Combination of aluminium and copper increases efficiency of electric drives

Combination of aluminium and copper increases efficiency of electric drives

Technology News |
By Christoph Hammerschmidt

A coil geometry with a flat conductor arrangement that can be produced using casting processes offers decisive advantages over conventionally produced wound coils. This manufacturing process enables specific and individual adaptation of the conductor cross-section, allowing maximum use to be made of the available installation space in electrical machines. In this way, groove filling factors of over 80 % can be achieved. By substituting copper with aluminum in the active part, a weight reduction of up to 50 % can be achieved with constant efficiency.

Due to the flat design, new cooling concepts also enable an additional increase in current density. The feasibility and technological advantages of cast coils have already been demonstrated in various applications.

The combination of a high filling factor, excellent thermal behavior and low current displacement losses can now be used in a variety of ways in machine design. If copper is replaced by aluminium, the cross-section of the aluminium conductor must be increased compared to a copper conductor due to the lower electrical conductivity. With the connection of the individual stator coils through the interconnection ring, however, this increase in the conductor cross-section leads to a higher installation space; this results in a larger drive design and thus a reduction in the volumetric power density. This disadvantage is compensated by the use of copper as conductor material in the circuit ring. This means that the conductors for the wiring can be made smaller and the necessary insulation and clearance distances can be maintained.

Up to now, it has been a challenge to reliably connect the cast aluminium coils to the copper conductors. A bad connection would lead to a heating of the contact point, which would lead to an increased power dissipation and possibly to a damage up to the defect of the drive. The electrical contacting between aluminium and copper and the use of this connection as an electrical conductor is the subject of research activities at the Fraunhofer IFAM, partly due to the ageing mechanisms (interdiffusion, friction corrosion, chemical corrosion, softening and electromigration).

The scientists at Fraunhofer IFAM tested various joining processes for their suitability for the efficient use of copper conductors as contacting elements. Various soldering, forming, adhesive, welded and assembled connections were compared. The joining process also had to meet the following requirements: environmentally friendly process (e.g. no use of substances harmful to health or the environment), minimum effort (no pretreatment or intermediate process steps), series production possible, joining temperature should correspond to thermal class H (max. 180°C) of the insulation class according to DIN EN 60085 and the joining should take place in the installed state, i.e. the coils are already mounted on the sheet metal package.

The results of the extensive investigations show that solid phase joining processes meet the high requirement profile. If the joining temperature is kept below 300°C, as a rule no mixed phases are produced and the joint has a low electrical resistance with sufficient strength. Detailed investigations with regard to strength and contact resistance have shown that joining by ultrasonic welding can be regarded as very promising. This process meets the requirements and offers great potential for further technology optimization. The two-dimensional tool arrangement allows the two contact partners to be joined with very good joining properties. Depending on the application, an optimization with regard to strength and formation of intermetallic phases can be achieved by varying the process parameters, with almost constant electrical resistance.

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