Advanced Welding Processes: Technologies and Process Control

Welding power source technology 35
in efficiency and enables normal air cooling to be used. The chopped waveform
appears at the output, but, if a sufficiently high switching rate is used, this
does not have any detrimental effect on the process. Switching frequencies
of 1000 to 25000 Hz are commonly used. Response rate is also determined
by the switching frequency but the higher-frequency supplies are capable of
responding within a few microseconds, which is significantly faster than
conventional supplies and approaching the rate achieved with series regulators.
GMAW and GTAW power sources of this type are available and offer high
precision at currents up to 500 A at an economical capital cost.
3.4.4 Primary rectifier–inverter
The methods of control outlined above use a conventional transformer to
achieve the step down in voltage required for welding. This transformer
operates at the incoming mains frequency of 50 Hz. The primary inverter
design utilizes the fact that transformer size may be significantly reduced if
its operating frequency is increased. The basic circuit is illustrated in Fig.
3.11 and the principle of operation in Fig. 3.12.
The primary AC supply is first rectified and the resultant high DC voltage
is electronically converted by the inverter to high-frequency AC. Only at this
stage does the supply enter the transformer. Since the frequency of operation
is between 5 and 100 kHz, the transformer is small; furthermore output
control is achieved by chopping or phase-shifting within the inverter and
very high response rates are achieved. The transformer output must be rectified
to avoid potential losses in the high-frequency AC circuit.
Rectifier
Driver 1
Inverter
Driver 2
Electronic
regulation
3.11 Typical inverter circuit.
Transformer
Feedback
Inductor
Rectifier