The Magnetics Project

Materials World magazine
1 Dec 2009

Hugh Stanbury and Stuart MacLachlan from the UK’s Materials Knowledge Transfer Network describe advances made in magnetic materials measurement and processes.

Advanced Electric Machines through Materials was a UK collaborative project, supported by the Technology Strategy Board, to produce new techniques and materials for the magnetics sector.

Property measurement

Improved magnetic measurement technology has been achieved with particular reference to assessing materials under test conditions relevant to operating machines by The National Physics Laboratory (NPL), Cardiff University and TRW Connekt. This has led to key improvements in the UK’s magnetic measurement facilities, including:

• A new measurement system for determining the DC magnetic properties of lamination materials under high mechanical stress.

• Characterisation of the behaviour of representative material samples under the magnetic excitation (flux density of complex waveforms and frequencies) experienced in operating machines in applications such as aerospace generators and automotive electric power steering motors.

An extensive magnetic measurements database for lamination materials under a range of test conditions has been established for reference and materials selection in defined applications.

Electric power steering motor

Automotive power steering systems driven by electric motors are more efficient and environmentally friendly than conventional hydraulic systems. However, for acceptable performance, the drag torque – the torque set up by the motor in opposition to its motion when it is not energised and is being driven by the self-centering action of the road wheels and contributes to the system’s overall friction – must be minimised.

Selection of the lamination material in the motor is critical in reducing the drag torque, so various non-oriented silicon steels have beeninvestigated by TRW Connekt and Cogent Power. Analysis shows that hysteresis loss is closely related to the drag torque, and coercivity, in particular, shows a clear linear relationship. This link has been supported by software modelling. Therefore, material selection on the basis of coercivity, which can be measured routinely, will be a key part of materials selection for power steering motors.

Soft magnetic composites

Development of a novel soft magnetic compact that can operate at temperatures of up to 250ºC has been conducted by CERAM. Ideally, the properties of a soft magnetic composite should have the highest physical density possible while ensuring that the metallic cores of the powders remain electrically isolated from each other.

In the process, a nanoscale magnetite has been used to coat the larger metal powders, which were then pressed and sintered. Since magnetite is an insulator and ferromagnetic it was expected that it would also contribute to the magnetic permeability of the compact.

The conventional method of providing a high electrical resistivity is to form an insulating layer by wet coating the powder particles before they are pressed and sintered to form a monolithic solid of high density. However, this process is unreliable so a method of fluidising the iron powder by rotation in a flow of CO2 at high temperature has been developed to coat the iron particles with magnetite. Hotisostatic pressing formed fully dense blocks.

At the conclusion of the project, the best material created using this process gave a DC magnetic flux density of 1.44 tesla at eight kiloamperes per metre, which is better than commercially available material options. Alternating current permeability was up to six times higher, although the AC power loss was worse. Further work could investigate higher purity Fe powder and scaling up of the coating equipment.

High strength, high temperature soft magnetic materials

Aerospace engine designers are looking to downsize electric machines. With novel designs and high strength magnetic materials, weight savings of 15% could be achieved if engines are located within the gas turbine to reduce the weight penalty of drive shafts and gearboxes. A key issue will be the development of the integrated starter generator, located co-axially inside the engine core.

This presents immense challenges to the producer of the magnetic material. Iron-cobalt (Fe-Co) cores, which are the traditional choice, have excellent soft magnetic properties, but when operating under extreme rotational forces and at high temperature, a reduction in their magnetic properties is inevitable.

UK companies QinetiQ and Rolls-Royce have devised a fabrication route for producing hoop-reinforced FeCo rings using a hybrid foil wire technique based on co-winding FeCo wire and silicon carbide fibre into double spirals of alternate wire and fibre, which are then interleaved with FeCo foil. This structure is encapsulated, evacuated and hot isostatically pressed to consolidate it into a fully dense composite material, which could be machined into the rotor of an electrical generator.

Further developments produced a fibre-reinforced foil which could form the FeCo laminated teeth of this composite rotor. The laminated pole pieces are diffusion bonded to the fibre-reinforced rotor hub. The addition of non-magnetic materials to the core can have a detrimental effect on the magnetic properties, but evaluations are ongoing into establishing the behaviour of the composite at high stress and temperature.

Software modelling of electric machines

A new generation of software tools to meet the needs of electrical machine designers was developed by Victor Fields (now known as Chobham Technical Services) and Motor Design. These include:

• The development of permanent magnet magnetisation and demagnetisation solvers in OPERA-2d and OPERA-3d allowing accurate modelling of the performance of permanent magnet machines, accounting for operation under increased temperatures and demagnetising armature currents.

