Electricity: electrical systems and devices – Safety and protection of systems and devices – Motor protective condition responsive circuits
Reexamination Certificate
1998-09-28
2001-01-09
Sherry, Michael J. (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
Motor protective condition responsive circuits
C361S103000, C361S093800, C340S643000
Reexamination Certificate
active
06172860
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a DC power source unit that can be used for cordless electrically powered tools.
2. Description of the Related Art
Cordless electrically powered tools, such as electrical drills, can be used any place and are easy to use because no power source cables are required. However, the use of batteries invites some disadvantages. When the charge remaining in the batteries drops to less than an allowable limit, the batteries need to be replaced or recharged. Further, because the battery voltage gradually drops as the use of the battery progresses, the tool's operation efficiency is not constant.
FIGS. 1 and 2
show a conventional DC power source unit suitable only for situations wherein the tool is used close to an AC power source and when work with the tool can be accomplished in a single location without the need to move the tool to different areas. The DC power source unit includes an AC/DC converting box
50
, an input cable
57
, and an output cable
54
. The input cable
57
has one end connected to the converting box
50
and another end attached to a plug
56
. The plug
56
is used for connection to an AC power source. The output cable
54
has one end connected to the converting box
50
and another end attached to an adapter plug
55
. The adapter plug
55
is detachably connected to the electrically powered tool. As shown in
FIG. 2
, the converting box
50
includes a transformer
51
for dropping the AC 100 volts to ten and several volts, a rectifier
52
for rectifying the AC voltage to a DC voltage, and a smoothing capacitor
53
for smoothing the waveform of the rectified voltage.
With the DC power source unit shown in
FIGS. 1 and 2
, the AC voltage supplied from a commercial outlet is converted to a DC voltage to be supplied to the tool. Such a DC power source unit allows the continuous use of tools, thereby ensuring constant work efficiency.
The motors contained in electrically powered tools do not heated up significantly when the tool is driven with batteries. For this reason, the battery powered tools are, generally, not provided with motor cooling fans. Even if the fans are provided, they are small size fans.
When a tool is driven using the conventional DC power source unit, the tool tends to be used continuously for a long period of time, causing the temperature of the motor to rise significantly. This shortens the service life of the motor or the tool. To overcome the above problem, one solution would be providing a large-size cooling fan within the electrically powered tool. However, a large-size cooling fan would increase the overall size of the tool. Large size tools are not desirable for the operator to handle or operate. Another solution to this problem would be providing a temperature sensor in the tool to monitor an abnormal temperature rise within the tool. Power supply to the motor can be controlled in response to the output of the sensor. However, it is difficult to add a new component to the electrical circuitry of the tool and to modify control circuitry of the tool. Even if such addition and modification of the control circuit were made, the resultant electrical arrangement would be complicated.
SUMMARY OF THE INVENTION
In view of the foregoing, it is an object of the present invention to provide an improved DC power source unit that is suitable for use in electrically powered tools.
Another object of the present invention is to provide a DC power source unit that minimizes shortening of the service life of electrically powered tools.
To achieve the above and other objects, there are provided several types of DC power source unit for supplying DC voltage to an electrically powered tool.
According to one aspect of the present invention, the DC power source unit includes current detecting means, control means and alarm means. The current detecting means detects a current flowing in the electrically powered tool and outputs a detection signal representative of a detected current. The control means computes accumulated electrical quantity based on the detection signal received from the current detecting means.
The control means predicts that the electrically powered tool is heated up to a first critical temperature based on the accumulated electrical quantity and outputs a prediction signal representative of the first critical temperature to the alarm means. In response to the prediction signal, the alarm means indicates that the electrically powered tool is heated up to the first critical temperature. This predication is made when the accumulated electrical quantity exceeded a first predetermined quantity.
The control means stops supplying the current to the electrically powered tool when the control means predicts that the electrically powered tool is heated up to a second critical temperature higher than the first critical temperature. In this case, the control means predicts that the electrically powered tool is heated up to the second critical temperature when the accumulated electrical quantity exceeded a second predetermined quantity larger than the first predetermined quantity. A reset switch may be provided for bringing the electrically powered tool to be operable when depressed. When the control means predicts that the electrically powered tool is heated up to the second critical temperature, the control means can control the current supply to the electrically powered tool so that the tool does not become operable until the rest switch is depressed.
Alternatively, the control means can temporarily stop supplying the current to the electrically powered tool in this situation, so the current supply to the electrically powered tool is resumed after expiration of a predetermined period of rest time. When the current supplied to the electrically powered tool is temporarily stopped for a predetermined number of times within a predetermined period of time, the control means may not resume supplying the current to the electrically powered tool.
The control means may determine every predetermined interval whether or not the current detected by the current detecting means exceeds a predetermined level. The alarm means indicates that the electrically powered tool is heated up when the current exceeding the predetermined level flowed a predetermined number of times.
The current detecting means detects the current every predetermined interval, for example, every 10 msec, and the control means computes the accumulated electrical quantity based further on the predetermined interval. The control means computes the accumulated electrical quantity every predetermined interval. Specifically, the accumulated electrical quantity is computed by multiplying the current and the predetermined interval. A predetermined value may be subtracted from the computed accumulated electrical quantity.
According to another aspect of the present invention, there is provided a DC power source unit including a heat generating element connected in series to a motor contained in the tool, temperature detecting means for detecting temperature of the heat generating element, and alarm means for alarming when the temperature detected by the temperature detecting means exceeded a first predetermined temperature. The heat generating element has a temperature characteristic substantially equal to the temperature characteristic of the motor. Control means may be further provided for stopping supplying current to the electrically powered tool when the temperature detected by the temperature detecting means exceeded a second predetermined temperature higher than the first predetermined temperature. An adapter plug is electrically coupled to the tool, and the alarm means is provided in the adapter plug.
According to still another aspect of the invention, a DC power source unit includes a main unit that has an output and produces DC voltage from the output, an adapter plug that is electrically coupled to the electrically powered tool in use, a connector that is electrically connected to the output of t
Funabashi Kazuhiko
Nakagawa Atsushi
Nakayama Eiji
Orikasa Hiroaki
Shimada Takahiko
Hitachi Koki Co., Ltd
Sherry Michael J.
Sughrue Mion Zinn Macpeak & Seas, PLLC
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