The most dramatic improvements in inverter-fed induction motors are achieved by the change of rotor slot geometry, which can reduce the harmonic winding losses of the rotor to a minimum. Based on an immense number of finite-element calculations, optimal values of different geometrical dimensions of the rotor slot with a U-shaped bridge have been determined. The slot can be designed step by step according to simple analytical expressions for any inverter-fed induction motor, regardless of power rating or speed. The basic principles of the loss mechanisms are explained qualitatively to support the results from the numerical calculations.