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Multipole Configurations and Halbach Arrays: When and How to Use Them in Modern Applications
Halbach array magnets and multipole configurations are advanced architectures used to optimise magnetic flux in electromechanical applications. Compared with individual blocks featuring axial or radial orientation, these solutions better meet the requirements of miniaturisation, power density and energy efficiency.
To assess which configuration is best suited to your project, contact the CIBAS technical team.
In this article, we take an in-depth look at two of the most important configurations: the standard multipole configuration and the Halbach array. We explain how they work, their physical and construction differences, and how to overcome production assembly challenges. For a broader overview of the systems that incorporate these configurations, read our guide to industrial magnetic assemblies.
The Fundamentals of Magnetisation: Beyond the North–South Pair
To fully understand the technological leap represented by advanced configurations, we must first take a step back and examine the limitations of conventional magnetisation.
Conventional Axial and Radial Magnetisation
In a conventional magnet with a single pair of poles, the flux lines emerge from the north pole and return through the south pole, following a symmetrical path through the surrounding space.
- With axial magnetisation, the poles are located on opposite faces of the magnet.
- With radial magnetisation, commonly used in ring magnets for motors, the flux travels from the centre outwards, or vice versa.
Although these solutions are cost-effective and easy to manufacture, they have a fundamental drawback: the magnetic field disperses equally in every direction. To channel the flux where it is actually needed, such as in a motor’s air gap, engineers must add a ferromagnetic yoke or return circuit made of a material such as soft iron or silicon steel. This adds weight, volume, hysteresis losses and eddy-current losses to the system.
The Evolution Towards Complex Systems
When applications require rapid spatial variations in the magnetic field or a localised flux density greater than an individual magnetic material can provide, within the limit set by its remanence Br, complex geometric configurations become necessary. This is where multipole structures and Halbach arrays come into play.
The Standard Multipole Configuration: Characteristics and Limitations
A multipole configuration consists of several magnetic poles with alternating polarities, north and south, arranged geometrically on a single structure or assembly.
In a multipole ring or bar, the magnets follow a linear or circular N-S-N-S sequence. This arrangement can be achieved in two ways:
- Selective magnetisation: A single block of magnetic material is placed inside a special magnetiser equipped with geometrically shaped coils that imprint the poles in a single electrical pulse.
- Segmentation: Several individually and conventionally magnetised magnets are bonded side by side on a support with alternating polarities.
Benefits of the Multipole Solution
- High switching frequency: In electric motors and generators, a high pole count makes it possible to achieve high torque at low rotational speeds.
- Economies of scale: When produced through the selective magnetisation of a single ring, manufacturing costs are relatively low.
- Field linearity: The generated magnetic-field profile can approximate a square or sinusoidal wave through pole shaping, making it ideal for position sensors and encoders.
Inherent Limitations
The main limitation of the standard multipole configuration lies in its symmetry. The generated magnetic field is identical on both sides of the structure. In an electric motor rotor, for example, half of the magnetic flux travels outwards towards the stator, where it is useful, while the other half travels inwards towards the motor shaft, where it is unnecessary or potentially harmful. A heavy metal backing yoke is therefore essential to prevent leakage and magnetic short circuits.
The Halbach Array: The Revolution of Asymmetric Flux
The Halbach array is a remarkable conceptual evolution of the multipole configuration. Originally theorised by John C. Mallinson in 1973 and later developed independently by physicist Klaus Halbach in 1980 to accelerate particle beams, this system breaks the symmetry of the magnetic field.
The Principle of Progressive Flux Rotation
The secret of a Halbach array lies in the orientation of the magnetisation vector of the individual elements forming the structure. Instead of simply alternating north and south, the magnetic orientation rotates progressively by a fixed angle, usually 90°, from one element to the next.
What happens from a physical perspective?
- On the active side: The magnetic-field components of the vertically and horizontally oriented magnets combine constructively. The flux becomes concentrated and intensified, theoretically reaching almost twice the values achieved by a standard configuration.
- On the passive side: The flux lines cancel one another through destructive interference. The residual magnetic field approaches zero.
Types of Halbach Array
Several geometric variations are based on this principle:
- Linear (Planar): Used in linear transport systems, magnetic guides and actuators.
- External Cylindrical (Ring): The vectors rotate to concentrate all the flux inside the ring’s central bore while cancelling it outside. This configuration is used in ultra-high-efficiency inner-rotor motors and portable NMR systems.
- Internal Cylindrical (Ring): The flux is concentrated exclusively outside the ring and cancelled within its central bore, making it ideal for outer-rotor motors.
Weight and Power-Density Analysis
One of the most striking benefits of a Halbach array is the elimination of the rear iron yoke. In conventional electric motors, the rotor iron accounts for a significant proportion of the total weight. Removing the iron and replacing it with lightweight structural materials such as carbon fibre, aluminium or titanium dramatically increases the motor’s power density in kW/kg. This makes the Halbach array the preferred choice wherever weight is a critical constraint.
The Manufacturing Challenge: How to Design and Assemble a Halbach Array
Although a Halbach array may seem like the perfect theoretical solution, its practical implementation presents significant engineering and manufacturing challenges. These factors must be carefully evaluated from an industrial perspective.
Internal Repulsive Forces
In a conventional N-S-N-S multipole arrangement, adjacent magnets attract one another laterally because the north pole of one sits next to the south pole of the other. This makes positioning relatively straightforward during assembly.
In a Halbach array, the 90° rotation of the poles forces the magnets into an unstable geometric configuration. The intermediate magnets literally push the adjacent magnets away. During bonding and assembly, the lateral repulsive forces are extremely high and can cause the array to bulge or mechanically burst unless it is properly constrained.
Conclusions for Designers
The decision to implement a standard multipole configuration or invest in the development of a Halbach array can be guided by three key considerations:
- Space and Weight Analysis: If the system volume is flexible and weight does not directly affect operating costs, as in fixed factory automation, a multipole configuration combined with an effective iron return circuit is the most cost-effective choice.
- Power Density and Dynamics: If the application requires extremely rapid acceleration, minimal rotor inertia and very low weight, as in articulated robotics, racing motors or aerospace systems, a Halbach array delivers performance that no other configuration can match.
- Development Budget: A Halbach array requires extremely tight geometric tolerances for the individual magnets, often ground to within one hundredth of a millimetre, as well as dedicated assembly tooling. It should be considered only when production volumes or the added value of the final application can absorb the high initial engineering cost.
Would you like to integrate Halbach or multipole technology into your systems?
Do not let flux leakage limit your application’s performance. Contact us, and our team will guide you towards the right technological solution.
