| 摘要 |
A position detector (11) detects the position of a rotor (8) of a brushless motor (6). A driving signal generator (12) generates a driving signal at a timing corresponding to the detected position. While the driving signal is received, a driving circuit (14) sequentially ON/OFF-operates switching elements (Q1 to Q6) of an inverter circuit (2). At this time, a rotational speed detector detects a rotational speed of the brushless motor (13). A rotational speed controller (15) controls the duty of ON/OFF operations of the switching elements (Q1 to Q6), performed by the driving circuit (14), through a chopper signal generator (17) in accordance with the difference between a detected rotational speed and a designated rotational speed. If the detected rotational speed does not reach the designated rotational speed even when the duty becomes a preset value or more, the rotational speed controller (15) generates a phase shift signal for advancing the phase of the driving signal from that of a driving signal in a normal operation, and supplies it to the driving signal generator (12). Upon this phase shift operation, in the brushless motor (6), a magnetic flux effectively acting from the rotor (8) to armature windings (7r, 7s, 7t) is reduced as compared with a magnetic flux in a normal operation. Thus, the rotational speed of the brushless motor (6) can be reliably set to a desired value without decreasing the operation efficiency. <IMAGE> |
