with Bus bar.
9. A method of repairing H.T. motors with enhanced repaired life of the motors as
claimed in anyone of claims 1 to 8 wherein in case of strip bars as jumpers routed
through framed rings such that the position of supply point from the stator
(motor) side is maintained unchanged by connection interchange at the jumper
rings only.
10. A repaired H.T. motor with enhanced life repaired following anyone of claims 1 to
9 comprising of interchanged connection of star point leads with supply leads and
vise-versa.
11. A method of repairing H.T. motors with enhanced repaired life of the motors
substantially as herein described and illustrated with reference to the
accompanying figures.