NVH ANALYSIS AND MEASUREMENT CORRELATION OF ELECTRICAL STARTER MOTOR FOR AUTOMOTIVE VEHICLES

Transcription

1 NVH ANALYSIS AND MEASUREMENT CORRELATION OF ELECTRICAL STARTER MOTOR FOR AUTOMOTIVE VEHICLES 1 VARATHARAJ NEELAKANDAN, 2 THULASIRAJAN GNAESAN, 3 PRAVEEN CHACKRAPANI RAO 1,2,3 Comstar Automotive Technologies Pvt.Ltd, India. 1 nvaratha@comstarauto.com, 2 g.thulasi@comstarauto.com, 2 c.praveen@comstarauto.com Abstract - Automotive passenger car technology demands more on the driving comfort and safety measures which are directly relates to the Vehicle durability. The vibration in starter motor which caused by the structural design involves the rotational and non-rotational components. To predict the vibration level for the whole starter motor as a system and test it to the NVH aspects for the customer requirement during the design stage is the critical needs. The method of analysis is followed by the Finite element method which includes the lumped mass concept, and modal analysis with harmonic analysis for the Vehicle level vibration loads. The paper work results the mode of the starter motor and resonance frequency of the starter motor system along with stress amplitude for the durability predictions. The many of the others paper study talks about the Stress analysis for the design indent during the conceptual stage. But this paper dealt the vibration aspect analysis to prevent comfortless during the engine start up and improve the Durability life.in the NVH simulation results, the mode shapes and resonance frequency was extracted and explained which are directly relates to the engine systems frequency and also the Starter motor components level design was focused for the NVH by the vibration dynamic simulation to meet the customer specifications. The simulation methodology for Starter motor is established and prediction process of NVH level for the starter motor is implemented. Keywords - FEA, Starter motor vibration, Dynamic analysis, Engine and Automotive Vehicle. I. INTRODUCTION Compressive the automotive engine is assisted by starter motor for starting the engine and the starter motor is mounted on the engine housing block by means of bolts and bracket as shown in the Figure 1: Automotive engine with starter motor. The starter motor is powered by the battery source about 12V and the electrical input is given to the starter motor for the starting of the engine. The starter motor consist of housing, solenoid, brush holders, armature, planetary gears, drive assembly consisting pinion gear and magnet poles. The pinion gear engage with engine ring gear while the solenoid transmit the linear force to the drive assembly. The engine cranking duration will be 0.3 sec to 0.5 sec which is required high torque at shorter period of the time. This is causing the impact load to the starter due to engine starting condition. In the engine starting duration, the combustion pressure is being activated in the engine cylinder and the flywheel inertial load is acting like a resistance to the starter while it engage to run and start the engine. During the cranking of the engine, while the speed of the engine starts gradually and attain the idle speed of the engine, the vibration starts and transferring to the starter motor. This vibration transfer is very harmful for the starter product as well as causing the noise and vibration to the driver in vehicle as shown in the Figure 2: Engine cranking trace. When the starter crank the engine, the whole system creates noise and vibration and so the prediction and optimization being importance to the starter motor design. Starter motor is the rotating element which consist of armature and gears that produce the adequate torque and speed with respect to the engine requirements. Figure 1: Automotive engine with starter motor When the starter crank the engine, the whole system creates noise and vibration and so the prediction and optimization being importance to the starter motor design. Starter motor is the rotating element which consist of armature and gears that produce the adequate torque and speed with respect to the engine requirements. Starter housing bracket is fixed with the engine with vdesigned overhanging hence it is acting as a cantilever member and it has rotating parts which involves in contributing the harmonic vibration. This 1

