Electromagnetically induced acoustic noise (and vibration), electromagnetically excited acoustic noise, or more commonly known as coil whine, is audible sound directly produced by materials

vibrating Oscillation is the repetitive or periodic variation, typically in time, of some measure about a central value (often a point of equilibrium) or between two or more different states. Familiar examples of oscillation include a swinging pendulum ...

under the excitation of

electromagnetic force In physics, electromagnetism is an interaction that occurs between particles with electric charge. It is the second-strongest of the four fundamental interactions, after the strong force, and it is the dominant force in the interactions of ...

s. Some examples of this noise include the

mains hum Mains hum, electric hum, cycle hum, or power line hum is a sound associated with alternating current which is twice the frequency of the mains electricity. The fundamental frequency of this sound is usually double that of fundamental 50/60 Hz, '' ...

, hum of

transformer A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...

s, the whine of some rotating

electric machine In electrical engineering, electric machine is a general term for machines using electromagnetic forces, such as electric motors, electric generators, and others. They are electromechanical energy converters: an electric motor converts electricity ...

s, or the buzz of

fluorescent lamp A fluorescent lamp, or fluorescent tube, is a low-pressure mercury-vapor gas-discharge lamp that uses fluorescence to produce visible light. An electric current in the gas excites mercury vapor, which produces short-wave ultraviolet lig ...

s. The hissing of high voltage transmission lines is due to

corona discharge A corona discharge is an electrical discharge caused by the ionization of a fluid such as air surrounding a conductor carrying a high voltage. It represents a local region where the air (or other fluid) has undergone electrical breakdown ...

, not magnetism. The phenomenon is also called audible magnetic noise, electromagnetic acoustic noise, lamination vibration or electromagnetically-induced acoustic noise, or more rarely, electrical noise, or "coil noise", depending on the application. The term electromagnetic noise is generally avoided as the term is used in the field of

electromagnetic compatibility Electromagnetic compatibility (EMC) is the ability of electrical equipment and systems to function acceptably in their electromagnetic environment, by limiting the unintentional generation, propagation and reception of electromagnetic energy whic ...

, dealing with radio frequencies. The term

electrical noise In electronics, noise is an unwanted disturbance in an electrical signal. Noise generated by electronic devices varies greatly as it is produced by several different effects. In particular, noise is inherent in physics, and central to the ...

describes electrical perturbations occurring in electronic circuits, not sound. For the latter use, the terms electromagnetic vibrations or magnetic vibrations, focusing on the structural phenomenon are less ambiguous. Acoustic noise and vibrations due to electromagnetic forces can be seen as the reciprocal of

microphonics Microphonics, microphony, or microphonism describes the phenomenon wherein certain components in electronic devices transform mechanical vibrations into an undesired electrical signal (noise). The term comes from analogy with a microphone, which ...

, which describes how a mechanical vibration or acoustic noise can induce an undesired electrical perturbation.

General explanation

Electromagnetic forces can be defined as forces arising from the presence of an electromagnetic field (electrical field only, magnetic field only, or both). Electromagnetic forces in the presence of a magnetic field include equivalent forces due to

Maxwell stress tensor The Maxwell stress tensor (named after James Clerk Maxwell) is a symmetric second-order tensor used in classical electromagnetism to represent the interaction between electromagnetic forces and mechanical momentum. In simple situations, such as a ...

magnetostriction Magnetostriction (cf. electrostriction) is a property of magnetic materials that causes them to change their shape or dimensions during the process of magnetization. The variation of materials' magnetization due to the applied magnetic field change ...

and

Lorentz force In physics (specifically in electromagnetism) the Lorentz force (or electromagnetic force) is the combination of electric and magnetic force on a point charge due to electromagnetic fields. A particle of charge moving with a velocity in an elect ...