• The development of a hysteresis solver in OPERA for hysteresis loss evaluation in electrical machines, while also quantifying drag force problems in actuators and machines employing permanent magnets.

• Accelerating the design of electrical machines, with a special focus on non-standard equipment such as axial flux geometries.

Corrosion effects on bonded magnets

Polymer bonded magnets are used in brushless DC motors for electric water coolant pumps in ‘more electric’ vehicles. The pump can be used in either de-ionised water, as in fuel cell applications, or in ethylene-glycol/water mixtures for automotive cooling systems where the temperature will reach 90ºC. Critically, the pump is designed so that the pumped liquid flows around the permanent magnet rotor as well as the impeller.

The magnet is normally encased in a stainless steel can to protect it from the liquid but this increases the air gap in the motor and can reduce its performance. One solution is to protect the magnet against the corrosive effects of the liquid. Two approaches to this problem were evaluated by Magnet Applications and Pierburg UK:

• A study of the effectiveness of sol-gel coatings.

• Research into fundamental examination of the corrosion mechanisms in neodymium-iron-boron (NdFeB) magnets.

The corrosion testing programme included accelerated environmental tests to duplicate the operating conditions of the motors with variations of epoxy and sol-gel magnet coatings in immersion tests. The coatings did not offer much protection.

The research has concluded that the decrease in coercive field strength is due to the absorption of hydrogen, which can diffuse rapidly from the aqueous corrosion of the NdFeB flakes. Also, PTFE (polytetrafluoroethylene) bonding provides the best protection against antifreeze, although there is some property deterioration which then stabilises.

Although coatings give some protection against surface corrosion, there is still a high deterioration of the magnetic properties. The ingress of hydrogen is also associated with this loss. Further work is planned to examine alternative sol-gel formulations and resins.

Insulating coatings for high temperature wire

A principal consideration in developing electrical machines that operate at higher temperatures isinsulating the conducting wires. Insulated wire needs to be produced in volume and at reasonable cost, compatible with machine designs, and able to withstand the mechanisms of winding and assembly.

A high temperature wire must toleratetemperatures over 450ºC in the long-term and have high electrical resistance under these conditions. Commercially available polyimide insulated wire has an upper, long-term temperature limit of 250ºC. Other alternatives, such as inorganic coatings (enamels or sol-gel silica), can be too brittle to survive assembly and may become conducting at high temperature.

The insulating coating developed at Teesside University, UK, considers the design needs at different stages in manufacture and service life. The requirement of mechanical flexibility is critical at the winding and assembly stage, followed by high temperature dielectric strength in service. The coating is therefore multilayered and based on hybrid organic-inorganic nanocomposites to allow functionality modification.

It is comprised of a base layer with low organic content to minimise shrinkage and outgassing, and a top layer with increased organic content to provide required flexibility. The suspension includes a ceramic filler of vermiculite particles in platelet form. On heating, the organics are burnt out and the inorganic bonding is formed to provide the insulated state for service. A patent application has been filed by Teesside University and future work will see this material assessed further.

History and aims

In 2004, PowdermatriX Faraday produced a technology roadmap to assess the UK magnetics and associated industries. The exercise identified key technologies to be addressed:

• Improved efficiencies in land-based and aerospace power generation systems.

• The development of new and improved soft magnetic composite processing technology for efficient motors.

In June 2005, PowdermatriX brought together 16 partners for a £2.4m UK Department of Trade and Industry (DTI) supported project called Advanced Electric Machines through Materials, which responded to the DTI Technology Programme call to improve fuel efficiency and reduce CO2 emissions. During the project, the work of the DTI was taken over by the Technology Strategy Board.

As well as the development of improved materials, a principal deliverable of the project was to optimise available materials and extend their use in machine technology. The work covered the processing and end-use of hard and soft magnetic materials. It addresses the demands of the aerospace and automotive industries for technology to produce more efficient power systems against the stringent UK and EU emissions legislations.

Within these objectives, the partners, drawn from leading UK companies, research and technical organisations, and universities specialising in magnetics, established strong technical links to develop:

• Improved methods for measuring the magnetic properties of magnetic materials under operating electrical machine conditions.

• Specialist software modelling packages applied to electrical machines and to extend their ranges of operation.

Further information: Stuart MacLachlan and Hugh Stanbury