Building efficient rotors for e-mobility starts with the first drop of melt
The demand for induction motors is growing. Scarcity of rare earths, climate change, electrification of mobility and other trends are driving this demand. However, the common industrial induction motor has weaknesses.
With its laminar die casting technology, German rotor manufacturer Wieland eTraction Systems boosts performance in asynchronous motors. While retaining the known advantages of asynchronous motors, these now aluminium and copper rotor types offer significant benefits in efficiency and safety.
Production difficulties in conventional rotor manufacturing
Every rotor consists of multiple individually punched electrical steel laminations that are stacked in a surrounding cast cage. This cage is typically manufactured using high pressure die casting. With cycle times of one to two minutes, this process can be highly automated. But from the casting perspective, rotors are very complex structures.
After placing the lamination stacks in the tool, the casting chamber is filled with molten metal. A piston forces the liquid metal into the casting tool at high speed. Once in the mould, aluminium or copper melt solidifies rapidly. Industrial technology therefore requires short filling times of less than 0.1 seconds with flow rates of more than 50 m/s.
To achieve this, the industry is using point gates. The disadvantage: these gates do not fill all slots in the stack at once. The melt first flows through the slots directly at the gate, then into the opposite ring, and finally fills the remaining slots from the rear with a so called back-filling. As a result, air, process gases, and oil and oxide contaminated melt fronts cannot escape.
High porosity wastes efficiency
During the transition from liquid to solid state, there is a decrease in volume, called solidification shrinkage. To compensate for this shrinkage, the piston continues to press, even after the mould has been filled to its full capacity. The combination of entrapped gas and shrinkage results in a high total porosity. Die casted rotors achieve up to 10% porosity, well above the five percent tolerance.
Every single pore reduces the conductive area, causes imbalance, and negatively impacts on the rotor’s mechanical properties. When the pores are centred at the transition from the slot to the ending ring, high current densities and maximum mechanical stress from centrifugal forces must be expected.
To reduce these potential pain points, many foundries limit the geometrics and reduce the number, length, and width of slots. Alternative manufacturing processes, like machining or welding, have similar limitations. This means that the full potential of asynchronous motor technology has remained untapped for a long time.
Laminar squeeze casting
The laminar squeeze casting process developed by Wieland eTraction Systems is designed to cast rotors with zero porosity - so called zero porosity rotors (ZPR). The patented gating system guarantees simultaneous filling of all slots and new geometries. In contrast to conventional turbulent filling, the filling process is ascending and laminar. Labour requirements are only slightly different from conventional casting, allowing cost-effective production.
With lower flow rates, the cast material remains liquid in the gate for a longer time and therefore eases refeeding. The active thermal management controls the solidification progress from the laminar core to non-critical areas. Since all the slots are filled at the same time in the laminar squeeze casting process, contaminated melt fronts are directed into the overflow.
Improving electrical conductivity
The laminar squeeze casting process results into a typical 3-5% increase in electrical conductivity, helping to significantly reduce the characteristic torque fluctuation and minimising the emitted noise.
The Institute for Metal Forming at the University of Aachen scanned different pore structures using computer tomography. In addition, the mechanical behaviour of the material at extreme limits was simulated. The results indicate that a speed increase of approximately 12.5% is technically possible with a porosity-free rotor. Actual burst tests confirmed this behaviour.
Besides the pore size, the slot geometry is an important factor affecting the strength of the rotor. Laminar squeeze casting also offers advantages by enabling complex slot geometries. Groove geometries with radial undercuts, which increase the bond to the laminated core, have already been successfully cast. In this context, electrical steel laminations with a high mechanical strength are used, which are optimally adapted to the centrifugal forces of the copper slots.
At circumferential speeds of more than 80 m/s, it is also customary to apply reinforcements such as carbon-fibre sleeves or bronze caps to the shading ring in an additional process step to enable these high speeds. Laminar squeeze casting allows this step to be integrated directly into the casting process due to the uniform and consistent filling. Rotors casted with this patented process, have achieved speeds of up to 200 m/s before bursting.
Peter Szilágyi is the Managing Director of Wieland eTraction Systems.