Electricmotor noiseand vibration

To visualize the acoustic wave propagation around the motor, the solution is transformed into the time domain with the harmonic wave relation (3):

In acoustics simulations, the wavelength of a sound wave needs to be resolved by a sufficiently fine mesh in order to get an accurate numerical solution. Here the max edge length is set to 12 nodes per , which means that there will be at least 12 elements per wavelength in each direction of the wave propagation.

Enginenoiseexamples

SymbolDescriptionUnit Rrevolutions per minute in a motor[rad/s] ρdensity of a medium[kg/m3] cspeed of sound in a medium[m/s] psound pressure[Pa] ωsound wave angular frequency[rad/s] fsound wave frequency[Hz] λsound wavelength[m] Xposition vector[m] rmotorradius of the motor[m] rdomainradius of the domain[m] δdeformation of the motor casing[m] dmaximum radial deformation[m] ϕpolar angle[rad] Γoutfar-field boundaryN/A Γcasingmotor casingN/A vvibration velocity[m/s] vibration amplitude[m/s] Ωcomputational domainN/A

Next, a sound barrier is added in the domain to reduce the motor noise in a particular direction. The barrier is placed at to minimize the noise in the direction.

Consider the motor spinning at an angular frequency : the magnetic forces between the rotor and stator will continuously deform the outer casing of the motor. This periodic deformation can be approximately described by the cosine function:

How to fixnoise in motor

Note that the noise level is significantly reduced in the direction between to for all three motor speeds. However, on the opposite region, the noise level is increased, due to the sound reflection from the barrier.

How to reducemotor noise

The SPL value is seen to be at the maximum around the motor casing and decays isotropically with the distance from the motor. Note that the noise level increases with higher motor speed.

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To model the time-harmonic deformation of the electrical motor, the acoustic model is built in the frequency domain. The propagation of harmonic sound waves can be described by the source-free Helmholtz partial differential equation (PDE) (1):

A related example that simulates the magnetic potential and the magnetic flux field within the electric motor is presented in a separate tutorial entitled "Solving Partial Differential Equations with Finite Elements".

Noise in motorwhen accelerating

To analyze the noise propagation in the surrounding area, the simulation domain is defined as a circular region enclosing the electric motor. The radius of the motor and the domain are and , respectively. The far-field boundary and the motor casing are symbolized as and , respectively.

3 phasemotormakingnoise

More information on mesh size requirements for acoustic models can be found in the "Acoustics in the Frequency Domain" tutorial.

Noise analysis is an important phase when designing electrical motor–driven systems. As a motor rotates, the harmonic electromagnetic forces between the rotor and stator continuously deform the outer casing of the motor. These periodic deformations lead to structural vibrations, which excite pressure waves to the surroundings as a noise.

Grindingnoisefrom engine when idle

As a theoretical background for acoustics, please refer to the information provided in "Acoustics in the Frequency Domain".

There are two types of boundary conditions involved in this example. On the motor casing , a normal velocity boundary condition is used to model the structural vibration.

In this model, the sound barrier is assumed to be made of a sound hard material, which allows it to reflect the incident sound waves. A sound hard boundary condition, which is an implicit default boundary condition, will be used to model the boundary of the barrier.

In this model, the maximum radial deformation of the motor is set as . Note that is assumed to be independent of the motor speed . This is a reasonable approximation, since the eigenfrequencies of the motor are packed so closely in the high-frequency range and have little impact on the structural deformation.

The acoustic wave radiates from the motor and propagates to the surrounding area in a spiral pattern. A plot of the sound pressure level (SPL) of the domain can be used to inspect the noise distribution. The noise level at distances of , and is shown as an SPL polar plot.

On the far-field boundary , an absorbing boundary condition is used to truncate the domain as if it had an infinite extension.

The following model simulates the acoustic wave radiation from a motor spinning at , and . First, the sound pressure level (SPL) in the surrounding field is computed to quantify the noise generation. In a subsequent step, a sound barrier will be placed in the domain to reduce noise in a specific direction. The resulting sound pressure field will then be compared to the free-space case to measure the effectiveness of the sound barrier.