This paper describes a brushless DC motor system without position or speed sensor. The brushless DC motor consists of a permanent magnet synchronous motor and a voltage-source inverter which is capable of controlling the amplitude and frequency of voltage. The rectangular-shaped stator current with a conducting interval of 120° (electrical) is controlled to be in phase with the trapezoidal back electromotive force. This results in producing maximum torque. Variable speed is achieved by adjusting the average motor voltage, just like chopper control of DC motors.
In this paper, two sensorless position detecting methods, “indirect method” suited at lower speed range and “direct method” suited at higher speed range, are proposed. The combination of the two makes it possible to detect the rotor position over a wide speed range. Furthermore, a speedsensorless PLL control is proposed, applying the principle of the direct method. Experimental results obtained from a prototype brushless DC motor are shown to confirm the validity of the sensorless drive. The starting procedure of the motor is also discussed because it is impossible to detect the rotor position at a standstill.