Power Source Characteristics

7/23/2019 Power Source Characteristics

1/4

Power source characteristics

Job Knowledge[ Zoom]

Fig. 1. Static arc characteristic

The prime objectie o! an arc welding power source is to delier controllable welding current at

a oltage demanded b" the welding process. The arc welding processes hae di!!erentre#uirements with respect to the controls necessar" to gie the re#uired welding conditions and

these in their turn in!luence the design o! the power source. $n order to understand how the

re#uirements o! the processes a!!ect the design o! the power source it is necessar" to understandthe interaction o! the power source and the arc characteristics.

$! the oltage o! a welding arc at ar"ing arc lengths is plotted against the welding current the

cures illustrated in Fig. 1 are obtained. The highest oltage is the open circuit oltage o! the

power source. %nce the arc is struc& the oltage rapidl" !alls as the gases in the arc gap become

ionised and electricall" conductie' the electrode heats up and the si(e o! the arc columnincreases. The welding current increases as the oltage !alls until a point is reached at which time

the oltage)current relationship becomes linear and begins to !ollow %hms *aw. +hat is

important to note !rom Fig. 1 is that as the arc length changes both the oltage and weldingcurrent also change , a longer arc giing higher oltage but with a corresponding drop in

welding current and ice ersa. This characteristic o! the welding arc a!!ects the design o! the

power source since large changes in welding current in manual metallic arc -/0 and T$welding is undesirable but is essential !or the $)/ and !lu2 cored arc welding processes.

[ Zoom]


2/4

Fig 3 4onstant current power source characteristic

/' T$ and submerged arc power sources are there!ore designed with what is &nown as a

drooping output or constant current static characteristic' $)/ and F4/+ power sources

with a !lat or constant oltage static characteristic. %n most power sources the slope o! the

characteristic can be changed either to !latten or ma&e steeper the cures shown in Fig 3 and Fig.5

Fig 3 shows drooping or constant current power source static characteristics' such as would be

used !or the / or T$ process' superimposed on the arc characteristic cures. +hen manual

welding is ta&ing place the arc length is continuall" changing as the welder cannot maintain aconstant arc length. +ith a constant current power source as the arc length changes due to the

welder6s manipulation o! the welding torch there is onl" a small change in the welding current ,

the steeper the cure the smaller the change in current so there will be no current surges and astable welding condition is achieed. Since it is primaril" the welding current that determines

such !eatures as the penetration and electrode consumption this means that the arc length is less

critical' ma&ing the welder6s tas& easier in achieing sound de!ect !ree welds. T"picall"' a 78oltchange would result in around a 79 amp change at 18:amp welding current.

$n some situations , !or e2ample when welding in the oerhead position or when the welder is!aced with ariable root gaps ; it is an adantage i! the welder has rather more control oer

deposition rates b" enabling him to ar" the rate b" changing the arc length. $n such a situation a

!latter power source characteristic will be o! bene!it.

Submerged arc welding also uses a drooping characteristic power source where the weldingcurrent and the electrode !eed rate are matched to the rate at which the wire is melted and

trans!erred across the arc and into the weld pool , the


3/4

Fig. 5 4onstant oltage power source characteristic

+ith $)/ and F4/+ power sources the welding current is controlled b" the wire !eed

speed' the welding current determining the rate at which the welding wire is melted and

trans!erred across the arc and into the weld pool , the


4/4

when a short circuit occurs the current rises. If this increase in the current is fast and uncontrolled then the electrode tip lows like an electrical fuse

resulting in e!cessive spatter " too slow a rise and the electrode may stu into the weld pool and e!tinguish the arc. #his is not too significant when using

the MMA process since the ma!imum current at zero voltage is controlled y the slope of the static characteristic curve and the welder can easily

estalish an arc gap. It is$ however$ important in the MIG/MAG process where a flat static characteristic power source is used and the current could rise toan e!tremely high value$ in particular when welding in the dip transfer or short circuiting condition.

An electrical component called an inductor is therefore introduced into the power source electrical circuit. #his device opposes changes in the weldingcurrent and hence slows the rate at which the current increases during a short circuit. #he inductance is variale and can e ad%usted to give a stale

condition as shown in &ig. '. Inductance in the welding circuit also results in fewer short circuits per second and a longer arc-on time - this gives a

smoother etter shaped weld ead. #oo much inductance$ however$ may result in such a slow rise in the welding current that there is insufficient time for

the arc to re-estalish and melt the wire tip so that the welding wire then stus into the weld pool. Inductance during spray transfer is also helpful inproviding a etter and less violent arc start.

Documents

Power Source Characteristics