EXPERIMENTAL ANALYSIS OF NOISE AND VIBRATION REDUCTION IN WINDMILL GEAR BOX FOR 5MW WIND TURBINE

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1 International Journal of Mechanical Engineering and Technology (IJMET) Volume 7, Issue 6, November December 2016, pp.76 85, Article ID: IJMET_07_06_008 Available online at Journal Impact Factor (2016): (Calculated by GISI) ISSN Print: and ISSN Online: IAEME Publication EXPERIMENTAL ANALYSIS OF NOISE AND VIBRATION REDUCTION IN WINDMILL GEAR BOX FOR 5MW WIND TURBINE K. Sunil Kumar, S. Arun, A. Mohan Assistant Professor in Mechanical Department, Veltech, Avadi, Chennai, India Dr. Sumathy Muniamuthu Associate Professor in Mechanical Department, Veltech, Avadi, Chennai, India ABSTRACT A Wind Turbine has many rotating parts making it a real challenge to make modal analysis in operating Conditions. The issue is to separate the structural modes from the harmonic components. In addition to that gearbox itself, there is also the generator, the pitch-drive and the yaw-drive producing harmonic vibration for a complete Wind Turbine in operating condition. The 63 ton gearbox tested in FFT test bench with a rotational speed of 1110 rpm and mounted in a test rig. Since the high speed shaft is running at Gearbox housing, the key point observed from FFT Analyzer report is noise in the form of whistling from gearbox housing. In this paper several approaches and techniques have been used in order to extract the structural modes. Modal analysis is applied here to get the natural frequencies of the gearbox housing itself. The FFT analyzer shows the results of excited frequency occurs inside the gearbox housing with the predicted noise level at 105 decibals (offshore) The noise is in the form on huge whistling which may cause severe damage to the living organisms in the sea. Hence gearbox housing is taken into account and applied necessary boundry conditions, we have proved the FFT Results with Ansys Results. Hence we added mass to overcome the resonance of the Gearbox housing. Key words: Gearbox modal analysis, FFT Analyser, Modal analysis of gearbox, Harmonic vibration, Natural frequencies. Cite this Article: K. Sunil Kumar, Dr. Sumathy Muniamuthu, S. Arun and A. Mohan, Identification Experimental Analysis of Noise and Vibration Reduction in Windmill Gear Box for 5MW Wind Turbine. International Journal of Mechanical Engineering and Technology, 7(6), 2016, pp INTRODUCTION The Entire gearbox set up was designed by pro-e version 5 software and analysed the model using Ansys workbench version 13. The problem identified is huge noise inside the gearbox in the form of whistling [1].The gearbox is designed with two stage planetary and one stage helical gear to deliver 5MW at Offshore Conditions. The weight of the entire gear box is 63 tonnes. The entire gear box setup is taken 76 editor@iaeme.com

2 Experimental Analysis of Noise and Vibration Reduction in Windmill Gear Box for 5MW Wind Turbine over FFT Analyzer Test bench[14][15].the test bench report shows that the Natural frequency is matching with excited frequency at Gearbox housing.since High speed shaft is running at Gear box housing with rotational speed of 1110 rpm and pinion teeth of 40.So we have taken gearbox housing in to account for fem simulation and extracted the Modes to determine exact the mode, where the resonance takes place. The weight of the gearbox housing is 12tonnes and the housing is splitted into two halves [6].The formula used to find excited frequency is given in equation 1 as fe = (N*Z)/60 (1) Fe = Gear Mesh Excited frequency in Hertz. N = RPM of the High speed Shaft= 1110 Rpm. Z = No of pinion teeth = 40 teeth Hence the excited frequency is 740 HERTZ. Here extracted n modes and determined the mode at natural frequency is matching with excited frequency of gearbox. Since natural frequency is a function of stiffness and mass. Mathematically expressed in equation 2 as fn = (1/2π )* k/m (2) fn = natural frequency in hertz (cycles/second) k = stiffness of the spring.(newtons/meter or N/m) m = mass(kg) 2. OBJECTIVES To determine resonance inside the gear box by extracting modes using ANSYS and to reduce WHISTLING noise of 5MW gear box in offshore condition. In the generator speed range between 1110 rpm and 1130rpm connect strongly dominanant, whistling noises in the area of high speed power train occurs[5].here natural frequency matches with excited frequency, hence resonance occurs therefore our scope is to shift natural frequency far away or below operating frequency. To reduce the decibel level of gearbox noise less than 105db,when operating speed from high speed shaft is 740 HZ. 3. METHODOLOGY Figure 1 Flow chart showing Methodology 77 editor@iaeme.com

