Drive control apparatus for electric motor

An electric power steering system for assisting in steering by a steering mechanism using an electric motor as a drive source, comprising

• a drive control apparatus configured to control drive of the electric motor, wherein
• the drive control apparatus includes:
• a drive circuit configured to control the drive of the electric motor;
• a first phase cutoff semiconductor relay arranged on a drive line between the drive circuit and the electric motor configured to cut off current supply from the drive circuit to the electric motor;
• a power supply relay arranged on a power supply line for power supply to the drive circuit configured to control supply and cutoff of power to the drive circuit, the power supply relay including a second powering cutoff semiconductor relay and a third regeneration semiconductor relay connected in series, the second powering cutoff semiconductor relay comprising a parasitic diode that permits forward flow in an electric-motor-to-power-supply direction, the third regeneration semiconductor relay comprising a parasitic diode that permits forward flow in a power-supply-to-electric-motor direction;
• a first driver configured to drive the first phase cutoff semiconductor relay for each phase of the electric motor or simultaneously drive the first phase cutoff semiconductor relay for the phases to perform phase cutoff, the first driver further configured to drive the second powering cutoff semiconductor relay;
• a second driver configured to drive the third regeneration semiconductor relay;
• and
• an active clamp circuit configured to turn on the semiconductor relay when the electric motor is rotated by an external force and a potential difference between the drive circuit side and the electric motor side of the first semiconductor relay is greater than or equal to a predetermined value, even when the first phase cutoff semiconductor relay and the second powering cutoff semiconductor relay are set by the first driver to be in a cutoff state.