Axial Flux Permanent-Magnet (AFPM) machine is a multivariable object due to its multivariable mathematical model which is defined by the matrix equations: voltage equation, flux equation, torque equation and motion equation. Especially, the complex inductance matrix in the machine’s mathematical description causes difficulties in analyzing to build its mathematical model. This paper proposes a method of applying perpendicular transformation to simplify the machine’s model to help the design of controller easier. | Đặng Danh Hoằng và Đtg Tạp chí KHOA HỌC & CÔNG NGHỆ 139(09): 175 - 181 STUDY TO APPLY PERPENDICULAR TRANSFORMATION IN ORDER TO BUILD MATHEMATICAL MODEL FOR AXIAL FLUX PERMANENT-MAGNET MACHINE Dang Danh Hoang1*, Duong Quoc Tuan1, Vu Duy Hung2, Nguyen Hai Binh2 1 College of Technology - TNU, 2Ha Noi University of Technology - Economics SUMMARY Axial Flux Permanent-Magnet (AFPM) machine is a multivariable object due to its multivariable mathematical model which is defined by the matrix equations: voltage equation, flux equation, torque equation and motion equation. Especially, the complex inductance matrix in the machine’s mathematical description causes difficulties in analyzing to build its mathematical model. This paper proposes a method of applying perpendicular transformation to simplify the machine’s model to help the design of controller easier. Keywords: Axial Flux Permanent, perpendicular transformation, PID controller. INTRODUCTION* Axial Flux Permanent - Magnet motor (AFPM) has many advantages such as: high performance, high ratio of power and size, high power density, long life, small moment of inertia, wide speed range, high ratio of torque and current, less affected by interference, and robust [1-4]. Thus, AFPM motors are used widely in high quality speed variable electrical drive systems such as industrial robots, CNC machines, medical equipment, and flywheels in energy storage systems and AFPM motors have the almost absolute advantages in electric cars. The basic differences between AFPM motor and other motors are that the electromotive force of AFPM motor is trapezoid wave form due to its centralized windings (the electromotive force of other motors are sinusoidal wave form due to distributed windings). Because of trapezoid electromotive force, AFPM motor has characteristics similar to characteristics of DC motor, high power density, high capability of torque generation, and high performance. When the application such as electrical drive system