2 vibration is induced by its own frequency and the operation frequency of the engine systems. energy (mass or inertia), and means by which the energy is gradually lost (damper). The starter vibration consists of lumped parameter systems composed of ideal springs, masses, and dampers wherein each element has only a single function. Linear displacement x, force F, spring constant k, damping constant c and mass. Spring: In the linear spring as shown in the Figure 3: Linear spring, the change in the length of the spring is proportional to the force acting along its length: Figure 2: Engine cranking trace II. VIBRATION ANALYSIS FOR STARTER MOTOR Vibration analysis is playing more important role in the starter motor design and development for its functions and working environments. The engine operating frequency and starter motor natural frequency should not coincide to operate for the starter engagement and disengagement. The 4 cylinder engine system frequency is 200 to 250 Hz Max given by engine spec sheet for the reference. The starter frequency should be well away from the engine system frequency nothing but starter motor frequency should be more than the engine system frequency. Vibration: Vibration, periodic back-and-forth motion of the particles of an elastic body or medium, commonly resulting when almost any physical system is displaced from its equilibrium condition and allowed to respond to the forces that tend to restore equilibrium. Free Vibration: Free vibration is a type of vibration in which a force is applied once and the structure or part is allowed to vibrate at its natural frequency Natural frequency: Natural frequency is the frequency at which a system tends to oscillate in the absence of any driving or damping force. Resonance frequency: Frequencies at which the response amplitude is a relative maximum are known as the system's resonant frequencies or resonance. Effect of resonance during starter engagement: Higher vibration level will be observed by driver. Higher noise level during engine cranking Start motor premature failure. Starter motor housing and other child parts failure. Lower in durability life. Performance degradation. The Vibratory systems comprise means for storing potential energy (spring), means for storing kinetic Figure 3: Linear spring F = k(x u).. (1) The ideal spring is considered to have no mass; thus, the force acting on one end is equal and opposite to the force acting on the other end. The constant of proportionality k is the spring constant or stiffness. Figure 4: Rigid mass Mass: A mass is a rigid body as shown in the Figure 4: Rigid mass whose acceleration x according to Newton s second law is proportional to the resultant F of all forces acting on the mass: F = mx (2) Undamped free vibration: Considering first the free vibration of the undamped system of Figure 5: Undamped single degree of freedom system, Newton s equation is written for the mass m. The force mx exerted by the mass on the spring is equal and opposite to the force kx applied by the spring on the mass: Figure 5: Undamped single degree of freedom system 2

3 mx + kx = 0. (3) Where x = 0 defines the equillibrium of the mass. Solution eq. (4) is x = A sin t + B cos t..(4) Where the term k m is the angular natural frequency deifend by The starter motor 3D model is ashown in the figure shown in the Figure 7: Starter motor total assembly - section view. The starter motor consist of Housing, endplate, armature assembly, drive assembly and solenoid assembly. These 3D model is thoroughly studied and simplified for the finite element analysis. During this process, few parts have been eliminated from the assembly and that has been considered as the lumped mass component concept. ω = rad/sec...(5) The reciprocal of the mass of the natural frequency is, f = = =...(6) Finite element Analysis: The starter motor has more than the 100 parts which involves on the performance and durability of the starter motor. The important parts are considered for the vibration analysis which can deal the dynamic load transfer and vibration stability. The following Figure 6: Modal dynamic analysis process is the defined which explains the vibration analysis for the Starter motor. The starter motor vibration analysis has two cases. 1. Modal analysis 2. Hamonic analysis. These two cases are involved to identify the natural frequency and stress amplitude at the excited vibration condition. Figure 7: Starter motor total assembly - section view The simplified model for the vibration analysis prepared and the lumped mass is calculated and applied for the total starter motor assembly effect. The starter assembly weight is 3.3 kg including the lumped masses of A: Plunger and solenoid coil masses, B: Brush holder mass, C: Armature coil and commutator masses and D: Drive pin masses as shown the Figure 8: Starter motor simplified- lumped masses. Material properties of the components considered for the analysis as below the Table 1: Material properties. Different type of material are used in the starter motor such as steel, aluminum, copper, backelite, fiber glass plastic and Magnet material. Figure 6: Modal dynamic analysis process Figure 8: Starter motor simplified- lumped masses Part Name Material Name Density(kg/m 3 Poisson s Young s ) ratio modulus(gpa) Housing Aluminum Alloy 2.77e Endplate Aluminum Alloy 2.77e

4 Terminals Copper Alloy 8.30e Magnetic Pole Magnet Material 5.0e Backcover Bakelite 1.24e Stationary Gear Glass Fiber Plastic 0.95e Remaining Parts Structural Steel 7.85e The mesh model has been prepared for the good quality of results and quick results setup as shown in the Figure 9: Starter motor Mesh model. The connections of the components relation is given for the better connectivity with contact constraint of contact-target option. Table 1: Material properties Figure 11: Starter motor 1st mode shape Figure 9: Starter motor Mesh model Modal dynamic analysis results: Modal analysis is to identify the natural frequency and mode shape of the starter motor. The first 10 numbers of natural frequency and mode shapes are calculated refer the Table 2: Starter motor natural frequencies.analyzed for the product rigidity with respect to the engine operating frequency. The 1 st frequency of the starter motor so called its fundamental or natural frequency is 572 Hz found from modal analysis as shown in the Figure 10: Starter motor 1st natural frequency. Figure 10: Starter motor 1st natural frequency The engine frequency is 200Hz is far away from the starter motor frequency. There are no opportunities to occur the resonance. The starter motor 1 st mode shape is pure bending like cantilever beam as the starter motor housing bracket is fixed with engine by means of the bolts.deformed shape of the starter motor mode shape shows that the cantilever effect as shown in the Figure 11: Starter motor 1st mode shape which involve the Inertia, density length of the motor and Elasticity of the materials used in the starter motor. Mode Natural frequency(hz) Table 2: Starter motor natural frequencies. Harmonic analysis: Harmonic response analysis is a method used to determine the vibration response of a linear structure to loads that vary sinusoidally with time. The idea is to calculate the structure's response at several frequencies and obtain a graph of some response quantity versus frequency. Peak responses are then identified on the graph and stresses reviewed at those peak frequencies. The load application on the starter motor is applying the G load to the starter motor mountings and mounting holes. Z and Y are the directions of load asper the engine layout and specific to customer standards. Z is the vertical and Y is the lateral of the start motor for the vibration analysis as shown in the Figure 12: Harmonic analysis vibration (G) load direction. 4

