This paper presents a multi-scale model of synchronous machine in phase-domain for efficient and accurate simulation of diverse transients. It makes use of the simulation methodology frequency-adaptive simulation of transients (FAST) in which the shift frequency appears as a simulation parameter in addition to the time-step size. Through appropriate setting of both shift frequency and time-step size, the proposed synchronous machine model enables the integrative simulation of electromagnetic and electromechanical transients. Moreover, this model is established in phase coordinates leading to a direct interface with the multi-scale models of external power systems. Test studies are performed in a single-machine-infinite-bus power system. The results show that FAST simulation gives superiority over the simulation using the electromagnetic transients program (EMTP). © 2012 IEEE.