Correlation of Test and Analysis Results for Alternating Current Machines

Induction motors and the generators ofen have problems associated with noise and vibration. Consequently,there is need for prediction of noise and vibration during the design of these machines.The Finite Element Modeling(FEM) method is used for the prediction and evaluation of natural frequencies and mode shapes of electrical equipment.However, FEM is ofen very difficalt,becouse of the influence of bolted joints,welded parts,shrink fits ,etc.It is useally required to validate and refine FEM results based on using Experimental Modal Analysis(EM4) test results. 1 . I N T R O D U C T I O N Vibration and noise of an electrical machine, such as an induction motor or generator,is mainly caused by electromagnetic force of the machine itself,and by centrifugal forces from another prime mover and itself. During the design and development of such equipment,we investigate vibration and noise caused by the electromagnetic force.Kowever,the initial prediction accuracy is usually not satisfactory.At this time,use is made of commercial FEM program(I-DEAS) and our own program in order to gain experience in accurately simulating vibration and noise.In order to achieve accurate predictions it is necessary to investigate the following items: (1) Construct an appropriate FFH modeling (2) Investigate the influence of bolted joints, weldings,and shrink fits at the model joints using a Building Block Approach(BBA) (3) accurate prediction of forces due to the machine’s electromagnetic force 2.Vibration a n d N o i s e o f A C M a c h i n e s An example of an Alternating Current machine is an induction motor.We are currently improving our analysis prediction accuracy,with regard to vibration and noise, prior to the design of this type of machine. The noise producted by induction motors can be classified into three types:windage noise, mechnical noise,and n o i s e d u e t o electromagnetic force.Each is briefly described below. (1) Windage Noise Three principal windage noise generating mechanisms are present: a) fan generated noise b) flow generated noise in ducting or cooler Pipes c) Mixing noise at the cooler outlet due to separate air jets and a coalesced air flow system The remainder of this paper does not explain with windage noise. (2) Mechanical noise Mechanical noise sources are a result of unbalance,misalignment,and noise from rolling element bearings. (3) Electromagnetic noise Magnetic noise should be minimized as a result of the original design.Magnetic noise is primarily structure-borne.