How to make sanity checks of e-NVH simulation results?

Introduction

Manatee e-NVH CAE collaborative platform contains multiphysic simulation models to perform numerical simulation of magnetic noise & vibrations. Whether you use Manatee or a general purpose simulation software, before interpretating the results it is important to check that the numerical simulation process performed well, and that results look physical. This article reviews some sanity checks that you can perform before starting the analysis.

Discrete spectra

As demonstrates in the MFSA (Magnetic Force Signature Analysis) feature of Manatee, magnetic excitations occur at some particular frequencies (except with randomized PWM). This means that all spectra should be discrete, whethere you look at harmonics of magnetic flux, forces, vibrations or noise. For instance, in a healthy synchronous machines, all magnetic forces are multiples of twice the electrical freqency.

Orders of magnitude

As e-NVH simulation involves several engineering profiles (electrical, mechanical, NVH) here a some orders of magnitude you should have:
– airgap flux density: max 2T peak, max 1.2T on fundamental flux in most electrical machines
– Maxwell stress: max 10e6 N/m^2
– displacement level: max 1mm at low frequency (remember that small displacements ~10e-6 m can generate high sound power level)
– vibration levels: max 150 dB Re 1e-9 m/s
– noise levels: max 130 dB Re 1e-12 W

Magnetic noise is only a part of the overall noise level, as it excludes aerodynamic & mechanical noise. In electrical machines, 70 dB of sound power level due to magnetic forces is very small (generally covered by non magnetic noise) and may not require any noise control actions. On the contrary, 120 dB of sound power level is the indication of a very noisy machine and magnetic noise control actions must be triggered.

Forces

Global forces should be observed the same way by stator & rotor with an opposite sign. In particular, stator torque should be the opposite of rotor torque. Stator torque can be calculated by summing the tangential forces times radius of application plus bending moments at tooth tips.

The largest magnitude of Maxwell stress harmonics or lumped force harmonics is always at twice electrical frequency in AC electrical machines.

 
 

Resonances

The amplification of vibration levels around a particular operating speed should be correlated with the excitation of a structural mode. The modal basis should contain a particular mode at the resonance frequency.