CHAPTER 7 CONCLUSION

Size: px

Start display at page:

Download "CHAPTER 7 CONCLUSION"

Sheryl Bates
6 years ago
Views:

1 125 CHAPTER 7 CONCLUSION 7.1 CONCLUSION Motors of rating less than 15 HP form 80 % of the motor population in India. In agriculture, the commonly used ratings of motors are 5 HP (3.7 kw) and 3 HP. The design for low capital cost is standardised in these motors. In India, rewinding is prevalent and most of the rewinders are not well informed. Because of the high probability that the value of the stator winding variables in a large population of motors, deviate from even the Standard efficiency design, performed according to the design for lower capital cost, enormous amount of needless energy consumption occurs. This takes place predominantly in the agricultural sector and to a lesser extent in the Industrial sector in India. In typical 1.5kW and 15kW three phase induction motors, copper loss (and particularly the stator copper loss) dominates. Hence, due to variation in the stator winding, in the form of reduction in conductor size and reduction in number of turns per coil from the optimal conductor area and turns per coil from the optimal number of turns per coil distribution, enormous amount of power is wasted. For example in the reported design of three phase induction motor of 3.7 kw under test, which is commonly used in Indian agricultural and

2 126 industrial sectors, an efficiency reduction of 5.1 and 5.35 percentage points from the original Energy-efficient design occurs due to a reduction of a single turn per coil and a reduction of one SWG respectively from the original design data. Hence, there is a large potential to avoid unwarranted energy consumption by operating-efficiency improvement in such motors. Unwarranted energy consumption and performance deterioration due to improper windings that occur in Custom-designed three phase squirrel cage induction motors Rewound three phase squirrel cage induction motors operating at End-user s site Conventional tests are not adequate to determine the quality of custom- designed motors in the manufacturing line before assembly of the motor itself. The other tests are not able to determine the unknown stator winding data in operational three phase induction motors. Hence, they cannot offer a solution to the above stated efficiency and performance reduction. This Reduction in efficiency and needless energy consumption can be avoided respectively by: Evolving a Quality Assurance approach. The approach is for non-destructive manufacturing-line Quality assurance of low horse power Custom-designed three phase squirrel cage induction motors. It consists of a proposed method to determine the standard stator for Surge comparison test. Evolving an Analysis Approach for improving the operatingefficiency of the motors. This includes a non-destructive

3 127 method, employable in the field, to determine three unknown winding data viz., Number of Parallel Circuits in each Phase (NPCP), Number of Turns per Coil (NTPC) and Conductor cross-sectional area (CA) used in the stator winding of a rewound low horse power three phase induction motor of a particular design. Then, these data are compared with the master file, which consists of winding data from manufacturers to major service providers, to determine if improper winding is present in the rewound motor of the particular design and if it is the cause for operation of the motor at low efficiency at the end-user s site. This thesis is basically an experimental work, which attempts at restoring the efficiency to the operating-efficiency level achieved by the original design and thus, to avoid the unwarranted energy consumption and performance deterioration that are caused due to improper windings, by implementing the winding as per designer s specification. It also explores for possible efficiency improvement in induction motors of Standard efficiency design above its original design efficiency level by using conductor of cross sectional area higher than the conductor cross- sectional area as per the Standard efficiency design, provided the slot size permits. The necessary tests for both the solutions are a proposed EMF test, Resistance measurement test, Visual inspection test for determination of approximate conductor cross-sectional area and determination of length(s) of mean turn(s). A mathematical proof for the theoretical validity of the proposed algorithms employed is expounded. It evolves that there exists a unique solution for determining three unknowns (the Number of parallel circuits in each phase, Number of turns per coil and Conductor area) that can take only

4 128 discrete values from two equality relations (in the form of EMF test and Resistance measurement tests) and a constraint (in the form of Visual inspection test). Experiments carried out on motors of three different configurations to determine the validity of the proposed quality assurance methods are reported. Experiments carried out on motors of five different configurations to determine the validity of the proposed methods in the analysis approach are reported. The methods provide accurate results Quality Assurance Approach In this Quality Assurance approach, a non-destructive method is carried out before assembling the stator and rotor of custom-designed motors. The search conductor that is placed inside the stator bore, for the EMF test is removed, once the test is over. Therefore, performing Quality Assurance on a stator by means of the proposed approach is non-intrusive. Once the tests are over, a Forward algorithm for determining the standard stator is used for the Quality assurance of Low horse power cage induction motors. The motor stators which have winding that deviates either in conductor SWG or in turn per coil implemented, from the designer s specification, is rejected during the Quality Assurance approach. Hence, the approach can assure the quality of the motors accurately. The manufacturer can opt for producing a model motor before going in for producing a batch of custom designed machines. If they produce a model machine then lead time will be introduced in execution of an order for custom-designed machines; Loss of time, energy and money will occur.

