Conventionally, there has been proposed a rotary tool for rotationally driving a motor in proportion to the operation amount of an operation switch (or a trigger switch). The rotary tool is provided with a feedback function so that a work can be smoothly performed even if the load is changed depending on the variation of workpieces or the like. (see, e.g., Japanese Patent No. 3391951).
With the feedback function, the load applied to a motor is monitored to control the motor in such a way that the rotation speed of the motor is automatically increased in response to the increase of the load while the rotation speed of the motor is automatically reduced in response to the decrease of the load. The feedback function is employed to realize a smooth work.
In the rotary tool with a feedback function, however, there is posed a so-called cogging torque problem in that the tip (i.e., jaw) of the rotary tool rotates with jerkiness. The cogging torque is likely to generate, e.g., in case where a trigger switch is slightly pulled in in a load-free state and fixed as it is.