Electric heating – Metal heating – By arc
Reexamination Certificate
2000-06-29
2001-12-25
Nguyen, Matthew (Department: 2838)
Electric heating
Metal heating
By arc
C323S237000
Reexamination Certificate
active
06333489
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to power supplies and, in particular to a method and apparatus for providing power with a reduced open circuit voltage.
BACKGROUND OF THE INVENTION
There are many known types of welding power supplies. One known type of welding power source is a phase controlled power source and includes a power circuit having controllable switches, such as SCRS, that are phase controlled. The phase controlled power supplies are relatively simple, robust and inexpensive.
Generally, phase controlled power supplies receive an ac input and provide an ac output, a rectified ac output, or a dc output. They control the amount of power provided by controlling the phase of an input ac cycle at which the controllable switches are turned on. Typically, the switches turn off at the next zero crossing. Thus, the portion of the input cycle from the on time to the zero crossing is provided as an output. (AC cycle length, as used herein, is the length of time from one zero crossing until the next zero crossing, or the length of time from one SCR being turned on, until the next SCR is turned on). A greater voltage output is provided by turning on earlier in each cycle, thus providing a greater portion of the input cycle as the output. A lesser output voltage is provided by turning on later in the cycle, and providing a lesser portion of the input cycle as the output. Current control can be obtained by feeding back output current to the phase angle regulator.
One such known phase control power supply is the Miller Mark VI™ power source. The Miller Mark VI™ power source is a multi-user power source that includes six phase controlled power circuits, one for each user output. The power circuits provide a dc cv (constant voltage) or cc (constant current)output. The power circuits include an SCR phase controlled rectifying bridge that receives an ac input. The output of the bridge is provided to an output circuit that includes an inductor to smooth the output. A controller receives a feedback signal, and compares the feedback signal to a user setpoint. The phase of the SCR firing is controlled in response to the comparison.
When the welding arc is initially created, a relatively high voltage (80V e.g.) is desirable. The relatively high voltage helps stabilize and create the arc. However, a higher voltage will possibly cause the user discomfort in the event the user touches both outputs. Accordingly, it is well-known in the art to provide circuitry which helps create a reduced open circuit voltage. Thus, when welding is not occurring the output voltage is reduced to a level which is less likely to cause the user discomfort. When the welding process begins a higher voltage is provided which is sufficient to start the arc. One such prior art circuit is shown in U.S. Pat. No. 5,513,093 (Corrigall et al), which is assigned to the owner of this invention, and is hereby incorporated by reference.
Other attempts have been made in prior art to provide a reduced open circuit voltage. However, such prior art attempts are generally not well-suited to use with a phase controlled power supply, are relatively complicated or do not provide a sufficiently reduced voltage without interfering with arc starting.
Accordingly, a phase controlled welding power supply that provides a reduced open circuit output voltage is desirable. Additionally, the reduced open circuit voltage should not interfere with arc starting and arc stability.
SUMMARY OF THE PRESENT INVENTION
According to a first aspect of the invention a welding power supply includes a phase controlled power circuit, an output sensing circuit and an OCV reduction circuit. The power circuit receives an ac input and provides output power in response to phase control signals. The output sensing circuit senses the output and provides a feedback signal. The OCV reduction circuit receives the feedback signal and provides a voltage reduction phase control signal as a phase control signal.
The output sensing circuit senses the output current in one embodiment and the phase controlled power circuit is disposed to provide an output voltage in response to at least two phase control signals in other embodiments
The controller receives the feedback signal and a user selected signal, and provides a set point phase control signal as a phase control signal in an alternative. The controller combines the set point phase control signal and the voltage reduction phase control signal to produce a single phase control signal in another embodiment.
An output circuit, including an RC circuit across two terminals, is provided in one embodiment. The RC circuit preferably has a time constant greater than the duration of one-half of SCR cycle length.
A method of providing welding power is provided in accordance with another aspect of the invention. The method includes receiving an ac input and phase controlling a power circuit to providing output power. An output feedback signal is provided and the power circuit is phased back in response to the feedback when the output current is below a threshold.
The feedback is current feedback in one alternative.
Phase controlling includes receiving at least two phase control signals in one embodiment, and one of the phase control signals is derived from a user setpoint in another embodiment. The phase control signals are combined to form a single signal in another embodiment.
The output is smoothed when the voltage is reduced in another embodiment.
A welding power supply includes a phase controlled power circuit, an output sensing circuit and a controller in accordance with a third aspect of the invention. The power circuit receives an ac signal having a cycle length and provides output power to an output circuit. The output circuit includes an RC circuit having a time constant greater than the cycle length. The output sensing circuit senses the output and provides a feedback signal. The controller receives the feedback signal and provide a phase control signal.
Other principal features and advantages of the invention will become apparent to those skilled in the art upon review of the following drawings, the detailed description and the appended claims.
REFERENCES:
patent: 4349720 (1982-09-01), Makimaa
patent: 4450340 (1984-05-01), Corrigall et al.
patent: 5355300 (1994-10-01), Zinn
patent: 6005220 (1999-12-01), Bunker et al.
patent: 6013892 (2000-01-01), Buda et al.
Corrigan George R.
Illinois Tool Works Inc.
Nguyen Matthew
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