The invention relates to an optimum design method for a flux leakage problem of an alternating-
current generator for a car. The optimum design method comprises the steps that the basic parameter of a finite 
element model is set, and the basic structure parameter of the alternating-
current generator is set; no-load electromagnetic models of a rotor, a 
stator, a 
stator winding and a field winding of the alternating-
current generator are set, and a transient 
electromagnetic field of a motor is solved through a three-dimensional 
finite element method; boundary adding is carried out on the dynamic parts of the models; excitation is added to the field winding; 
mesh generation is carried out on the finite 
element model, and a solving option is defined, and comprises the 
simulation time of the no-load electromagnetic models, step length and motor rotating speed; leakage flux is measured quantitatively by calculating 
leakage inductance, the no-load leakage coefficient of the alternating-current generator is calculated, and motor structure parameter optimization is carried out according to the flux leakage problem. A novel design concept is built, the leakage flux of a permanent 
magnet generator is analyzed and calculated again, a 
magnetic structure and a 
control system are improved, the leakage flux of the permanent 
magnet generator is reduced, the running efficiency of the motor is improved, and production cost is controlled.