from the strong mode to the night mode the target number of rotations is controlled to be decreased the control unit 190 controls the motor 130 to be rotated by the target number of rotations set by the selected course the control unit 190 senses an electric current of motor s 212 the more load is applied to the motor the higher electric current is applied the control unit 190 compares the sensed electric current of the motor 130 with a maximum permissible limit of an electric current s 213 if the sensed current of the motor is higher than a maximum permissible limit the control unit 190 controls the motor 130 to be stopped s 214 the maximum permissible limit is an electric current when the motor 130 is overloaded if the motor is overloaded the control unit 190 controls a power supply to stop an electric power supply to the motor 130 the control unit 190 checks if the electric current of the motor 130 exceeds a maximum permissible limit less than 3 times s 215 if the excess number is less than 3 times the control unit 190 controls the motor to be rotated in reverse direction s 216 the motor 130 may.