Aerodynamic noise in electrical machines

Introduction

Acoustic noise of electrical machines includes the contribution of electromagnetic, mechanical and aerodynamic sources. Aerodynamic noise level may mask electromagnetically-excited noise, so it is important to understand how it is generated and its evolution with speed.

Aerodynamic noise in electric machines

Mechanisms of flow noise generation include: -* Monopolar noise (harmonic) due to quick variations of flow rate imposed by flow obstables, evolving in 60log10(V) where V is the linear fluid speed -* Dipolar noise (harmonic or broadband noise), evolving in 50log10(V) where V is the linear fluid speed -* Quadripolar noise (broadband noise) directly generated inside the flow due to shear strains induced by turbulences

Aerodynamic noise frequencies in electric machines

Characteristic frequencies of aerodynamic noise in electric machines include:

  • vortex frequency
  • blade passing frequency
  • rotor duct passing frequency

Open-slot induction machines with Zr rotor slots can generate aerodynamic noise at Z_rf_R which overlaps with acoustic noise due to electromagnetic forces.

Aerodynamic noise mitigation in electric machines

Aerodynamic noise mitigation actions includes

  • Change of blade geometry (circular to aerofoil cross-section) -> no more vortex frequency
  • Increase of minimum distance between fan blades and stationary obstacles
  • Reduction of number of rotor vents and lining air chambers with sound absorbent insulation
  • Use of unevenly spaced fan blades (caution with unbalance)
  • Reduction of fan diameter
  • Texturing of blades
  • Use of porous material for fan blades
  • Use of axial fan rather than radial fan

Use in MANATEE software

MANATEE software aims at calculating acoustic noise and vibrations due to magnetic forces in electrical machines. However, the user can add the contribution of aerodynamic noise as a function of speed specifying its own aerodynamic noise model.

Read more