Electric heating – Metal heating – By arc
Reexamination Certificate
2000-02-09
2001-10-09
Paschall, Mark (Department: 3742)
Electric heating
Metal heating
By arc
C219S121540, C219S121570, C363S124000
Reexamination Certificate
active
06300589
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to plasma arc cutting and welding power supplies and more particularly to a power supply with a plurality of secondary switchers.
BACKGROUND OF THE INVENTION
Plasma arc cutting is a process in which an electric arc is used to cut a metallic workpiece. Generally, plasma arc cutting uses an electric arc between an electrode and the metal to be cut. The arc creates a plasma that cuts the metallic workpiece.
It is generally accepted that approximately 250 volts (open circuit) is desirable to initiate a plasma arc cutting process. After the process has been initiated, the cutting arc voltage is approximately 90-125 volts. Of course, the cutting arc (load output) voltage is dependent upon the length of the cutting arc. The greater the length of the arc, the greater the load voltage, and conversely, the lesser the length of the arc the lesser the load voltage. Similarly, the cutting arc voltage varies with the magnitude of the output current.
A typical prior art plasma arc cutting power supply receives an input voltage (from a power line or generator) and provides an output voltage to a pair of output terminals, one of which is the electrode and the other of which is connected to the workpiece. The power supplies provide about 250 volts open circuit and about 90-125 volts under load. There are numerous types of known plasma arc cutting power supplies, such as magnetic power supplies, inverter power supplies, phase control power supplies, and choppers or secondary switches. The present invention relates to chopper based power supplies.
A typical prior art chopper shown in
FIG. 5
includes a voltage source
601
, a switch
602
, a diode
604
and an inductor
603
which provide output current to load
605
. Voltage source
601
may be a transformer receiving line voltage and a rectifier. Voltage source
601
should be capable of providing input power, generally at a desired voltage level. Of course, the source need not be a constant voltage source but merely should be suitable for use with a chopper power supply. When switch
602
is on current flows from the positive output of voltage source
601
through load
605
, inductor
603
and switch
602
. During this time the load current (and inductor current) is increasing and inductor
603
is storing energy, dependent upon the volt seconds applied to inductor
603
while
602
is on. The volt seconds are determined by the time switch
602
is on, and the source are load voltages. When switch
602
is off current freewheels through diode
604
, load
605
and inductor
603
. While switch
602
is off the load current (and inductor current) is decreasing and inductor
602
in returning energy, again dependent upon the volt seconds (the time and load voltage) inductor
603
applies to diode
604
and load
605
. In some high current applications a single voltage source may have more than one chopper connected in parallel. The choppers are operated in-phase with one another, thus the load current is merely twice the output current of either chopper.
The output voltage applied to load
605
is dependent upon the duration of time switch
602
is on, relative to the duration of time switch
602
is off. Specifically, the output voltage is equal to the ratio of on time of switch
602
to the on time plus off time of switch
602
, multiplied by the output voltage of voltage source
601
. Thus, if switch
602
is on 50% of the time the load voltage will be 50% of the source voltage. Typically, a current feedback element is used in conjunction with a control circuit to control the on and off time of switch
602
.
Thus, a chopper will have a triangle (sawtooth) current output having predetermined, but variable amplitude or frequency according to the switching frequency. Their operation is based on the controlled switching of a DC voltage input to a desirable DC current output. The sawtooth output may be characterized as having a ripple current, dependent on the maximum current less the minimum current.
One drawback of chopper power supplies is the ripple current rating of the output. When in-phase parallel choppers are used, the output ripple is twice the ripple of a single chopper. In a switching type (chopper) power supply, the output cutting capability of the air plasma arc cutter is adversely affected if its ripple current capability is not adequate for the job, i.e., cutting occurs best at relatively constant current, not with a sawtooth output. The life span of input capacitors in the voltage source is also affected if the ripple current is greater than the ratings. The ripple current generates internal heat in the capacitor, with the attendant changes in temperature dependent parameters. Elevated temperatures may reduce the life expectancy of any electrochemical component. It has often proved difficult to ensure a moderate ambient temperature for capacitors, much less to aggravate the situation by permitting excessive ripple currents. Even with an appropriately rated capacitor, dangerous internal temperatures can develop when there is no provision for heat removal from the external surface of the case. Additionally, the magnitude of the peak current is dependent upon the percent of ripple. Because high peak current can erode consumables, a low ripple current is desirable.
Chopper power supplies, however, are relatively inexpensive, controllable, and not lossy. Also, choppers are well suited for receiving an input voltage, and provide a load current at a lesser output voltage. Accordingly, it is desirable to provide a chopper based power supply to take advantage of the positive aspects of choppers, yet avoid a major drawback—excessive ripple current in the load.
Welding power supplies have many similarities to plasma arc cutting power supplies. Specifically, the welding process is best initiated with a generally accepted fixed open circuit voltage (approximately 80 volts). After initiation, however, the operating load output voltage is generally in the range of 20-45 volts, and often 25 volts. Finally, as in the plasma arc cutting process the actual arc voltage varies with the current and the length of the arc.
In some welding applications it is desirable to have a single power supply provide current to a number of welding stations connected in parallel. Thus, more than one welder can use a given power supply with this sort of arrangement. Typically, to provide the necessary open circuit voltage to initiate the welding arc the power supply will be a constant voltage, 80 volts source. Each welding station includes a variable resistor in series with the welding electrode (or workpiece). The resistor dissipates sufficient power to provide a typical load output voltage, 20-45 volts, e.g. As may be readily seen this is very wasteful of power—as little as 25% of the power is delivered to the arc load, while 75% of the power is dissipated in the resistor.
Accordingly, it is desirable to provide a welding power supply that is capable of providing 80 volts open circuit and in the range of 25 volts load voltage. Preferably, such a welding power supply would be capable of providing multiple parallel welding stations, without wasting power.
SUMMARY OF THE PRESENT INVENTION
According to one embodiment of the invention a power supply, such as a plasma cutting power supply or a welding power supply, provides an output to a pair of output terminals. The power supply includes a source of voltage and a plurality of choppers. The choppers are connected in parallel between the voltage source and the output terminals. A controller controls the choppers so that they are out-of-phase with respect to each of the other of the plurality of choppers. The choppers preferably include a freewheeling diode, an inductor and a switch.
According to a second embodiment of the invention the voltage source includes a transformer for receiving line voltage and a rectifier.
According to a third aspect of the invention a user selected current input is provided and current feedback is provided. The control circuit receives the
Li Stephen H.
Reynolds Jon O.
Schneider Joseph C.
Corrigan George R.
Illinois Tool Works Inc.
Paschall Mark
LandOfFree
Plasma cutting or arc welding power supply with phase... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Plasma cutting or arc welding power supply with phase..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Plasma cutting or arc welding power supply with phase... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2609206