Electric heating – Metal heating – By arc
Reexamination Certificate
1999-11-18
2001-08-21
Shaw, Clifford C. (Department: 1725)
Electric heating
Metal heating
By arc
C363S141000
Reexamination Certificate
active
06278080
ABSTRACT:
This invention relates to a power supply apparatus for, for example, welders, cutters and projectors, which alternates between a running mode of operation and a pausing mode of operation in a plurality of cycles having a relatively short time period.
BACKGROUND OF THE INVENTION
An example of such power supply apparatus includes an input-side AC-to-DC converter which converts a commercial AC voltage to a DC voltage. The DC voltage is applied to an inverter where it is converted to a high-frequency voltage, which, in turn, is applied to a voltage transformer. The voltage transformer transforms the high-frequency voltage to a high-frequency voltage having a predetermined value. The voltage-transformed high-frequency voltage from the transformer is converted back to a DC voltage in an output-side high-frequency-to-DC converter. The resulting DC voltage is applied to a load.
The input-side AC-to-DC converter and the output-side high-frequency-to-DC converter each include at least one diode. The inverter includes at least one semiconductor switching device, e.g. an IGBT.
Since the described power supply apparatus includes an inverter, it can use small-sized reactors in the transformer and the output-side high-frequency-to-DC converter, which enables the downsizing of the power supply apparatus.
The power supply apparatus is placed in a casing, and, therefore, Joule heat generated by some components tends to accumulate in the casing. The diodes used in the input-side and output-side AC-to-DC converters and the power semiconductor device used in the inverter, in particular, generate a large amount of heat. A fan is disposed in the casing for the purpose of forcibly cooling such devices.
When this power supply apparatus is used with, for example, a welder or a cutter, of which the load includes a torch and a workpiece, it is used for a plurality of operation cycles. Each cycle consists of one running period in which the power supply apparatus operates in a running mode to generate an arc between the torch and the workpiece, and one pausing period in which the apparatus operates in a pausing mode. In the pausing mode, the apparatus is temporarily stopped. The time period of one cycle is predetermined for a particular apparatus. For example, it may be 10 minutes. Also, the proportion of the period of the running mode in one cycle (i.e. running ratio) is predetermined on the basis of the rating of the particular power supply apparatus.
Although the fan is driven to rotate in both the running and pausing periods, the power semiconductor devices including the above-mentioned diodes and semiconductor switching device, generate heat only during the running periods, causing the temperature in the casing to rise. During the pausing periods, the temperature within the casing rapidly decreases because the generation of heat by the power semiconductor devices is stopped and the fan is operating to cool the interior of the casing.
Experiments have revealed that the lifetime of power semiconductor devices decreases as the difference &Dgr;Tc between the highest temperature and the lowest temperature during their operation increases. Temperature difference frequently occurs in a power supply apparatus for an arc welder or an arc cutter, in particular, which has a predetermined running ratio, which results in decrease of the lifetime of power semiconductor devices used therein. In addition, the power consumption of such power supply apparatus is large because the fan continuously operates even during a time interval between one welding operation and another or between one cutting operation and another, during which the fan need not be operated.
There is a power supply apparatus in which the temperature within the casing is measured, and the rotation of the fan is controlled in accordance with the measured temperature, so that unnecessary power consumption can be reduced. However, this arrangement cannot provide a solution to the reduction of the lifetime of diodes and semiconductor switching devices used in the apparatus.
The present invention is based on a discovery that power semiconductor devices can be used for a longer time as the difference between the highest and lowest temperatures to which they are subjected during operation is smaller. An object of the present invention is to prolong the lifetime of power semiconductor devices used in a power supply apparatus and reduce unnecessary power consumption of the apparatus.
SUMMARY OF THE INVENTION
A power supply apparatus according to the present invention includes a casing within which a power supply unit is housed. The power supply unit includes heat generating components including power semiconductor devices. A fan is provided for the casing to cool the heat generating components. A start switch is closed and opened alternately and repeatedly to thereby cause the power supply apparatus to operate in a plurality of operating cycles each consisting of one running period and one pausing period following the running period. Each cycle lasts a relatively short time period. Fan control means responds to the closure and opening of the start switch by operating and stopping the fan, respectively.
Thus, according to the present invention, the fan is operated while the power semiconductor devices of the apparatus are generating heat so that excessive temperature rise of the power semiconductor devices is prevented. The fan is stopped when the power supply apparatus is placed in the pausing mode. Therefore, the temperature of the semiconductor devices at the instant when the power supply apparatus is brought into the pausing mode is highest. Since the fan is not operating during the pausing mode, the temperature of the power semiconductor devices does not fall much. Thus, the temperature at the instant when the start switch of the power supply apparatus is next closed is lowest. Since the temperature difference between the highest and lowest temperatures is small, the lifetime of the semiconductor devices can be prolonged.
The fan control means may be switching means for coupling and decoupling a power source to and from the cooling fan. The power source for the fan may be an AC power source or a DC power source.
The cooling fan may operate upon receiving an AC signal. In this case, the fan control means is provided by converter means which converts a DC signal from the power supply unit into an AC signal in response to the closure of the start switch and supplies the resulting AC signal to the cooling fan. The converter means stops supplying the AC signal when the start switch is opened.
The fan control means may be arranged such that it does not stop the fan immediately after the start switch is opened, but it may allow the fan to rotate for a predetermined time period after the opening of the start switch.
REFERENCES:
patent: 3059164 (1962-10-01), Johnson
patent: 5045670 (1991-09-01), Gilliland
patent: 5250786 (1993-10-01), Kikuchi et al.
patent: 5825642 (1998-10-01), Ishii et al.
patent: 703291 (1965-02-01), None
Ikeda Tetsuro
Ishii Hideo
Kinoshita Atsushi
Moriguchi Haruo
Duane Morris & Heckscher LLP
Sansha Electric Manufacturing Company Limited
Shaw Clifford C.
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