1997-02-24

Experimental and Numerical Modal Analysis of a Blower Motor Support 971129

About ten years ago the major noise and vibration sources in a vehicle were the engine and the aero dynamic noise sources. During these last years the noise and vibration levels induced in the vehicle by these two sources have decreased in very high proportions. The consequence of that is that former secondary noise sources like the air-conditioning system have now become major noise and vibration sources. One of the most important noise and vibration sources in the air-conditioning unit is the blower. This component is composed of an electric motor, a fan wheel and a motor support in which the electric motor is fixed. This work only deals with the vibrations generated which are due either to the wheel unbalance or due to electric motor vibrations. The unbalance can generate vibration discomfort on the steering wheel and the electric motor vibrations are very often solid borne noises generated especially for low speeds of operation. In order to solve these problems we need uptodate tools. The experimental modal analysis and the structural analysis are two tools that we try to marry with each other and solve our noise and vibration problems. The experimental modal analysis is the tool that gives the vibration properties of a component under test. It's force is to be able to give the exact results concerning the system under test. The difficulty of this method is to make sure of the configuration of the product under test, the prototype has to be very near to the final product! This is very often impossible since the prototype is often far from the final product in terms of vibration properties. The second difficulty is the time it takes to build a setup and to do complete modal analysis on the initial and the optimized components. The numerical simulation can be helpful in simulating and extrapolating very rapidly conditions for which no prototype exists. What is for the moment difficult is to simulate all the vibration properties and conditions. This is a long-term research work. The only solution is to adopt a strategy based on marrying the two methods by completing and adjusting the numerical results by test results. This document gives an example of application of the two methods used in parallel. First modal and numerical analysis are done in parallel. Then the numerical results are adjusted in order to be as near as possible to the experimental results. What we present in this document is the application of this method on our blower motor support.
The results of the motor support alone show that the numerical analysis can easily simulate a plastic component of our product. The exact number of modes are found, the deformations are right and some uncertainties exist in terms of frequencies.

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