MATERIALS
Comparison of casting technologies
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 CONDUCTIVTY 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.
GEOMETRICAL FREEDOM 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.
www.wieland.com
www.engineerlive.com 13
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44
