The evolution mechanism of inclusions in Al-killed alloyed steel during secondary refining process was studied by industrial experiments and thermodynamic calculations. It is found that during the tapping process, Al-O deoxidization reaction is very close to equilibrium with the formation of many Al2O3 clusters. With the slag/steel reaction, inclusions vary with the route as Al2O3 inclusions→MgO-Al2O3 system inclusions→CaO-MgO-Al2O3 system inclusions, and finally change into globular inclusions surrounded by CaO-Al2O3 outer layer, of which the melting point is lower than liquid steel temperature. Since MgO is less stable than CaO and it is easier to be reduced by Al, dissolved Mg is generated earlier and faster than Ca before LF refining, thus the MgO-Al2O3 system inclusions form at first. The mapping photos of inclusions show that the evolution mechanism of MgO-Al2O3 system inclusions into CaO-MgO-Al2O3 system inclusions is Ca element substitution for Mg element in MgO-Al2O3 inclusions. The line scanning shows that there is also the reaction of Ca element substitution for Al element in the outer CaO-Al2O3 layer without MgO.
Novel permanent magnet (PM) motor drives have been successfully developed to fulfil the special requirements for electric vehicles such as high power density, high efficiency, high starting torque, and high cruising speed. These PM motors are all brushless and consist of various types, namely rectangular-fed, sinusoidal-fed, surface-magnet, buried-magnet, and hybrid. The advent of novel motor configurations lies on the unique electromagnetic topology, including the concept of multipole magnetic circuit and full slot-pitch coil span arrangements, leading to a reduction in both magnetic yoke and copper, decoupling of each phase flux path, and hence an increase in both power density and efficiency. Moreover, with the use of fractional number of slots per pole per phase, the cogging torque can be eliminated. On the other hand, by employing the claw-type rotor structure and fixing an additional field winding as the inner stator, these PM hybrid motors can further provide excellent controllability and improve efficiency map. In the PM motors, by purposely making use of the transformer EMF to prevent the current regulator from saturation, a novel control approach is developed to allow for attaining high-speed constant-power operation which is particularly essential for electric vehicles during cruising. Their design philosophy, control strategy, theoretical analysis, computer simulation, experimental tests and application to electric vehicles are described.
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