(also called Laplace force). Maxwell forces, also called reluctances forces, are concentrated at the interface of high magnetic reluctivity changes, e.g. between air and a

ferromagnetic Ferromagnetism is a property of certain materials (such as iron) which results in a large observed magnetic permeability, and in many cases a large magnetic coercivity allowing the material to form a permanent magnet. Ferromagnetic materials ...

material in electric machines; they are also responsible of the attraction or repulsion of two magnets facing each other. Magnetostriction forces are concentrated inside the ferromagnetic material itself. Lorentz or Laplace forces act on conductors plunged in an external magnetic field. Equivalent electromagnetic forces due to the presence of an electrical field can involve

electrostatic Electrostatics is a branch of physics that studies electric charges at rest (static electricity). Since classical times, it has been known that some materials, such as amber, attract lightweight particles after rubbing. The Greek word for amber ...

, electrostrictive and reverse piezoelectric effects. These phenomena can potentially generate vibrations of the ferromagnetic, conductive parts, coils and permanent magnets of electrical, magnetic and electromechanical device, resulting in an audible sound if the frequency of vibrations lies between 20 Hz and 20 kHz, and if the sound level is high enough to be heard (e.g. large surface of radiation and large vibration levels). Vibration level is increased in case of a mechanical

resonance Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied periodic force (or a Fourier component of it) is equal or close to a natural frequency of the system on which it acts. When an oscillatin ...

, when electromagnetic forces match with a structural mode

natural frequency Natural frequency, also known as eigenfrequency, is the frequency at which a system tends to oscillate in the absence of any driving force. The motion pattern of a system oscillating at its natural frequency is called the normal mode (if all par ...

of the active component (magnetic circuit, electromagnetic coil or electrical circuit) or of its enclosure. The frequency of the noise depends on the nature of electromagnetic forces (quadratic or linear function of electrical field or magnetic field) and on the frequency content of the electromagnetic field (in particular if a DC component is present or not).

Electromagnetic noise and vibrations in electric machines

Electromagnetic

torque In physics and mechanics, torque is the rotational equivalent of linear force. It is also referred to as the moment of force (also abbreviated to moment). It represents the capability of a force to produce change in the rotational motion of th ...

, which can be calculated as the average value of the Maxwell stress tensor along the airgap, is one consequence of electromagnetic forces in electric machines. As a static force, it does not create vibrations nor acoustic noise. However

torque ripple Torque ripple is an effect seen in many electric motor designs, referring to a periodic increase or decrease in output torque as the motor shaft rotates. It is measured as the difference in maximum and minimum torque over one complete revolution, ge ...

(also called cogging torque for permanent magnet synchronous machines in open circuit), which represents the harmonic variations of electromagnetic torque, is a dynamic force creating torsional vibrations of both rotor and stator. The torsional deflection of a simple cylinder cannot radiate efficiently acoustic noise, but with particular boundary conditions the stator can radiate acoustic noise under torque ripple excitation. Structure-borne noise can also be generated by torque ripple when rotor shaft line vibrations propagate to the frame and shaft line. Some tangential magnetic force harmonics can directly create magnetic vibrations and acoustic noise when applied to the stator teeth: tangential forces create a bending moment of the stator teeth, resulting in radial vibrations of the yoke. Besides tangential force harmonics, Maxwell stress also includes radial force harmonics responsible for radial vibrations of the yoke, which in turn can radiate acoustic noise.

Electromagnetic noise and vibrations in passive components

Inductors

In inductors, also called reactors or chokes, magnetic energy is stored in the airgap of the magnetic circuit, where large Maxwell forces apply. Resulting noise and vibrations depend on airgap material and magnetic circuit geometry.

Transformers

In transformers magnetic noise and vibrations are generated by several phenomena depending on the load case which include Lorentz force on the windings, Maxwell forces in the joints of the laminations, and

inside the laminated core.

Capacitors

Capacitor A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals. The effect of ...

s are also subject to large electrostatic forces. When the capacitor voltage/current waveform is not constant and contains time harmonics, some harmonic electric forces appear and acoustic noise can be generated. Ferroelectric capacitors also exhibit a piezoelectric effect that can be source of audible noise. This phenomenon is known as the "singing capacitor" effect.

