Magnetic noise and vibrations in electrical machines

Introduction

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Noise and vibrations of electrical machines include electromagnetically-excited sources, aerodynamic sources and mechanical sources. This article reviews electromagnetic noise and vibration sources in electric machines, also called e-NVH in automotive applications (hybrid/electric vehicles).

Explanation of electromagnetically-excited noise in electric machines

Electromagnetic noise and vibrations come from the vibrations of the electric motor active parts in the audible range (20 Hz to 20 kHz) under the excitation of electromagnetic forces, namely magnetostriction and Maxwell forces. A low-speed, high-torque, direct-drive permanent magnet synchronous generator for wind turbine applications may be responsible for a "low frequency" 100 Hz humming noise, while a high-speed brushless DC motor used for instance in model making might create a high-pitch, "high frequency" whining noise up to 5000 Hz.

The role of magnetostriction in noise generation can be generally neglected: based on EOMYS consulting experience on more than 50 electrical machines, it is not the root cause of electromagnetic noise and vibrations in rotating machines.

Maxwell stress is the formalization of how magnetic forces arise from electromagnetic fields: it includes well-know attraction/repulsion forces between two magnets, and also reluctant forces which tends to shorten the magnetic field lines (law of minimum magnetic reluctance); it also includes Laplace forces which apply on an electric current plunged in an external magnetic field.

The Maxwell stress tangential net effect on the rotor gives the average electromagnetic torque of the electrical machine, but its tangential and radial harmonics can produce parasitic vibration and acoustic noise.

Resonance effects

When the shape of magnetic forces along the airgap (called a wavenumber) match with a structural mode of the stator and travel at the right speed, vibrations and acoustic noise are amplified significantly: this is the resonance effect. Resonance occurs when the electrical frequency (not the mechanical frequency) of the travelling force wave of wavenumber r match with the circumferential mode (r,0) natural frequency. As an example, the following animation shows the modal shape (2,0) of a stator lamination stack:

A rotating radial force of wavenumber r=2 is illustrated here:

Finally the match between exciting force and modal shape when the electrical frequency of the force wave equals the natural frequency of the mode is illustrated here:

The modal participation factor of magnetic forces to rotor and stator structural modes can be more rigourously quantified using the concept of Modal Force Matrix (projection of magnetic force on structural modes).

Magnetic noise and vibration reduction techniques

A special article discusses electromagnetic NVH mitigation techniques at design stage of electric machines. This article deals with techniques such as

  • topology, pole / slot / phase numbers, winding
  • skewing
  • pole magnetization, pole shaping, pole width and position
  • slot and tooth shape / position
  • stator slot opening optimization, magnetic wedges
  • notches, flux barriers
  • airgap increase
  • control
  • harmonic current injection
  • switching strategies
  • structural response, damping
  • asymmetries

EOMYS can help you to identify the root cause of noise and vibrations, design and validate appropriate noise control actions while keeping electromagnetic performances unchanged.

Experimental highlights

To know if an electrical machine is noisy because of electromagnetic noise and not because of aerodynamic or mechanical noise, some simple experiments can be carried.

In induction machines for instance, electromagnetically-excited noise stops when the machine is current-free (null stator/armature current). For permanent magnet synchronous machines, cancelling the currents does not cancel all sources of magnetic fields ; it is therefore necessary to drive the machine with demagnetized magnets, or carry more detailed spectral analysis to check if the frequency of the acoustic noise match with the theoretical content of harmonic magnetic forces.

Based on its experience on more than 60 electric powertrains, EOMYS has developped some specialized experimental test methodologies to quickly characterize and troubleshoot e-NVH issues in rotating machines (e.g. spatiogram post-processing). The combination of MANATEE simulation software and advanced e-NVH tests allows the company to efficiently identify e-NVH sources and transfer paths, propose and validate noise control actions.

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