What can explain differences between experimental tests and MANATEE simulation results?

Introduction

MANATEE software specialized in NVH simulation of electrical machines has been initially developed by EOMYS to support its consulting activities around electric motor noise mitigation. MANATEE has therefore been successfully applied to to analyze more than 100 electric motors in very different applications.

MANATEE software is a specialized software solution and its use requires to be trained on e-NVH phenomena. Comparison between simulation and tests must be carried carefully as explained in this article.

Sound pressure versus sound power level

Comparison between tests and simulations must be carried on comparable figures: MANATEE acoustic noise level can be given as a sound power level (SWL), or as a sound pressure level (SPL). SPL measurements are easy to carry but can include significant uncertainties (e.g. background noise, reverberant field, directivity). Using SPL comparison requires more attention. If analytical SPL model is used, the room constant where NVH tests have been carried must be specified to include reverberation effects.

When comparing measured and simulated sound pressure levels, be sure to input in MANATEE

  • the right distance from the outer surface of the machine to the microphone in Input.Simu.type_micro_distance
  • the right directivity coefficient in Input.Simu.directivity_coeff
  • the right room constant for reverberant field in Input.Simu.room_cstt

Electrical machines are usually tested at factory where directivity coefficient is not ideal and reverberation field is uncertain, in that case simulation results may differ from experiments with up to 10 dB difference.

If you want to estimate your room constant to improve quantitative noise calculations with MANATEE, EOMYS can run a specific measurement campaign to quantify reverberation effects in your test facilities.

Sound power level measurements run according to acoustic standards (e.g. ISO 3745:2012, ISO 3744:2010, ISO 3746:2010) are recommended for a valid comparison. In loaded case, sound power level of tested machine must not include background noise and loading machine noise. Intensimetry technique is therefore recommended and EOMYS can help you to carry such NVH measurements.

Mechanical and aerodynamic parasitic sources

When comparing measured and simulated SWL due to electromagnetic forces, be sure that you have also excluded non-magnetic acoustic noise sources (aerodynamic noise, mechanical noise) that might be present in experiments. Note that aerodynamic noise may occur at same frequencies as electromagnetic noise in some specific applications. If necessary, MANATEE allows to add an external source of noise function of speed.

Air-borne Vs structure-borne noise

MANATEE software includes different modelling levels suitable for early design and detailed design stages. In early design phase based on semi-analytic vibroacoustic models, only air-borne noise radiated by outer structure is included. Experiments always include a part of structure borne noise (SBN) which can explain differences between test and simulations. SBN due to rotor excitation by magnetic forces can be included when using MANATEE combined with a 3D FEA mechanical model during Electromagnetic Vibration Synthesis process. The rotor FEA model (in particular bending modes) should be fit with experiments for good estimation of SBN.

Damping

A key value to have a good estimation of the absolute sound and vibration values is structural damping (from 0.5 to 4% in electrical machines) which cannot be calculated numerically, and depends on several parameters (temperature, resin type, winding type, etc). Without damping measurement, MANATEE uses a default average damping value of 2% and calculations may lead to 20log(0.5/2)=-12 dB to 20log(4/2)=6 dB errors at some frequencies. A step by step Experimental Modal Analysis is highly recommended to quantifiy tmodal damping of your application. When measured damping is used in your simulation, vibration and sound levels accuracy can be brought down do +/-3 dB. Other uncertainties are detailed below.

Eccentricities and asymmetries

It is known that eccentricities and geometrical / magnetic asymmetries can introduce new resonances due to magnetic forces, and might change significantly vibration and noise levels. In particular eccentricities modulate all pulsating forces with UMP harmonics, which can easily excite different structural modes. If you have simulated a symmetrical machine in MANATEE some resonances may be missing compared to experiments. EOMYS recommends to carry stator bore radius measurements to check airgap uneveness, as well as eccentricity measurements.

Modelling accuracy

MANATEE software proposes different modelling levels suitable for early design stage and detailed design stage applications. When comparing MANATEE results with experiments, the degree of modelling details should be progressively increased in case of mismatch. In particular, electromagnetic loads should be calculated with FEMM instead of SDM or PMMF. Structural response should be calculated with a 3D mechanical FEA model that has been fit with experiments. Acoustic response should be caclulated with 3D acoustic FEA or acoustic BEM (case of free field).

Besides, MANATEE default electromagnetic models include radial, tangential and moments on tooth tips. Axial magnetic forces may induce additional vibration and noise, so be sure that they are included in your model especially in case of skewed lamination.

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