Abstract: Based on the analysis of the operation of an AC-AC frequency converter-multiphase synchronous motor speed control system, this paper proposes an analysis method that decomposes the d-axis and q-axis variables of the multiphase synchronous motor into low-frequency components (including aperiodic components) and high-frequency components for separate processing, while neglecting the resistance of the rotor circuit to the high-frequency components and the effect of the damping winding on the low-frequency components. An equivalent circuit model suitable for steady-state and quasi-steady-state analysis of the AC-AC frequency converter-multiphase synchronous motor system is established. Experimental results prove the correctness of the model. Keywords : Equivalent circuit, Mathematical model, Multiphase synchronous motor, AC-AC frequency converter 1 Introduction Establishing a mathematical model of an AC-AC frequency converter-multiphase synchronous motor speed control system is the theoretical basis for analyzing the dynamic and steady-state operating performance of this type of system. Because the mathematical processing of such strongly nonlinear systems is very difficult, digital simulation is usually used. However, digital simulation methods are difficult to obtain regularities, especially in terms of designing corresponding control laws for the system. This paper takes a twelve-phase (four Y-shifted by 15 degrees) synchronous motor system as an example. Starting from the perspective of small disturbances, the d-axis variables of the AC-AC inverter-multiphase synchronous motor system are decomposed into low-frequency and high-frequency components and processed separately. The equivalent circuit and mathematical model of the multiphase synchronous motor suitable for steady-state and quasi-steady-state process analysis are derived. 2. d- and q-axis operational equations of the twelve-phase synchronous motor The twelve-phase synchronous motor adopts four sets of star-connected three-phase windings, denoted as Y1, Y2, Y3, and Y4. They are each 15 degrees phase apart. The relative positions of the stator windings are shown in Figure 1. [b][align=center]For more details, please click: AC-AC inverter-multiphase synchronous motor steady-state and quasi-steady-state equivalent circuit model[/align][/b]