This paper investigates operation characteristics of direct-driven Interior Permanent Magnet Synchronous Machines (IPMSMs) for electrical vehicles designed with the given specifications. The rotor structure of IPMSM consists of permanent magnet layers inserted in the rotor laminations, resulting in saliency, where the inductance of q-axis is larger than that of d-axis. To estimate how motor parameters and inverter rating affect the output power capability, an analytical method was developed according to dq-axis equivalent circuit model of IPMSM. The inductances of d-axis and q-axis characteristics of the designed IPMSM were calculated using the finite element method. These results provide the design criteria to obtain an optimal combination of motor parameters to expand the wide speed range of constant power operation. Finally, the designed IPMSM and its control drive will be prototyped and tested to verify the performance for EV traction applications.