5 Figure 12: Harmonic analysis vibration (G) load direction Analysis Results: The harmonic vibration analysis have been conducted for the starter motor with respect to the engine load of 20G at Z direction and 8G at Y direction with the frequency band of 50Hz to 1000Hz. The starter motor excited by the applied load at the mounting bracket and the stress amplitude is calculated from the finite element analysis. The stress at Z direction load is 139MPa induced due to the vibration load as shown in the Figure 13: Harmonic analysis stress plot at Z direction 20G Load. The stress at the Y direction is 72 MPa as shown in the Figure 14: Harmonic analysis Stress plot at Y direction 8G Load. The maximum stress induced at the housing bracket which considered as the weakest link in vibration test based on the components included in the analysis. The material stress limit of the housing bracket which is made up of aluminum alloy is 160 MPa. The resonance test up as shown in the Figure 15: Resonance test setup for starter motor has been made for the starter motor frequency analysis at lab and measured the natural / resonance frequency. The test set up has the provision to test at both the direction of X and Y. Starter is mounted on the fixture and fixture with starter is mounted on the vibration shaker machine. One accelerometer is fixed on the starter end and other accelerometer fixed on the fixture portion for the reference G Load calibration. These accelerometer are connected to the data acquisition instruments. The measurement has been carried out with this set up and the results of the resonance test have been extracted from the data acquisition as shown in the Figure 16: Resonance test results for starter motor. The starter motor measured resonance frequency is 565Hz. Figure 15: Resonance test setup for starter motor Figure 13: Harmonic analysis stress plot at Z direction 20G Load Figure 14: Harmonic analysis Stress plot at Y direction 8G Load Figure 16: Resonance test results for starter motor CONCLUSIONS The starter motor resonance frequency is calculated as 572 Hz using the finite element method and along with that the mode shapes is extracted and found that it is bending mode. Bending deflection occur at the end of the starter motor. The Lab test up is prepared and the resonance frequency analysis conducted in lab and found that the frequency is 565 Hz which is a good correlation the finite element analysis results. The calculated starter motor resonance frequency is compared with the engine frequency and found that the starter frequency is far away and almost two time more. The prediction of resonance frequency 5

6 primarily avoiding the NVH problem during the engine cranking. The Harmonic analysis results in predicting the stress level of the Starter motor due to G load at various directions. The maximum stress is 139MPa at Z Direction G load of 20G and starter motor housing bracket. This improves the starter motor durability and assisting to maintain the starter motor performance. Hence the method of vibration analysis for the prediction of resonance frequency and vibration load stress for the starter is established and implemented. The lab test for the resonance frequency is reduced for the particular variant of start stop starter motor at proto development stage of product development cycle. REFERENCES [1] Ralph E. Blake. Basic vibration theory, Chapter 2, Harris shock and vibration handbook, McGraw Hill Professional, 01-Oct [2] Free vibration of undamped 1D cantilever beam, ANSYS Tutorial, University of Connecticut, School of Engineering [3] Ashwani Kumara, HimanshuJaiswala, Free Vibration Analysis of Truck Transmission Housing Based on FEA,ELSEVIER- Procedia Materials Science 6 ( 2014 ) [4] B. Vidhya, K.N. Srinivas, Vibration analysis including stator, rotor, housing and dynamic response analysis of Flux Reversal Generator, JESIT , [5] Mr.vijaykumar, Mr.shivaraju, Mr.srikanth, Vibration Analysis for Gearbox Casing Using Finite Element Analysis,IJES, ISSN (e): [6] Gopali S Lamani, Prof: S.R.Basavaraddi, Static And Modal Analysis of Tractor Power Take Off (PTO) Gearbox Housing, IRJET, p-issn: