AN INTEGRATED APPROACH FOR THE IMPROVEMENT OF DYNAMIC CHARACTERISTICS OF VENTILATION FAN OF A TRACTION MOTOR

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1 International Journal of Mechanical Engineering and Technology (IJMET) Volume 7, Issue 1, Jan-Feb 2016, pp , Article ID: IJMET_07_01_012 Available online at Journal Impact Factor (2016): (Calculated by GISI) ISSN Print: and ISSN Online: IAEME Publication AN INTEGRATED APPROACH FOR THE IMPROVEMENT OF DYNAMIC CHARACTERISTICS OF VENTILATION FAN OF A TRACTION MOTOR Bishnu Pada Pal Deputy Manager, Experimental Mechanics and Virtual Reality Lab, BHARAT HEAVYELECTRICALS LIMITED, Corporate Research & Development Division, Vikasnagar, Hyderabad , India K. Ramakrishna Additional General Manager, Experimental Mechanics and Virtual Reality Lab, BHARAT HEAVYELECTRICALS LIMITED, Corporate Research & Development Division, Vikasnagar, Hyderabad , India G. Jayaraman Additional General Manager Metallurgy Lab, BHARAT HEAVYELECTRICALS LIMITED, Corporate Research & Development Division, Vikasnagar, Hyderabad ABSTRACT Ventilation fan constitutes one of the critical components of traction motor. As the weight of the component is to be kept minimum, it is made up of aluminium alloy casting. But it has to withstand high centrifugal acceleration and shock loads. Any failure of the fan shall lead to consequential damages to the motor resulting in long outages and huge expenditure. This paper presents an integrated approach with an improved quality plan to meet the operational criteria of the fan. It consists of a review of all the aspects like design analysis, material composition, casting process and control, heat treatment, post casting operations, aerodynamic and mechanical performance testing in a specially built test rig followed by field trials, which resulted in achieving a longer and trouble free performance. Key words: Accelerometer, Experiment, Finite Element Analysis, Slip Ring, Strain Gauge, Testing, Ventilation fan editor@iaeme.com

2 An Integrated Approach for The Improvement of Dynamic Characteristics of Ventilation Fan of A Traction Motor Cite this Article: Bishnu Pada Pal, K. Ramakrishna and G. Jayaraman. An Integrated Approach for The Improvement of Dynamic Characteristics of Ventilation Fan of A Traction Motor, International Journal of Mechanical Engineering and Technology, 7(1), 2016, pp INTRODUCTION The ventilation fan impellers of the motor were observed to be giving limited performance and resulting in premature failures. Fig1. Shows a sample failed ventilation fan. Detailed investigation was carried out on some failed and new samples of the same lot, which resulted in the following observations and recommendations. Scanning Electron Microscope (SEM) studies on the impeller with failure in the blade portion shows shrinkage porosity with dendrite lobes. This behaviour can be caused by an excessively high pouring temperature or by failure to provide proper risers. Figure 1 Failed sample of the ventilator In the case of umbrella type impeller failure, SEM studies reveal cleavage facets indicating brittle failure and also presence of cavities. This could have been due to entrapped air being carried into the mould. Radiography requirement is class 5 for thickness greater than 10mm and class 4 for thickness less than 10mm and full radiography analysis indicated class 8 in some portions. It is recommended to maintain radiography of Class 3 or better During mechanical testing some samples failed without ductility. Though the material specification is GAl7SiMg which stands for sand casting, to further improve the performance the casting process is revised to low pressure die casting. Microstructures are not as per T6 heat treatment and traces of cavities are seen. Fatigue strength evaluation on test bars is found to be necessary Dimensional accuracy is found to be further improved by measuring dimension using CMM. 2. FE ANALYSIS OF VENTILATION FAN As failures of fan impellers in operation were observed in the umbrella mode, where the back plate gets cracked, its minimum thickness was increased by 50% in the prototype development Finite Element Analyses was carried out on the Fan impeller using ANSYS tools to obtain natural frequencies and stress distribution with over speed runs and appropriate shock loads. Fig.2 shows the 3D model of the ventilator. Fig.3 shows the vonmises stress plot of the ventilator and fig.4 shows Campbell diagram generated from the FE modal analysis for first 10 modes editor@iaeme.com

3 Bishnu Pada Pal, K.Ramakrishna and G.Jayaraman It is seen from the FEA results that stresses and deformations are within limits even at over-speed runs with appropriate shock loads and the umbrella mode occurs at a very high modal frequency, which does not impose high stresses/deformations. Hence, the failures are not due to these reasons and so they must be due to inadequacies in the casting process and/or heat treatment. Figure 2 3D model of the ventilator Figure 3 vonmises stress plot Figure 4 Campbell diagram 3. CFD ANALYSIS OF FAN IMPELLER CFD analysis was carried out to study the aerodynamics of the fan and to find out any possibilities that flow induced stresses may be the cause of failures. It can be concluded from the results obtained that the flow within and around the impeller is in order. Neither pressure variations nor the velocities observed can be termed potential sources for the kind of damage shown in impeller. Fig.5 and fig.6 shows the total pressure variation over ventilator and static Pressure variation over the ventilator, respectively editor@iaeme.com

