Field magnet position detection method for electric motor

A field magnet position detection method for an electric motor including a rotor including a permanent magnet field system, a stator including a three-phase coil, an output unit configured to perform bidirectional energization on the three-phase coil via a half bridge type invertor circuit, a control unit configured to perform PWM control on a coil output in response to an instruction from a host controller, store energization angle information and energization pattern information in units of 60° energization interval in which continuous rotation is attainable, and switch an energization state by performing switching control on the output unit based on the information, and a measurement unit configured to perform A/D conversion on the three-phase coil voltage and transmit resultant voltage to the control unit, the method comprising, using the electric motor and by the control unit:

• performing 120-degree energization including an off cycle periodically in an energization pattern in which a point of self-excited stop occurring with two-phase fixed energization through the output unit matches a start point position of the 60° energization interval;
• and
• performing sensorless driving while detecting a field magnet position of the electric motor by measuring energization phase voltage and non-energization phase voltage in an on cycle of the PWM energization with the measurement unit, wherein
• the control unit selects an optimum duty ratio for sensing the non-energized phase coil voltage during the on-duty period and operates by varying the duty ratio of the predetermined drive voltage applied to the three-phase coil through the output unit, and
• when driving with the optimum duty ratio during a current conduction period of one cycle, the predetermined drive voltage is applied through the output unit during the on-duty period of the optimum duty ratio, and
• when driving with a duty ratio exceeding the optimum duty ratio during a current conduction period of one cycle, the predetermined drive voltage is applied through the on-duty period of one cycle dividing into a plurality of on-duty periods including the optimum duty ratio.