Resonance effect in electrical machines

In radial flux rotating electric machines, resonance due to electromagnetic forces is particular as it occurs at two conditions: there must be a match between the exciting Maxwell force and the stator or rotor natural frequency, and between the stator or rotor modal shape and the exciting Maxwell harmonic wavenumber (periodicity of the force along the airgap). Stator elliptical mode 2

As an example a resonance with the elliptical modal shape of the stator can occur if the force wavenumber is 2. Under resonance conditions, the maxima of the electromagnetic excitation along the airgap and the maxima of the modal shape displacement are in phase.

Numerical simulation

Methodology

The simulation of electromagnetically induced noise and vibrations is a multiphysic modeling process carried in three steps: * calculation of the electromagnetic forces * calculation of the resulting magnetic vibrations * calculation of the resulting magnetic noise It is generally considered as a weakly coupled problem: the deformation of the structure under electromagnetic forces is assumed not to change significantly the electromagnetic field distribution and the resulting electromagnetic stress.

Application to electric machines

The assessment of audible magnetic noise in electrical machines can be done using three methods: * using dedicated electromagnetic and vibro-acoustic simulation software (e.g. MANATEE ) * using electromagnetic (e.g. Flux, Jmag, Maxwell, Opera), structural (e.g. Ansys Mechanical, Nastran, Optistruct) and acoustic (e.g. Actran, LMS, Sysnoise) numerical software together with dedicated coupling methods * using multiphysics numerical simulation software environment (e.g. Comsol Multiphysics, Ansys Workbench)

Examples of device subject to electromagnetic noise and vibrations

Static devices

Static devices include electrical systems and components used in electric power storage or power conversion such as *

inductor An inductor, also called a coil, choke, or reactor, is a passive two-terminal electrical component that stores energy in a magnetic field when electric current flows through it. An inductor typically consists of an insulated wire wound into a c ...

s *

power inverter A power inverter, inverter or invertor is a power electronic device or circuitry that changes direct current (DC) to alternating current (AC). The resulting AC frequency obtained depends on the particular device employed. Inverters do the opp ...

s *

capacitor A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals. The effect of ...

s *

resistor A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active el ...

s: the braking resistors of electric trains, used to dissipate electrical power when the catenary is not receptive during braking, can make electromagnetically induced acoustic noise * coils: in

magnetic resonance imaging Magnetic resonance imaging (MRI) is a medical imaging technique used in radiology to form pictures of the anatomy and the physiological processes of the body. MRI scanners use strong magnetic fields, magnetic field gradients, and radio wave ...

, "coil noise" is that part of total system noise attributed to the receiving coil, due to its non-zero temperature.

Rotating devices

Rotating devices include radial and axial flux rotating electric machines used for electrical to mechanical power conversion such as *

induction motor An induction motor or asynchronous motor is an AC electric motor in which the electric current in the rotor needed to produce torque is obtained by electromagnetic induction from the magnetic field of the stator winding. An induction mot ...

s *

synchronous motor A synchronous electric motor is an AC electric motor in which, at steady state, the rotation of the shaft is synchronized with the frequency of the supply current; the rotation period is exactly equal to an integral number of AC cycles. Sync ...

s with

permanent magnet A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, steel, nickel, ...

s or DC wound rotor * switched

reluctance motor A reluctance motor is a type of electric motor that induces non-permanent magnetic poles on the ferromagnetic rotor. The rotor does not have any windings. It generates torque through magnetic reluctance. Reluctance motor subtypes include synchro ...

s In such device, dynamic electromagnetic forces come from variations of magnetic field, which either comes from a steady AC winding or a rotating DC field source (permanent magnet or DC winding).