4 An Integrated Approach for The Improvement of Dynamic Characteristics of Ventilation Fan of A Traction Motor Figure 5 Total pressure variation over ventilator Figure 6 Static Pressure variation over the ventilator 4. REVISED QUALITY PLAN Subsequent to the analysis dealt in the previous sections, a revised quality plan is made that is discussed in the following sections 4.1. Component Geometry and Dimensions: Strictly as per drawing. Quality Plan: Supplier has to arrange measurement of dimensions of each impeller using computer-aided co-ordinate measurement machine (CMM) to ensure quality and provide the results. The supplier has to also provide with each impeller a certificate of compliance with the desired dimensions or indicate any deviations in the dimensions. These measurements are to be made in the presence of representatives Material Aluminium Alloy G-Al Si 7 Mg Wa (DIN 1725) or equivalent as per Product Standard Quality Plan: Supplier has to certify the quality of the material. Virgin material (not re-melted from scrap) to be procured from reputed suppliers to ensure quality Material Specifications As per Product Standard with modified elongation of 7 9% on Test Bar and minimum 5% on casting editor@iaeme.com

5 Bishnu Pada Pal, K.Ramakrishna and G.Jayaraman Quality Plan: Test bars are to be made along with each lot of impeller castings. Supplier has to arrange for tests on Test bars and at least one impeller casting in a lot to ensure that elongations as specified above are obtained and provide the results. The supplier has to also provide with each lot of impellers a certificate of compliance or deviation. Spare Test bars are to be supplied to carry out tests independently Material Testing As per specification Product Standard Quality Plan: Test bars are to be made along with each lot of impeller castings. Supplier has to arrange for tests on Test bars and at least one impeller casting in a lot to ensure that other material properties and quality as specified in Product Standard No. TM are met. All the results are to be provided and also provide with each lot of impellers a certificate of compliance or deviation. Spare Test bars are to be supplied to carry out tests independently Casting Process Preferably Gravity/Low Pressure Die Casting. Quality Plan: Supplier has to give details of casting process being followed. 4.6 Melting process Preferably Induction melting Quality Plan: Supplier has to give details of the melting process being followed Heat Treatment T6 condition Quality Plan: Supplier has to certify that heat treatment is carried out as per T6 condition Microstructure Globular micro-structure Quality Plan: Supplier has to arrange for micro-structure tests on each lot and provide the results along with certification about achievement of Globular microstructure Casting tolerance As per IS 4897 CL-1. Quality Plan: Suppler has to certify that casting tolerances are as per IS 4897 CL Radiography Class 3 or better (100 % radiography to be done on all castings as per Product Standard at positions marked A in the drawing and DP Testing as per Corporate R&D Standard AA & AA for remaining portion). Quality Plan: Supplier has to arrange to carry out cent per cent radiography and DP testing as specified above and provide the test results along with certification about actual micro-structure obtained editor@iaeme.com

6 An Integrated Approach for The Improvement of Dynamic Characteristics of Ventilation Fan of A Traction Motor Balancing Dynamic balancing to be done on each impeller by addition of suitable weights at slot X shown in drawing at a minimum speed of 500 RPM. Quality Plan: Supplier has to arrange to get dynamic balancing done at a minimum speed of 500 RPM on each impeller and provide compliance certificate Stress relieving Fan impellers to be stress relieved as per stress relieving cycle given in the drawing Quality Plan: Supplier has to arrange to get stress relieving done on each impeller and provide compliance certificate 5. MANUFACTURE OF PROTOTYPES The Die & prototype ventilation fans manufactured with utmost quality control following the above revised quality plan and specifications have resulted in improved fan impellers, which may be expected to perform much better in service. Fig.7a and fig.7b shows two views of the Die made by the vendor for casting the ventilation fan impeller. Fig.8 shows the prototype ventilation fan. Figure 7a Figure 7b Figure 7a and 7b Two views of the Die for casting of ventilation fan Figure 8 One prototype ventilation fan 6. TEST RIG FOR VENTILATION FAN In addition to regular spinning & vibratory loads in operation, traction machine ventilation fan is subjected to shock loads when the train passes over track joints. A editor@iaeme.com

