Abstract: The increasing penetration of wind power generation displaces a major portion of conventional generating plant reducing system inertia and deteriorating system frequency dynamics. In this sense, the induction machines（IMs） including induction motor loads and induction generators, which occupy a fair proportion in power consumption and generation, should be viewed with some concern in frequency dynamics of inertial response timescale. To estimates IMs available inertia in electromechanical timescale, the internal voltage vector of IMs was defined and its phase motion equation was deduced. It is shown that the IMs available inertia is affected by both rotor speed and slip frequency regulation and presents a time-varied property. Furthermore, a power system constituted of a SG, an aggregate Type 3 WTs and an IM load was established. The effect of IMs available inertia on frequency dynamics of the system with wind power was analyzed. Time-domain simulation results of the studied system were conducted to verify IMs available inertia and the correctness of the analyses.