Electric heating – Metal heating – By arc
Reexamination Certificate
2000-02-22
2001-07-31
Shaw, Clifford C. (Department: 1725)
Electric heating
Metal heating
By arc
C219S13700R, C363S025000
Reexamination Certificate
active
06268587
ABSTRACT:
The present invention relates to the art of electric arc welding and more particularly to a rapid switching type inverter power supply for electric arc welding with an improved current feedback device for use in such welder.
INCORPORATION BY REFERENCE
The present invention relates to an electric arc welder having a rapid switching inverter type power supply including an output transformer with a primary stage and a secondary stage. In the preferred embodiment of the invention, the primary stage includes two separate primary windings through which are passed current pulses of opposite polarity determined by closing a set of switches causing the current flow through separate portions of the input stage. Such an electric arc welder is disclosed in Blankenship 5,351,175 incorporated by reference herein as background information. Such electric arc welder normally includes a shunt in the output welding circuit for sensing the arc current as a voltage level used as a feedback signal for controlling the current directed to the secondary stage of the output transformer. Details of this type of electric arc welder and arrangements for rapidly switching the input DC current to provide positive and negative current pulses are well known power supply technology and need not be repeated as background to the present invention.
BACKGROUND OF INVENTION
An inverter welder for electric arc welding normally includes a rapid switching inverter including a transformer having a primary stage magnetically coupled with the secondary stage that creates an AC or pulsating current. The AC output current is rectified and used in the electric arc welding process. The primary stage of the transformer is a winding which is subject to a series of positive polarity current pulses alternating with a series of negative polarity current pulses. In some inverters, the primary winding is divided into two sections, one receiving the positive polarity pulses and the other receiving the negative polarity pulses so that the secondary stage, or secondary winding, outputs an AC current. This AC current is rectified and filtered to produce a DC welding current. Rapidly created series of positive current pulses and negative current pulses are caused by closing a switch in a series circuit with an input DC link. The switching rate is normally about 20 kHz; however, other super audio frequencies are used. The “switch” may include two or more switches in series so that the switch for the positive polarity pulses and the switch for the negative polarity pulses operate in sequence to create the AC primary current. In this type of power supply for an arc welder, the arc current is sensed at the welding operation and is used as a feedback signal to control the length of time the two switches are closed to control the output welding current at a set current. Such control systems also involve an inner loop for sensing the current in the primary stage of the output transformer to create another feedback parameter which is combined with the arc current to control the two switches driving the primary stage of the transformer. The outer loop current feedback is normally sensed by a shunt in the output circuit. A shunt reads the actual current being used in the welding operation to give an accurate current feedback signal. The inner control loop presents more challenges. The current used in the primary stage of the output transformer must be determined in a real time manner. In the inverter, a shunt can not be used because the pulsating current has a very high frequency and relatively high power. Such factors cause heating of the shunt, as well as causing inaccuracy in the high frequency response of a shunt. Another drawback of a shunt is that it provides no electrical insulation. Consequently, the inner loop feedback control for current in a rapid switching inverter has heretofore used a current transformer responsive to high frequency pulses and capable of handling high current power signals. A current transformer in the primary stage has been suggested due to its high frequency response, high power capability and the ability to isolate the sensed current signal form the input stage of the inverter. The use of a current transformer in certain applications has not been completely successful in obtaining the inner loop component of a current mode control system. As long as the AC current being sensed by the current transformer is symmetrical, there is no net DC component and the current transformer provides a satisfactory feedback signal for the inner loop of the welder. However, if the AC current being measured becomes asymmetrical or unbalanced for any reason, the current transformer tends to saturate and distort the signal. The signal is distorted by adding a DC shift that is proportional to the DC component in the main transformer. This shift is caused by the current transformer's inability to produce an output with a DC component. By adding this DC component the “area” of the positive pulse equals the “area” of the negative pulse and the current transformer's flux is forced to be balanced by resetting itself during the switch off-time. The current signal in the inverter can become asymmetrical when the main transformer to the inverter is pushed into saturation or when the pulses to the main transformer are not equal for each half cycle. In order to prevent an asymmetrical signal from saturating the current transformer used to measure the primary current in the inverter, it is common practice to employ a DC blocking capacitor in combination with the current transformer. The DC blocking capacitor is either used in series with the primary or secondary of the current transformer. Although such a DC blocking capacitor in the secondary of the current transformer will prevent the secondary from causing the transformer to saturate, such capacitor will not prevent an asymmetric primary current from causing a distorted output signal. Consequently, a current transformer in the primary stage of the main transformer to sense the current through the transformer coil often experiences imbalanced current pulses. The positive polarity pulses are different from the negative polarity pulses. The blocking capacitor in the current transformer will eliminate the DC component by shifting the current transformer output signal. Consequently, the current transformer will shift the current pulses to balance the DC component. When the current transformer signal shifts, the current mode control will regulate the pulse width based upon a distorted signal. The control system will continue creating current pulses having different widths which continues the unbalanced process. This unbalance is initially quite small; however, the difference continues to increase causing the output signal of the current transformer to shift further to compensate for the current unbalance. Consequently, the current imbalance or unbalance will continue to increase until the main transformer is pushed into saturation. Running the transformer in saturation causes the switch elements to heat up by being subjected to high currents. Also, a saturated main transformer will reduce the ability to deliver power to the secondary output.
THE INVENTION
In accordance with the present invention, two current transformers are placed in the primary circuit of the main transformer of an inverter used in an electric arc welder. Each current transformer is used to measure either the positive polarity pulses or the negative polarity pulses. When using the two current transformers in a push pull inverter, half bridge inverter or full bridge inverter, each of the current transformers encircles a lead in the pulsing circuits so that each current transformer senses the pulses of a given polarity. The location of the current transformers is chosen so that the current flowing through each of the current transformers is unidirectional. The output signals of the two unidirectional current transformers can be combined with a blocking diode for each current transformer to give a signal that is represent
Kooken Todd Eric
Luo Lifeng
Lincoln Global Inc.
Shaw Clifford C.
Vickers Daniels & Young
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