Reaction Wheel (RW) is the most popular actuator in the Attitude Control System (ACS) of satellites which has a significant role in performing satellite missions. Reaction wheel supplies the torque and the momentum that is required for satellite attitude control. The RW model which is used in the ACS simulations and FDI algorithms should be an accurate model. On the other hand, each physical module has a special role in the reaction wheel modeling. In this regard, based on elements in the internal structure of this actuator, a precise model of RW is presented. Furthermore, the proposed model of RW is more general and can be used in other types of RW. Moreover, in this paper proposed a new model of the reaction wheel for ACS simulation. The proposed model of RW is validated by a Real-Time HIL test bed in the form of set point commands and profile command.