The commutatorless motor with an armature induced voltage commutation has found recently many practical applications. However, it has a drawback that its commutation ability is smaller than that of a motor with forced commutation.<br>Examples of the most commonly used position sensor of a rotor are a device composed of proximity switches or a device composed of lamps and photo-transistors. These devices, however, have a drawback of a fixed leading angle.<br>The effective leading angle which determines the reverse bias period of commutating thyristor decreases with an increase of load current. If a leading angle is controlled according to the load current automatically, the commutation ability is, much increased than in the case of fixed leading angle.<br>Along this idea we have conducted an experiment about a new detection method of the rotor's position by detecting the armature induced voltage. This control method compensates the decrease of a leading angle due to the armature reaction by detecting the self adjusted cross point of the induced voltages and requires no mechanical position sensor such as proximity switches, photo-transistors or a sector.<br>Therefore, this control method enables us to control a conventional synchronous motor as a commutatorless motor only with an addition of control circuits.<br>According to the theoretical considerations of the criterion of the commutation ability, its maximum current becomes 3.75 times as much as that for a mechanical position sensor and its maximum torque is 3.24 times as much. The most suitable leading angle of this control method is 45∼50 degrees.<br>We have proposed the starting method of the commutatorless motor which is controlled by this control method and have shown that this starting method enables us to start successfully the commutatorless motor.<br>We have introduced theoretically the relation between the angular velocity of the motor, the power source voltage, the current and the phase angle in a state of starting and have shown a permissible range of the power source voltage for a stable starting.