3 K. Sunil Kumar, Dr. Sumathy Muniamuthu, S. Arun and A. Mohan The above figure1 shows the Methodology adopted for design and analysing the Gearbox housing using pro-e and Ansys Workbench. Preprocessing stage defines Geometry, material properties and boundry conditions of Gearbox housing. Post processor stage defines the graphical representation of results and Solver stage defines the Results in terms of frequencies by modes. 4. GEARBOX HOUSING MODEL 4.1. Pro E Model, Figure 2 Pro-e model of windmill gearbox housing The above Figure 2 shows that 3D Model of Gearbox housing using Pro-E software. Wildfire 5 is used to model the gearbox housing and the gearbox housing is splitted into two halves[7] Ansys Model Figure 3 Ansys model of Gearbox housing 78 editor@iaeme.com

4 Experimental Analysis of Noise and Vibration Reduction in Windmill Gear Box for 5MW Wind Turbine 4.3. Boundry Condition Figure 4 Boundry condition The next step is to define the flow boundaries for the model. Based on the natural environmental conditions, the boundaries condition (displacement is arrested by fixed support, fully defined condition) is given to model at the face of gearbox housing. The boundary condition given for the study area is shown in the figure 4. The displacement is fully defined at two faces of the gearbox housing. The Figure 4 shows that the boundary condition assigned at the face of the gearbox housing. Since the load is distributed through torque arm to the gearbox housing. Hence we are arresting the displacement at the face of the gearbox housing as shown in the arrow Mark in the figure 4.Load is distributed from main shaft [9] to first stage planetary gear and first stage planetary gear to second stage planetary gear. The second stage planetary gear receives the load with increased rpm and delivers to high speed shaft running at gearbox housing through Torque arm [10], which is attached to the face of gearbox housing Contacts Assigned To get the contact pair to work, the details settings for contact Face and for target Face for the contact pair can either be left as Program controlled, or set by the user to choices of Top and Bottom, so that the red Contact face and the blue Target face will look towards each other. This is required to get the resulting target and contact elements in ANSYS to attach to each other without implied penetration and solution failure. continuing with the details definition of the contact pair, the contact will be set to Bonded, which permits both Symmetric contact with penalty-based formulations. Since the gearbox housing is splitted into two halves and symmetric in design. 5. GEARBOX HOUSING MATERIAL PROPERTIES The table 1 shows the gearbox housing material properties that assigned during preprocessing stage in ansys simulation. Materials used here is structured steel with elastic properties mentioned below. Table 1 Material properties of gearbox housing. Property Young s Modulus Poisson Ratio Density Symbol E m ρ Units mpa - kg/m 3 Cover Housing editor@iaeme.com

5 K. Sunil Kumar, Dr. Sumathy Muniamuthu, S. Arun and A. Mohan 5.1. Meshed Model of Gearbox Housing Figure 5 Meshed model of full gearbox housing Figure 6 Meshing Stragety. Figure 6 shows the results of Fine mesh with 17443Nodes and elements is formed during (preprocessing) stage of gearbox housing Equations of Motion The steady state solution of this problem can be written as Ø = Phase shift angle X(t) = Xcos(2π ft - ø) (3) 80 editor@iaeme.com