5 129 Further, since the drive in industry is for reducing the overheads, the winding work is also sub-contracted. Hence, producing model motors for custom designed motors are not in practice. Hence, adopting the proposed approach of quality assurance can be an easier approach for quality assurance. Even if the conventional end-of-line Quality Assurance test may detect that a motor operates below the design efficiency level, it can be done so only after building the entire machine. This involves time and energy wastage. Thus the Quality Assurance approach helps in avoiding unwarranted performance reduction that includes efficiency as well as avoids unwarranted reduction in life of the motors Analysis Approach for Efficiency Improvement On the other hand, while determining the winding configuration in a motor that is operational in the field as per the proposed method for determination of unknown winding data, the stator has to be disassembled from the rotor. Here, the test may be termed intrusive. However, the level of intrusion can be minimised, if the planning for the test is done when the motor is taken up for scheduled preventive maintenance work. Once the tests are over, a Reverse algorithm for determination of unknown winding data is carried out in the method to determine the improper winding in the Rewound motors. The proposed algorithm for determining the winding data can differentiate a winding with either a slight deviation of Conductor crosssectional area (CA) from the designer s specified SWG or deviation in the Number of turns deviating from the designer specified NTPC. Hence, the

6 130 method is effective in determining winding data accurately. This measure will be applicable in countries like India, Brazil etc Proposed Measures for Achieving Higher Operational Efficiency The winding data for motors of common ratings are provided by manufacturers to major service providers. If such winding data is not available, they may be arrived at through design. Therefore, suitable recommendations for corrective measures can be made for restoring the efficiency to the operational efficiency level achieved through the original design or in the case of Standard efficiency design, for improving the efficiency of the motor above the original design-efficiency level, to the enduser. Restoration to Operating-Efficiency Level Achieved through Original Design: For an improperly Rewound motor that has fewer turns than that in the winding specification for the particular design efficiency level, the recommendation should be to rewind the motor so that it has the required number of turns as per the original winding specification. If stator slot size does not permit larger winding conductor than the winding specification for the design, then a conductor whose cross sectional area sticks to the requirement of the design for that particular efficiency category is used for rewinding. The above recommendation will be applicable to improperly rewound energy efficient motors also. Efficiency Improvement Measure: In the case of Standard efficiency design motors, if the improperly rewound motor has Conductor cross sectional area smaller than that required by the winding specification for

7 131 the design efficiency level, then the following recommendation is to be adopted. If the stator slot size permits, winding conductor with crosssectional area higher than the requirement for that Standard efficiency design can be employed. This recommendation will be mainly applicable to improperly rewound motors of Standard efficiency. These recommendations are also applicable to Standard efficiency design motors that have not been rewound. If slot size permits insertion of conductors of increased cross-sectional area, the ohmic losses in the stator will be reduced and there will be efficiency improvement. All the measures discussed here improve the efficiency of the motor from its earlier operating efficiency. This could be achieved even without any modification of the core. Throughout the thesis work, the discussions are based on three phase squirrel cage induction motors. It is obvious that a machine can be used either as a motor or a generator. Hence, the discussions in the thesis may also be applicable for all machines with three phase windings in the stator such as Induction generators, Synchronous generators and Synchronous motors. The time needed for testing and calculation in the proposed nondestructive methods is about 20 minutes. The time may be further reduced by computerizing the calculation part. Further reduction of time for testing may be achieved by automating the solutions. Energy requirement for the proposed non-destructive methods are minimal as the tests necessary for the solutions do not belong to the load test category, which requires comparatively large amount of power.

