GB1591185A - Welding power sources - Google Patents

Description

(54) WELDING POWER SOURCES (71) We, BOC LIMITED, of Hammersmith House, London W6 9DX, England, an English company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and Qy the following statement: This invention relates to adjustable power sources for electric welding.

Conventional power sources have relied on the use of tap-changing transformers, magnetic amplifiers or thyristor-controlled converters for controlling the voltage, the amperage, or both, of the output. The disadvantages of these known systems is that they have all necessitated the use of bulky, heavy and expensive welding transformers intended to operate at. the normal mains supply frequency of 50 Hz.

The present invention aims at providing an adjustable welding power source of reduced size and weight compared with existing power sources of the same rating.

Accordingly the present invention provides an adjustable power source for electric welding, which is as claimed in the appended claims.

The present invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a schema of the power source of the present invention, and Figure 2 is a block diagram illustrating the mode of operation of the source shown in Figure 1.

The apparatus shown in Figure 1 is intended to be connected to a three-phase power supply at the normal supply fresuency of 50 Hz. The three phases are connected to a full-wave rectifier bridge including diodes D1 to D6. The output of the rectifier bridge is smoothed by capacitors C1 and C2 which also serve to provide the half-voltage supply point 2 for the sinewave inverters A and B. As the two inverters are identical, the components of only one of them will be discussed and referenced.

Each inverter consists of a pair of asymmetrical thyristors T1 and T2 shunted by diodes D7 and D8 and connected in series with inductors L1 and L2, of which the mid-point is connected through a capacitor C3 to the point 2.

The capacitors C3 are both shunted by the primary winding of a ferrite-cored transformer TR. The secondary winding of the transformed is connected across a pair of output terminals 4.

The two inverter circuits are intended to operate at a so-called 'ultrasonic frequency', which in this specification means a frequency within the range of about 18-30 kHz. It has been found that below the lower limit the power source is too noisy, both in the power source itself and in the arc produced by it. Above the high limit the ferrite transformed TR gives unacceptably-high eddy current losses. In addition, the thyristors and any transistors will not work above 30 kHz.

In operation, the control electrodes of the thyristors T1 and T2 of each inverter are operated at the desired frequency. In addition, the timing of the operation of the two inverters is controlled so that the phase of one inverter relative to the other varies over the range 0" to 1800 i.e. from wholly in-phase to wholly out-of-phase.

It is a feature of the present invention that the inverters produce a sine-wave output, and not a square-wave output.

The method by which the two inverters are controlled relatively to each other is shown in Figure 2. A clock generator 6 produces pulses at the desired ultrasonic frequency. The pulses are fed directly to an amplifier 8 connected to the control electrodes of the thyristors in inverter A.

The pulses from generator 6 are also fed through an adjustable phase-delay circuit 10 having its output amplified by amplifier 12 coupled to the control electrodes of the thyristors in inverter B.

The extent of the delay introduced by circuit 10 is controlled by a direct signal derived from an error signal amplifier 14 having as one input a reference signal and having a feedback signal as the other input.

The reference signal is under the control of the operator, and the feedback signal is derived from the power source itself when connected to a welding torch effecting the welding.

The combination of the two inverters can be regarded as providing a fixed output signal which is adjustably decremented or incremented so that the output current or voltage appearing across terminals 4 can range between zero and the maximum respective capability of the power source.

It is the use of the ferrite transformer operating at an ultrasonic frequency which gives rise to considerable saving in size and weight compared with transformers able to produce the same effective output but at a frequency of 50 Hz.

It will be appreciated that the power source of the present invention may be used for DC welding, as well as for AC welding, by adding a bridge rectifier across the output terminals 4.

Accordingly it will be seen that the present invention provides a welding source which is easily adjustable and which is of small size and weight compared to known welding power sources of the same capacity.

WHAT WE CLAIM IS: 1. An adjustable power source for electric welding, including a rectifier bridge for a power supply at normal mains frequency; two identical sine-wave inverters connected across the outlets of the rectifier bridge to produce an output alternating at an ultrasonic frequency; means for combining algebraically the outputs of the two inverters, such that when wholly òut-of-phase the nett current is at a maximum value, and when wholly in-phase the nett current is zero and means for connecting tre combined outputs of the two inverters for delivery to a welding torch or like electrical load.

2. A power source as claimed in claim 1, in which the phase-adjusting means includes a thyristor in each of the inverters, with the control electrode of one thyristor being adapted to be supplied with gating pulses at an ultrasonic frequency, and with the control electrode of the other thyristor being fed with similar pulses which have been delayed by an adjustable amount.

3. A power source as claimed in claim 2, in which extent of the delay is controlled by the output from an error signal amplifier having as one input a control signal sunder the control of an operator, and having as the other input a feedback signal derived from the power source itself when connected to a suitable electrical load.

4. A power source as claimed in any preceding claim, in which the means for combining together the outputs of the two inverters includes a ferrite-cored transformer primary winding connected across the two mid-points of the inverters, and its secondary winding connected to a pair of outlet terminals.

5. A power source as claimed in claim 4, in which, for direct current welding, a bridge rectifier is connected across the outlet terminals.

6. An adjustable power source for electric welding substantially as described herein with reference to, and as shown in, the accompanying drawing.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (6)

**WARNING** start of CLMS field may overlap end of DESC **. The reference signal is under the control of the operator, and the feedback signal is derived from the power source itself when connected to a welding torch effecting the welding. The combination of the two inverters can be regarded as providing a fixed output signal which is adjustably decremented or incremented so that the output current or voltage appearing across terminals 4 can range between zero and the maximum respective capability of the power source. It is the use of the ferrite transformer operating at an ultrasonic frequency which gives rise to considerable saving in size and weight compared with transformers able to produce the same effective output but at a frequency of 50 Hz. It will be appreciated that the power source of the present invention may be used for DC welding, as well as for AC welding, by adding a bridge rectifier across the output terminals 4. Accordingly it will be seen that the present invention provides a welding source which is easily adjustable and which is of small size and weight compared to known welding power sources of the same capacity. WHAT WE CLAIM IS:

1. An adjustable power source for electric welding, including a rectifier bridge for a power supply at normal mains frequency; two identical sine-wave inverters connected across the outlets of the rectifier bridge to produce an output alternating at an ultrasonic frequency; means for combining algebraically the outputs of the two inverters, such that when wholly òut-of-phase the nett current is at a maximum value, and when wholly in-phase the nett current is zero and means for connecting tre combined outputs of the two inverters for delivery to a welding torch or like electrical load.

2. A power source as claimed in claim 1, in which the phase-adjusting means includes a thyristor in each of the inverters, with the control electrode of one thyristor being adapted to be supplied with gating pulses at an ultrasonic frequency, and with the control electrode of the other thyristor being fed with similar pulses which have been delayed by an adjustable amount.

3. A power source as claimed in claim 2, in which extent of the delay is controlled by the output from an error signal amplifier having as one input a control signal sunder the control of an operator, and having as the other input a feedback signal derived from the power source itself when connected to a suitable electrical load.

4. A power source as claimed in any preceding claim, in which the means for combining together the outputs of the two inverters includes a ferrite-cored transformer primary winding connected across the two mid-points of the inverters, and its secondary winding connected to a pair of outlet terminals.

5. A power source as claimed in claim 4, in which, for direct current welding, a bridge rectifier is connected across the outlet terminals.

6. An adjustable power source for electric welding substantially as described herein with reference to, and as shown in, the accompanying drawing.