6 Experimental Analysis of Noise and Vibration Reduction in Windmill Gear Box for 5MW Wind Turbine The equation 3results states that the mass will oscillate at the same frequency of the applied force,but the phase shift Ø and amplitude of the vibration x is defined by formula: X (t) = (Fo / K)*((1/ (1- r ) * (2 r) 2 ) (4) The Ratio of the harmonic force frequency over undamped natural frequency is given by equation 5 2 ς R = f/fn (5) At a given frequency ratio,the amplitude of the vibration x is directly proportional to the amplitude of the force with little or no damping,the vibration is in phase with the forcing frequency when the frequency ratio r<1 and 180 degrees out of phase when r>1,when r<< 1 the amplitude is just the deflection of the spring under static force F0. This deflection called static deflection δst. Effects of the mass and damper are minimal, when r >> 1 the amplitude of the vibration is actually less than static deflection δst. In this region the force generated by mass (F = ma ) is dominating coz acceleration seen by the mass increases with its frequency as shown in the figure 7. Figure 7 Frequency ratio vs amplitude ratio. 6. INFLUENCE OF EXCITING DISPLACEMENT For determination of response by direct integration of the FE structure, it is suitable to use sine excitation functions whose frequencies are equal to the frequencies of modal shapes whose response is determined [5].The frequency of excitation force should be varied so that it coincides with modal frequencies. The gearbox is the part with the most excitations during operation. A second group of frequencies are the tooth mesh frequencies of each stage. For both groups you could find also several harmonics. In a typical FFTanalysis of a gearbox under operation could find at least round about 10 different frequency peaks in the spectrum editor@iaeme.com

7 K. Sunil Kumar, Dr. Sumathy Muniamuthu, S. Arun and A. Mohan 7. RESULTS 7.1. FFT Analyzer Results-Test Bench Figure 8 FFT Analyzer results The figure 8 showing about FFT Analyzer results showing that resonance is happening at operating frequency of 740 Hertz with 100 decibals, From above figure X axis implies frequencies in Hertz and y axis implies noise in decibals [11] Ansys Results Figure 9 Ansys results 82 editor@iaeme.com

8 Experimental Analysis of Noise and Vibration Reduction in Windmill Gear Box for 5MW Wind Turbine Table 2 Results Obtained From Ansys MODES FREQUENCIES IN HERTZ Transverse mode at x Transverse mode at y Transverse mode at z Bending mode at x Bending mode at x Bending mode at z Axial mode at x Axial mode at y Axial mode at z Shearing mode at x The Ansys results from figure 9 showing that resonance is happening at gearbox housing with respect to mode four-bending mode with respect to x direction with excitation frequency of 740 Hertz. Table 2 shows the results obtained from Ansys model. Results clearly showing that natural frequency of the gearbox are matching with excited frequency at mode 4- Bending mode with respect to X direction. 8. CONCLUSION From the comparison of test bench report with Ansys results, the results showing that excited frequency is matched with natural frequency of gearbox housing at mode 4 (bending mode about x axis)therefore in order to overcome the resonance mass is added with respect to mode 4. Considering the feasability of design it was adviced and added mass with respect to mode four thereby the noise was reduced from 105 decibals to 85 decibals which is permitted level as per offshore environmental impact regulations. REFERENCE [1] N-J. Jacobsen, Separating Structural Modes and Harmonic Components in Operational Modal Analysis, Proceedings IMAC XXIV Conference, [2] N-J. Jacobsen, P. Andersen, R. Brinker, Eliminating the Influence of Harmonic Components in Operational Modal Analysis,conf [3] P. Andersen, R. Brincker, C. Venture, R. Cantieni, Estimating Modal Parameters of Civil Engineering Structures subject to Ambient and Harmonic Excitation, Proceedings ICSV 15 Conference [4] N-J. Jacobsen, P. Andersen, R. Brinker. Applications of Frequency Domain Curve-fitting in EFDD Technique, Proceedings IMAC XXVI Conference, 2008 [5] N-J. Jacobsen, P. Andersen, Curve-fitted Enhanced Frequency Domain Decomposition A Robust Technique To Harmonic Excitation in Operational Modal Analysis, Proceedings ICSV 15 Conference, editor@iaeme.com

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