Today, several kinds of brushless synchronous machines are widely used because of minimum maintenance requirement. In 1958, Nonaka et al. proposed a new brushless excitation system called half-wave rectified brushless excitation and developed a brushless self-excited type single phase synchronous generator, which has very simple brushless structure. In 1987, Oyama et al. applied this brushless excitation to an AC servo-motor and proposed a half-wave rectified brushless synchronous motor. This paper describes principles and characteristics of the half-wave rectified brushless synchronous motor. It also present a new type half-wave rectified brushless synchronous motor with permanent magnet to improve the power factor and efficiency. Finally, a new sensorless (position and speed sensorless) control system using half-wave rectified brushless synchronous motor, which can operate in wide speed range form zero to the rated speed, is proposed.