1. Field of the Invention
The present invention relates to an electric lawn mower of a battery driven type, comprising a DC motor provided at an output shaft thereof with a mowing blade, and a battery connected to the DC motor.
2. Description of the Related Art
The demand for electric lawn mowers has significantly increased in recent years, particularly in the suburbs because of the fact that electric mowers make less noise and produce less pollution than gas-powered lawn mowers.
Electric lawn mowers include a battery driven type (DC type) and a power cord type (AC type). The AC type is popular due to the fact that a reliable power source can easily be insured. However, the power cord or AC type is inconvenient to handle, because a power cord must be dragged around the lawn during mowing, and additionally, the power cord has a drawback that its working area is limited due to a limit in the length of the power cord.
By contrast, the battery driven type (for example, see Japanese Patent Application Laid-open Nos. 4-360615 and 5-192027) is easy to handle, because it does not have a cord which must be dragged around, and moreover, the working site is not limited. On the other hand, however, the battery driven type has a drawback that the battery capacity is limited, whereby the working time or the area to be mowed must be limited, and in actuality lawn mowing can only be carried out in a small area with an amount of electricity charged in the battery in single charge.
In a conventionally known electric lawn mower disclosed, for example, in Japanese Patent Application Laid-open No. 5-192027, there is provided a revolution-number sensor mounted to an output shaft of a motor or to a driving shaft for a mowing blade in order to maintain the number of revolutions of the motor at a certain constant value during working by the lawn mower. An electric current supplied to the motor is controlled in response to a signal output from the sensor. In such a lawn mower, however, the revolution-number sensor and the wiring connected thereto are typically located in rugged environments where there is dust, dirt, mud, water, and lawn cuttings scooped up by the mowing blade. For this reason, dust-proof and water-proof means must be provided for the sensor and the wiring. A significant increase in cost is incurred as a result, which is in addition to the expense of the sensor.