Regarding a hand-held power tool, a cordless impact tool that is driven by the electric energy accumulated in a battery is widely used. In the impact tool where a tip tool such as a drill or a driver is rotationally driven by a motor to thus perform a required operation, the battery is used to drive a brushless DC motor, as disclosed in PTL 1, for example. The brushless DC motor refers to a DC (direct current) motor that has no brush (brush for rectification). The brushless DC motor employs a coil (winding) at a rotor-side and a permanent magnet at a stator-side and has a configuration where power driven by an inverter is sequentially energized to a predetermined coil to thus rotate the rotor. The brushless DC motor has higher efficiency than a motor having a brush and can obtain a high output while using a rechargeable secondary battery. Also, since the brushless DC motor includes a circuit on which a switching element for rotationally driving the motor is mounted, it is easy to achieve an advanced rotation control of the motor by an electronic control.
The brushless DC motor includes a rotor having a permanent magnet and a stator having multiple-phase armature windings (stator windings) such as three-phase windings. In the brushless DC motor, a position detection element configured by a plurality of Hall ICs configured to detect a position of the rotor by detecting a magnetic force of the permanent magnet of the rotor and an inverter circuit configured to drive the rotor by switching a direct current voltage supplied from a battery pack and the like with semiconductor switching elements such as FET (Field Effect Transistor) or IGBT (Insulated Gate Bipolar Transistor) and changing energization to the stator winding of each phase are used. The inverter circuit is configured to be controlled by a microcomputer and to set energization timing of the armature winding of each phase on the basis of position detection results of the rotor by the position detection elements such as Hall ICs.