Sources of magnetic noise and vibrations in electric machines

The harmonic electromagnetic forces responsible for magnetic noise and vibrations in a healthy machine can come from *

Pulse-width modulation Pulse-width modulation (PWM), or pulse-duration modulation (PDM), is a method of reducing the average power delivered by an electrical signal, by effectively chopping it up into discrete parts. The average value of voltage (and current) fed ...

supply of the machine * slotting effects * magnetic saturation In a faulty machine, additional noise and vibrations due to electromagnetic forces can come from * mechanical static and dynamic eccentricities * uneven air-gapLe Besnerais, J. (2015). Effect of lamination asymmetries on magnetic vibrations and acoustic noise in synchronous machines. In 2015 18th International Conference on Electrical Machines and Systems (ICEMS). http://doi.org/10.1109/icems.2015.7385319 * demagnetization * short circuits * missing magnetic wedges Unbalanced Magnetic Pull (UMP) describes the electromagnetic equivalence of mechanical

rotating unbalance Rotating unbalance is the uneven distribution of mass around an axis of rotation. A rotating mass, or rotor, is said to be out of balance when its center of mass (inertia axis) is out of alignment with the center of rotation (geometric axis). Unbal ...

: if electromagnetic forces are not balanced, a non-zero net magnetic force appears on stator and rotor. This force can excite the bending mode of the rotor and create additional vibration and noise.

Reduction of electromagnetic noise and vibrations

Reduction of magnetic noise and vibrations in electric machines

NVH mitigation techniques in electrical machines include * reducing the magnitude of electromagnetic excitations, independently of the structural response of the electrical machine * reducing the magnitude of the structural response, independently of the electromagnetic excitations * reducing the

resonances Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied periodic force (or a Fourier component of it) is equal or close to a natural frequency of the system on which it acts. When an oscillat ...

occurring between electromagnetic excitations and structural modes Electromagnetic noise and vibration mitigation techniques in electrical machines include: * choosing the right slot/pole combination and winding design * avoiding resonances match between stator and electromagnetic excitations * skewing the stator or the rotor * implementing pole shaping / pole shifting / pole pairing techniques * implementing harmonic current injection or spread spectrum PWM strategies * using notches / flux barriers on the stator or the rotor * increasing

damping Damping is an influence within or upon an oscillatory system that has the effect of reducing or preventing its oscillation. In physical systems, damping is produced by processes that dissipate the energy stored in the oscillation. Examples incl ...

* increasing the frequency outside the audible frequency range

Reduction of "coil noise"

Coil noise mitigation actions include: * add some glue (e.g. a layer of glue is often added on the top of television coils ; over the years, this glue degrades and the sound level increases) * change the shape of the coil (e.g. change coil shape to a

figure eight Figure 8 (figure of 8 in British English) may refer to: * 8 (number), in Arabic numerals Entertainment * ''Figure 8'' (album), a 2000 album by Elliott Smith * "Figure of Eight" (song), a 1989 song by Paul McCartney * '' Figure Eight EP'', ...

rather than a traditional coil shape) * isolate the coil from the rest of the device to minimize structure-borne noise * increase damping

Experimental illustrations

Effect_of_a_rotating_magnet_on_a_ferromagnetic_material_less_frames

A varying electromagnetic force can be produced either by a moving source of DC magnetic field (e.g. rotating permanent magnet or rotating coil supplied with DC current), or by a steady source of AC magnetic field (e.g. a coil fed by a variable current).

Forced vibration by a rotating permanent magnet

This animation illustrates how a ferromagnetic sheet can be deformed due to the magnetic field of a rotating magnet. It corresponds to an ideal one pole pair permanent magnet synchronous machine with a slotless stator.

Acoustic resonance by a variable frequency coil

The resonance effect of magnetic vibration with a structural mode can be illustrated using a tuning fork made of iron. A prong of the tuning fork is wound with a coil fed by a variable frequency power supply. A variable flux density circulates between the two prongs and some dynamic magnetic forces appear between the two prongs at twice the supply frequency. When the exciting force frequency matches the fundamental mode of the tuning fork close to 400 Hz, a strong acoustic resonance occurs.

Examples of audio files

PMSM motor (traction application)

References

{{Reflist

External links

Video of a resonating tuning fork magnetically excited by a variable frequency current
on YouTube
Video of a tuning fork magnetically excited by a fixed frequency current
on YouTube
Video of a ferromagnetic cylinder deformed by a rotating magnet
on YouTube Acoustics Electromagnetism Noise Mechanical vibrations Noise pollution Sound