8 132 A simple and a sophisticated mechanical set-up used for aligning the rotor concentric to the stator are developed. The simple mechanical set-up is portable and can easily be employed in the field Benefits to the End-User By following the efficiency restoration / improvement measures, efficiency is restored to the operational efficiency level achieved through the original design or even better efficiencies can be achieved in the motor, which had improper winding. In the reported motors B and C of 3.7 kw Energyefficient design that had improper winding in them, a net efficiency improvement of over 4.5 percentage points could be achieved by rewinding them with proper winding. If the reported motor after the efficiency restoration measure operates in the industrial sector for 7200 hours per year, the potential electricity savings in kwh per motor per year is The value of electricity saved works to the tune of INR 3,596, with the electricity rate considered to be INR 3 per kwh. The pay back period for the amount spent towards the efficiency restoration measure is 1.8 years. Avoiding the unwarranted temperature rise in motors with improper winding, by rectifying the impropriety in the motor, will result in avoiding the unwarranted reduction in the life of the motor. It may even lead to reduced temperature rise in cases when the level of efficiency is raised by employing conductor of cross-sectional area higher than the design data for low capital cost design during the proposed efficiency improvement measure. This will ultimately result in increased life of the motor. Therefore, by avoidance of improper winding in both the solutions presented in the thesis, efficiency and life of the motor are either restored to the operational efficiency level achieved through the original design or even improved further above the original design-efficiency level. Thus, energy

9 133 efficiency can be achieved with the help of the proposed non-destructive approaches Feasibility of Implementation The proposed approaches are applicable to motors of all winding configurations employed for low horse power three phase squirrel cage induction motors. The proposed Quality Assurance and Analysis Approaches are within the capability of winders. By issuing easy-to-understand literature and hands-on training for a day, they will be able to carry out the proposed approaches for efficiency improvement. The service providers and industries who carry out the proposed analysis approach on three phase stators can maintain an inventory of test exciters for each of the common combinations of possible stator bore and number of poles. This procedure will be particularly suitable for motors of lower ratings, as they occur in the agricultural and industrial sectors, where lower-power motors are used in large numbers. The efficiency restoration / improvement measure is applicable to motors of Standard efficiency and energy-efficient designs. The manufacturer s who manufacture custom-designed motors, can obviously maintain an inventory of test exciters for each of the common combinations of possible stator bore and common number of poles they produce. The cost for this will be relatively less compared with the benefits of the quality assurance approach, in terms of reducing overheads and good will it can earn.

10 134 A preliminary conclusion from the simplified cost-benefit analysis is that the measure is likely to be cost-effective in industrial motors but not in agriculture. However, the above statement does not take into account additional benefits from the rewinding procedure. For instance, the life of the motor will also be restored or improved further due to reduced heating Recommendations The Analysis approach for efficiency improvement is highly recommended for Indian industrial sector, where the operating hours per year of motors is relatively higher than that of agricultural sector motors and where rewinding is prevalent. Lowest initial cost for a rewind motor should not be the major selection criterion as a low quality rewind will cost more because of wasted electrical energy. Standards of motor repair should be strictly adhered to so as to ensure that the rebuilt motor has the correct bearing fit, correct cooling, correct winding, and that it will run without vibration. All motor stators should be screened by a core loss test prior to stripping to identify motor with heavily damaged stator iron for possible replacement or to have the iron repaired and reinsulated if it is a critical motor that is not immediately replaceable. All the motors should have a core loss test after stripping as a proof that no damage was done in the stripping process. Good rewinding practices should be adopted while rewinding the motors to avoid unwarranted efficiency reductions.

11 135 There is no substitute for quality. Demand for quality should be there. Inspection for quality should be there. Testing for quality in both new and rebuilt electric motors has to be done. Hand winding may be followed. This will reduce the length of overhang and also may reduce stray load loss. Incentives to be given by the government so that it encourages those who wish to implement energy conservation measures. 7.2 FUTURE SCOPE A possible scope to carry out further research is to determine the winding data in motors that have Number of In hands greater than one. Automation of the proposed non-destructive methods is another possible future scope.

CHAPTER 6 RESULTS AND DISCUSSION

72 CHAPTER 6 RESULTS AND DISCUSSION 6.1 INTRODUCTION The earlier chapters discuss the problems that occur due to improper winding in three phase induction motors, the methodological basis for the approaches