7 Bishnu Pada Pal, K.Ramakrishna and G.Jayaraman Test Rig has been established to test prototype ventilators subjected to simulated operational loads. In this test rig, shock loads are simulated (in addition to regular loads) through a cam-operated spring-loaded hammer. The fan impeller along with shaft & bearings is mounted on a base plate and connected to the drive motor (which is not mounted on the said base plate) through a flexible coupling. This base plate is subjected to intermittent shock loads through a shock-frame which gets intermittent hits from the cam-operated spring-loaded hammer. Springs of different stiffness can be used to get different shock load conditions. The cams are operated with a geared motor, whose speed can be varied to change the frequency of the shock loads. The base plate is mounted on a spring matrix to simulate vibratory loads coming on the impeller in operation. The basic spinning of the impeller is done by the main motor, whose speed can be varied using the VF drive controller. Thus, different load conditions can be simulated to get a complete picture of the operational performance of the fan impeller. Fig.9 show the solid model of the ventilation fan test rig and fig.10 shows the photograph of the actual test rig established. Figure 9 Solid model of Fan Test Rig Figure 10 Photograph of fan test rig Assembly with Air Duct 7. LAB TESTING OF THE FANS To evaluate the structural performance of the prototype impeller, it was strain gauged at the critical location and the spin & vibration/shock test of one of the prototype DE ventilator fans in the lab. To simulate the actual service conditions, the impeller was rotated at over speed and intermittent shock loads as per IEC standard were given at about 30 seconds intervals using the cam & Hammer arrangement to simulate the track-joint jolts in actual operation. Fig.11 shows the strain gauging carried out on the fan for lab testing editor@iaeme.com

8 An Integrated Approach for The Improvement of Dynamic Characteristics of Ventilation Fan of A Traction Motor Figure 11Strain gauged prototype ventilator Fig.12. shows the connection of strain gauges and slip ring with the ventilator while rotation. Figure 12.The ventilator while rotation. Strain gauges are placed at the locations where the maximum stresses occur based on Finite Element Analysis and also that is the location of the failure reported in the site. Two strain gauges are placed in the radial direction and two are for hoop direction. Each strain gauge is connected with the slip ring and to the data acquisition system. All four strain gauges reading are monitored in the dynamic strain indicator system and daq software is used to record the test data online. Fifth channel is connected with an accelerometer to see the shock level in vertical direction. Fig.13. shows the Instrumented set-up for data acquisition via Slip Rings assembly using dynamic strain indicator Figure 13 Instrumented set-up for data acquisition via Slip Rings assembly editor@iaeme.com

9 Bishnu Pada Pal, K.Ramakrishna and G.Jayaraman 7.1. Results summary The maximum Hoop and radial strain levels during steady state operation is less than 300 micro strains The maximum Hoop and radial strain levels during shock loading is less than 350 micro strain. The steady state strain levels are found to be stable and well within the limits after the completion of the testing, indicating that no plastic deformation is taking place. Based on the above results of spin and shock testing, it is concluded that the maximum strain levels are well below the yield strain limit of 2600 micro strain, which corresponds to the yield stress limit of 190MPa for Aluminum and also no plastic deformation in the ventilator is observed during/after the prolonged testing. Further as against the IEC standard requirement of shock loads for 18 cycles, the testing was done extensively for more than 1000 cycles. Thus it can be concluded from the test results that the ventilator is fit for prolonged operation at rated speed and designed shock loads. 8. CONCLUSIONS Detailed investigation was carried out on some failed and new samples of the same lot. FE and CFD analyses carried out on fan impeller not only give insight into the loading and flow distribution in the present fan, but also will be helpful in new design. Indigenous vendor developed can be expected to still improve the quality in view of the experience gained from this work and give still better products in the future. The Die & prototype fan impellers manufactured with utmost quality control in various aspects including material, casting process, dimensional accuracy, mechanical strength, heat treatment & microstructure have resulted in improved fan impellers, which may be expected to perform much better in service. The results of the aerodynamic performance and endurance tests carried out on the prototype impellers for varying speed and vibratory/shock loads as experienced in actual operation have given more confidence on the better performance of these fan impellers in actual operation. REFERENCES [1] Detailed technical Report on the project Development of Design methodology, Analysis, Manufacture and Endurance testing of Ventilation Fans for Traction machines by M. Mohan Prabhu and Bishnu Pada Pal. [2] Strain Gauge manual. [3] IEC standard. [4] Ansys user manual. [5] ASTM standard and for radiography. [6] Sanjib Kalita. Uses of Mems Accelerometer in Seismology, International Journal of Advanced Research in Engineering and Technology, 4(6), 2013, pp [7] Dr. N.R. Bhasme and Mr. Pranit Durge. Comparative Study of Electrical Motors for Ceiling Fan Application, International Journal of Electrical Engineering and Technology, 6(5), 2015, pp