With the increased demand for electric vehicles (EVs) using large-capacity electric motors, a motor's thermal management has been deemed essential because thermal characteristics are closely related to its efficiency and lifespan. The conventional commercial winding synchronous motor system has a disadvantage in terms of efficiency and vehicle applicability since it has a separate packaging structure for the motor with an inverter in the perspective of the design for electric vehicles. Therefore, single-system packaging research has been regarded as a technical trend to enable size optimization, and the high-efficiency cooling structure has been recognized as an essential design issue accordingly. This study investigated the effectiveness of the coolant flow rate of a single packaged WFSM-inverter unit with dual cooling passage through computational fluid dynamics and experimental verification. Initially, temperature saturation experiments were conducted on the rotor model, involving the variation of coolant temperature (40°C, 50°C and 60°C) under continuously rated conditions at 5 A and instantaneous operation conditions at 13 A. These experiments aimed to assess the effectiveness of shaft cooling and determine the appropriate coolant temperature, which was found to be 60°C. CFD base model was established and verified with the temperature saturation test, achieving analysis consistency within a 4~5% deviation. The WFSM full model was developed from this verified base model to conduct a coolant effectiveness analysis regarding flow rate variations. This analysis finally determined the suitable flow rate range from 5 to 20 LPM. Furthermore, a normalized cooling performance index was suggested, and the coolant flow rate (10 liters per minute) for optimal operation was determined, enhancing the performance of the WFSM motor.