Cast inductors increase the efficiency of electrical machines

A variety of demands are made of the coils in electrical machines. They are expected to be more efficient when the same or even less material is used. High product quality is demanded and individualised solutions are sought in their production.

The Fraunhofer Institute for Manufacturing and Applied Materials Research IFAM (Institut für Fertigungstechnik und Angewandte Materialforschung) has developed a casting process with which more efficient high-performance coils with improved cooling properties can be manufactured. The inductors are used in industrial drives and traction machines, in particular in the areas of electromobility and energy generation.

The casting technology employed to manufacture the designed inductor has a high slot fill factor and this makes a 90 percent improvement in the utilisation of the available construction space possible.

In addition to the high slot fill factor, the flat arrangement of the conductors offers further advantages from an electromagnetic and thermal perspective. When compared to a coiled wire with the same cross-sectional area, the thermal characteristics are improved while the negative impact of the current displacement is minimised. Depending on the particular size of the inductor, a constant current density of between 18 and 24 A/mm² can be realised in the case of water-cooled applications.

When compared with a coiled inductor in the same construction space, the electric resistance and, as a result, the current heat losses are reduced by up to 50 percent.

When the increased slot fill factor is taken into account, the current density in the groove, which is decisive for the performance of electrical machines, can be increased up to threefold compared to conventionally manufactured inductors.

Significant increase in torque and power density

This combination of high slot fill factor, excellent thermal characteristics and limited current displacement losses can now be used in many ways in machine design. A significant increase in torque and power density can be achieved when cast inductors are used in a similar construction space. Alternatively, the same torque level can be maintained while the construction space and weight could be reduced.

The increase in the slot fill factor also allows aluminium to be used as conductor material. The disadvantage of aluminium when it comes to its electrical conductibility is offset by the higher fill factor, while the coil weight can be reduced by 50 percent and the raw material costs by 85 percent.

Further application potential is there to be taken advantage of with respect to the geometric design freedoms when developing innovative cooling concepts. It is in this way that the build-up of thermal hotspots can be avoided in areas that are subject to high increases in temperature (e.g. on the coil head) through the targeted increase on the cross-section of the conductor.

Thanks to the different casting options available, the inductors can be manufactured in various sizes and in any batch numbers required. When precision casting is employed, conductor heights of less than a tenth of a millimetre at inductor lengths of between 10mm and 200mm can be realised, while inductors destined for installation in large machines, produced using the Lost-Foam process, can be as much as one metre long. For very large batch numbers, the production of the inductors in permanent moulds using high-pressure casting or low-pressure casting results in surprisingly low costs per individual product manufactured.

The selection of the correct casting process depends on the specific application under consideration of the batch sizes being aimed for and the quality demands which this entails. A tailor-made insulation coating is then applied. The optimisation goals in this case, in addition to ensuring the insulation strength when minimising coat thickness, are increasing the temperature resistance as well as avoiding the indications of attrition due to partial discharges.

In order to make it possible to create prototypes quickly, a complete process chain is available at Fraunhofer IFAM, which can make it possible to manufacture coated inductor geometry as hardware within just a few days.

Cast inductors have an advantage

The process developed at Fraunhofer IFAM for the manufacturing of inductors using casting technology offers decisive advantages compared to conventionally manufactured coils.

Through the combination of slot fill factors of in excess of 80%, the excellent thermal characteristics and the limited additional losses as a result of current displacement, a clear increase of the current density of electrical machines compared to the latest technology developments and standards is achieved.

The high slot fill factor makes it possible to substitute a conventional copper coil with a cast aluminium coil when the construction space remains unchanged.

Design and construction of prototypes

The »Electrical Drives« department offers a tailor-made design and construction of the inductors corresponding to the manufacturing facilities available.

There are various design tools available for the electromagnetic and thermal simulation as well as the constructive design. Besides the design of the inductor itself, this also involves the design of the electrical machine while considering the assembly concept and the overall losses.

Application examples

In various applications, the feasibility and the technological advantages of cast inductors have been demonstrated:

- Cast inductors made from copper for wheel hub drives:
The increase of the slot fill factor and the improved thermal characteristics mean a significant increase in the efficiency and torque density.

- Cast inductors made from aluminium for use in electric machines: A significantly higher slot fill factor makes a constant torque or performance density possible in contrast to a conventional wire-wound coil with a concurrent reduction of the coil weight.

- Cast conductors for use as commutating inductors in direct current machines (in cooperation with the Lloyd Dynamowerke GmbH & Co. KG. headquartered in Bremen): In the exact same construction space, it was possible to replace a copper inductor while also decreasing the operating temperature to the tune of 30 Kelvin.

Aluminium versus copper

Through using aluminium, raw materials costs were able to be reduced by over 85 percent compared to copper inductors. As a consequence of the increased slot fill factor, the electrical resistance of the aluminium inductor is not higher as is the case with the copper inductor despite the lower specific conductibility.

Taking the possibilities of the process into account, the completed commutating inductors of the 330kW direct current machine have been geometrically designed in such a way that in addition to the maximisation of the conductor surface, the cooling compared to the previously installed copper inductor has been improved.

In a test carried out on the test stand, the temperatures of the aluminium commutating inductors in constant operation were compared with the temperatures of the copper inductors before operation.

The temperature increase of the commutating poles in constant operation has been reduced from 75 down to roughly 45 Kelvin thanks to improved cooling. The reduced temperature range in this case leads to a light reduction in the machine's overall losses.

"The improvements of the cooling performance achieved as a result of the new casting technology surpassed our expectations," says Michael Jakob, development engineer at Lloyd Dynamowerke. "With the freedoms in terms of design that this technology offers, we see the potential for increasing the efficiency and performance density of electrical machines in future applications."

Frank Lindner