Patent ID: 8018191

Claim:
An electric power steering system for generating steering assist force by driving a motor using a main power supply and an auxiliary power supply for auxiliary power as required, the electric power steering system comprising: a first switch including a first switching element provided in a path for supplying power from the main power supply to the motor and a first diode arranged in parallel with the first switching element such that a current flow direction of the first diode coincides with a current flow direction at a power supplying time; a second switch including a second switching element provided in a path for supplying power from the main power supply and the auxiliary power supply to the motor and a second diode arranged in parallel with the second switching element such that a current flow direction of the second diode is opposed to the current flow direction at the power supplying time; a voltage detector which detects a sum voltage (V B +V C ) based on a voltage V B of the main power supply and a voltage V C of the auxiliary power supply; a rotation speed detector which detects the rotation speed of the motor; and a control and drive unit which computes a voltage V G of power generated by the motor based on the rotation speed detected by the rotation speed detector, compares the voltage V G with the voltage detected by the voltage detector, and if the relation V G >V B +V C holds, turns off both the first switching element and the second switching element to stop the power supply from the main power supply and the auxiliary power supply to the motor for charging the auxiliary power supply using the voltage V G through the second diode.