Magnetic noise and vibrations in electrical machines

Introduction

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Magnetic 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.

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 to 20000 Hz) 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 forces describe the magnetic forces attracting or repulsing two magnets, they tend in electric motors to bring the stator closed to the rotor and reciprocally. They are also called reluctance forces as they concentrate at the interface of magnetic reluctivity changes, so mainly at the interface between lamination and air.

Their tangential net effect on rotor gives the electromagnetic torque of the machine, but their tangential and radial harmonics can produce parasitic vibration and 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

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.

Some videos and audio files of electromagnetically-excited noise can be found on our resources webpage.

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