Patent ID: 8134326

Claim:
A motor drive apparatus for an electric motor, which provides steering assist torque in an electric power steering system, the motor drive apparatus comprising: a three-phase inverter for applying three phase-voltages of three phases to the motor, the three-phase inverter having an upper arm element and a lower arm element connected in series in each phase; a current detection sensor for detecting a phase current flowing in the lower arm element in each phase; and a controller for controlling the three-phase inverter, wherein: the controller is configured to calculate a q-axis current or a function value, which has a direct correlation to the q-axis current, the controller is configured to control the three-phase inverter to implement a three-phase sine-wave modulation method when the q-axis current or the function value is less than a predetermined threshold value, the controller is configured to control the three-phase inverter to implement a two-phase modulation method when the phase current detected by the current detection sensor is greater than a predetermined threshold current and the q-axis current or the function value is greater than a predetermined threshold value, the controller is configured to control the three-phase inverter to implement the three-phase sine-wave modulation method when the phase current detected by the current detection sensor is greater than the predetermined threshold current and the q-axis current or the function value is less than the predetermined threshold value, and the controller is configured to control the three-phase inverter to implement the three-phase sine-wave modulation method irrespective of the q-axis current or the function value relative to the predetermined threshold value when the phase current detected by the current detection sensor is less than the predetermined threshold current.