251 – Vibration Analysis of an Alternator

Application Note / Model Data

Overview

Electromagnetic force in the form of an electromagnetic excitation force acting on a motor or an alternator causes vibration and noise. Vibration and noise also are produced when this electromagnetic excitation force resonates with an eigenmode of the motor.
In order to accurately evaluate this phenomenon, it is necessary to grasp each frequency and spatial mode in detail about the electromagnetic force and eigenmodes of the motor.
In this example, acceleration is evaluated by analyzing the electromagnetic force generated in the stator core of a claw-pole alternator and coupling this with the eigenmodes of the alternator. In addition, the frequency components of the electromagnetic force and eigenmodes and the spatial modes are determined, and the resonance modes are confirmed.

Electromagnetic Force Calculations

The electromagnetic force distribution for one period of electrical angle at steady-state is shown (Fig. 1). If the 16-pole rotor is rotated at 1,500 r/min, the fundamental frequency is 200 Hz.
In general, the electromagnetic force acting on the stator teeth has a frequency component (secondary component) which is twice the fundamental frequency, but if the rotor is a claw-pole rotor, since the N-pole and S-pole opposing widths are different, the fundamental component is included as well.
In particular, the size of the fundamental components in the axial center and ends are different (Fig. 2).

Eigenmode Calculations

The stator is fixed between the front and rear bracket. Since each part is connected using a screw hole, these connections are modeled using rigid body conditions. Also, the installed state with the rear bracket installation screw socket completely constrained is simulated (Fig. 3).
Fig. 4 shows the spatial 2nd to 4th order eigenmodes and each eigenfrequency.

Vibration Analysis and Sound Pressure Level Evaluation

Vibration analysis is performed for each frequency component of the electromagnetic force obtained by magnetic field analysis to obtain sound pressure levels (Fig. 5). By obtaining the spatial mode of vibration from the acceleration of the stator outer circumference at 2.4 kHz and 7.2 kHz, the ring modes can be confirmed (Fig. 6). In addition to the 0th order, it can be seen that the influence is large from the 2nd and 4th orders. Fig. 7 shows the sound pressure level distribution at this time.

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