Automated design of squirrel-cage induction machines by predefined torque-speed-characteristic

The operation of a squirrel-cage induction machine as a traction motor requires the consideration of an operating range in the design process. The machine has to provide an almost constant mechanical power on the shaft within a speed range which is defined by the Constant Power Speed Ratio CPSR. This is the main difference compared to the conventional design process of electrical machines which takes only the nominal operation point into account. The herein introduced design method formulates an optimization problem. The solution is the full draft of the squirrel-cage induction machine. For this purpose the optimization algorithm NSGA-II is used which requires a complete mathematical description of the induction machine. The optimization algorithm minimizes the difference between the desired Constant Power Speed Ratio CPSRdes and the calculated Constant Power Speed Ratio CPSRcal. CPSRdes is defined by the intersection point between the torque-speed characteristic of the inner torque and the torque-speed characteristic of the breakdown torque. CPSRcal is calculated by the ratio between the nominal torque and the break-down torque. Both torques are expressed by the equivalent circuit parameters of the squirrel-cage induction machine which is the result of a general derivation of the inner torque. The lumped parameters of the equivalent circuit are calculated in turn by the corresponding geometrical, electric and magnetic parameters of the machine. To verify the design methods functionality a comparison to a full classical design of a squirrel-cage induction machine is carried out.