To change the characteristics of an electric motor in three stages through a simple circuit.

In a first state in which a switch 86 for low speed is on and a switch 88 for high speed is off, the electric motor 58 has low-revolution, high-torque characteristics. In a second state in which the switch 88 for high speed is on and the switch 86 for low speed is off, the electric motor has high-revolution, low-torque characteristics. In a third state in which both the the switch 86 for low speed and the switch 88 for high speed are on, the electric motor 58 has medium-revolution, medium-torque characteristics. Thus, the characteristics of the electric motor 58 can be changed in three stages by changing on and off of the switch 86 for low speed and the switch 88 for high speed, that is, through a simple circuit.

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