Advanced Welding Processes: Technologies and Process Control

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Advanced welding processes
Table 10.4 Difference between mean and RMS current values for rectangular
waveforms
Pulse Background On–off MA value True RMS Scaled RMS
current (A) current time ratio (A) value (A) value* (A)
600 0 1/1 300 424 333
550 50 1/1 300 390 333
500 100 1/1 300 360 333
450 150 1/1 300 335 333
300 300 1/1 300 300 333
600 0 1/3 150 300 167
*The scaled RMS value is typical of the reading obtained with a low-cost meter
which derives the RMS from the measured mean absolute current.
waveforms, there may be a substantial difference between MA and RMS
values, [206] as shown in Table 10.4. It is most important to specify the
methods being used to measure current and to adopt the same techniques
when comparing equipment or processes.
Voltage. The voltages used in welding usually need to be attenuated before
measurement either by oscilloscope or by analogue or digital meters. Most
metering systems do however, incorporate suitable attenuation for the normal
levels of voltage to be measured. The system of measurement and type of
instrumentation must again be specified since variations will occur between
different waveforms or instrumentation as described above. The simplest
method of checking the output of power sources is by the use of an ammeter
and voltmeter or a combined instrument. This may comprise a Hall-effect
current-sensing device and a digital voltmeter.
Speed and time. Linear travel speed and wire feed speeds are often measured
manually by timing over a measured travel distance or amount of wire fed.
Measurement using electrical and mechanical tachometers may also be used,
but it is necessary to translate the linear motion to rotational movement.
Suitable sensors are available; these usually use either slotted disc optical
tachometers or small DC generator devices. The voltage generated may be
displayed on analogue or digital instruments or in the case of the slotted disc
encoder the frequency pulses may be easily converted into rotational speed.
Stability monitors
The measurement of process stability is of interest when monitoring the
performance of the consumable electrode arc welding processes. It may be
used to assess the process performance during production as well as assisting
the development of consumables and equipment.
The operating performance of conventional SMAW and GMAW processes
may be evaluated by welding trials conducted by an experienced welder.