When converting the design of three-phase squirrel cage rotor induction motors into the design of threephase line start permanent magnet synchronous motors, they have usually its stator fixed, mainly its rotor changed. In this case, the pieces of permanent magnet have been inserted in the squirrel cage rotor. Thus, the air-gap flux is mainly produced by the magnet. Hence, it is very important to determine exactly the airgap flux density. This paper presents a novel method for a detailed analysis of the air-gap flux density determination related to the steel lamination B-H characteristics. Finally, the results are verified by the FEM tool in Ansoft Maxwell software.