Based on permanent magnet DC motor used in an electronic power steering system（ EPS）

the mathematical model for DC motor is analyzed to find out the reason why the armature resistance and the reverse electromotive force coefficient change in the actual work of the motor. Ansoft finite element electromagnetic simulation software is used to establish the equivalent model for the motor structure and simulate the armature reaction state under different working conditions. The variation of motor resistance and induced electromotive force coefficient with operating conditions is derived

and the mathematical expression of the variation parameter of rotating speed is estimated. The motor test platform is set up

and the motor voltage

current and temperature are measured in real-time by the motor controller

and the speed is estimated according to the expression. The results show that the motor speed can be estimated accurately when the motor load and ambient temperature change.