exercises in welding process and equipment ---part 3

1

Takayoshi OHJI

Professor Emeritus, Osaka University

Dr. of Engineering

VIRTUAL WELD CO.,LTD

[email protected]

Exercises in Welding Process and Equipment

--- Part 3: Power-source and Equipment ---

JICA_JICA_JICA_JICA_OHJI

© OK 0912

Ex. Explain briefly the following technical terms.

①①①① Reactance

②②②② Power factor

JWES_JWES_JWES_JWES_OHJI

2

Ex. Power sourceWelding processes are given in the column A in the following table. The external

characteristics of the power sources and the wire (including rod) feeding systems

or electrode adjusting systems are given in Group B and Group C below.Select and put a mark of the appropriate word in the column B and C respectively.

Group B: Power source

(a) Drooping (or constant current) characteristics

(b) Constant potential characteristics

Group C: Wire feeding system or electrode adjusting system

(1)Manually feeding or adjusting

(2)Voltage feed-back control feeding or adjusting

(3)Constant speed feeding or adjusting

A Welding process B Power source C Wire feeding system

SMAW

TIG welding (GTAW)

MIG welding (GMAW)

MAG welding (GMAW)

SAW

© JWES 0808


Output characteristics of welding power source ①①①①

The output characteristics of welding power source is classified into two types, i.e.,

the constant current (or the drooping characteristics) and the constant voltage types.

The constant-current type of power source (CC) is generally applied to manual arc

welding processes such as shielded metal arc welding, TIG welding or plasma arc

welding, where the variation in arc length caused by the manual operation is inevitable.

The constant-voltage type of power source (CV) is generally applied to MAG or

MIG welding process where a consumable electrode-wire is used and the electrode is

fed at a constant speed to make use of the self regulation effect.

Fig.1 Constant voltage and constant current welding power sources

Output current

(a) Drooping characteristics (b) Constant voltage characteristics

Constant current

characteristics

Output current

Output

voltage

Output

voltage

Drooping

characteristics Constant voltage

characteristics


3

Ex. Power Sources for Welding _No.1

A constant voltage type of power source is applied to MAG arc welding.

The power source with a constant-speed wire feeder sustains a nearly

constant arc voltage (arc length) during the welding operation. Explain the

mechanism of the self regulation of arc length.


Ｉ０ Current

Self regulation of arc length in constant voltage type power source

Arc characteristics

Ｌ０ＭＲ

ＷＦ

ＷＦ＝ＭＲ

ＷＦ：ＷＦ：ＷＦ：ＷＦ： const. ＭＲＭＲＭＲＭＲ≒≒≒≒ｆ（Ｉ）ｆ（Ｉ）ｆ（Ｉ）ｆ（Ｉ）

Output characteristics

of power source

Ｌｌ

Ｌｌ

Voltage

Arc length：

Ｌｓ＜Ｌ０＜Ｌｌ

Ｌｓ

ＷＦ＜ＭＲＷＦ＞ＭＲ

Ｌ０

Ｌｓ


4


In AC Welding Power Sources mostly used in Japan, the drooping external

characteristics are normally provided by means of the movable iron core

installed in the welding transformer. The welding current adjustment is made by

taking in or taking out the movable iron core. Explain this principle, showing a

schematic diagram of the welding transformer and an external characteristics

diagram of the welding power source.


In AC Welding Power Sources mostly used in Japan, the drooping characteristics are

generally realized by using a movable iron core, installed in the welding transformer.

The welding current, the output is adjusted by taking in or taking out the movable core.

For example, as shown in Fig.1, a third iron core M3 is inserted between the cores M1

and M2 of a normal transformer, so that when the load current flows, the magnetic flux

partially leaks and passes through the iron core M3, which then acts as a series reactor.

Since the magnetic leakage flux, i.e. reactance, becomes larger as M3 is inserted deeper

into the main transformer, the external characteristics of the power source changes, as

shown in Fig.2, and the output current (arc current) becomes smaller.

Fig. 1 Movable iron core type Fig.2 External characteristic of power source

Primary input

Primary coil Secondary coil

Movable core

Electrode

Base metal

M3

M1M2

Output current

Leakage flux: small

Leakage flux: large

Outp

ut

voltag

e

A.C. Welding Power Source JWES_JWES_JWES_JWES_OHJI

5

There is an A.C. welding power source of rated secondary current: 300A and rated duty cycle: 40%.

Check the possibility of burning (thermal damage of welding transformer) when this power source

is used for welding with current of 200A and duty cycle of 60%.

© JWES 0808

Ex. Duty cycle in arc welding power source


Load

Time t

on on onoff off off

period

Rated duty cycle in welding power source

Generally, welding power sources are used intermittently. The components of the welding

power source are heated up during arc-on time.

Duty cycle is expressed as a percentage of the maximum time that the power source can

be operated at its rated output successively without overheating as shown in the figure.

In Japan, a ten minute period is used.

The following equation is applied to estimate the duty cycle at other than rated output,

TR×IR2 = T×I2 (1)

where, TR: rated duty cycle (%), T: duty cycle (%) under the output required,

IR: rated current, I: required output current.

Duty cycleDuty cycleDuty cycleDuty cycle JWES_JWES_JWES_JWES_OHJI

6

The power source of three-phase is generally used as the input power of a D.C. welding machine.

There is a three-phase D.C. welding machine of the rated input voltage: 200V, rated output: 300A

and the rated primary input: 21 kVA. Calculate the rated primary current of this machine.

© JWES 0808

JICA_JICA_JICA_JICA_OHJI

Ex. Primary current of arc welding power source

Most electric power systems are three-phase where they involve three voltage

sources having the same amplitude and frequency but displaced in time from

each other by 120°.

(1)

(2)

(3)

Fig. 1 Three-phase power and its expression by complex vectors

For a three-phase power source, the following expressions of power are given:

：apparent power (VA)：active or real power (W)

：reactive power (var)where Vl：line voltage (effective value), Il：line current (effective value).

[J.J.Cathey and S.A.Nasar, Basic Electrical Engineering, McGraw-Hill,1997 ]

tVv ma ωsin=

)120sin( °−= tVv mb ω

)120sin()240sin( °+=°−= tVtVv mmc ϖω

ll IV ××3

θcos3 ll IVP ××=

θsin3 ll IVQ ××=

Three-phase circuits JWES_JWES_JWES_JWES_OHJI

7


Explain the principle of the inverter controlled welding machine, and give two

items of the advantage of the inverter controlled machine compared with the

conventional SCR-type welding machine.


DC power source of SCR-type

Fig.1 Single phase DC power source with SCR

Figure 1 shows a single phase DC power source of SCR type. The silicon-controlled

rectifier (SCR), so called a thyristor, is a sort of diode with a trigger, called a gate.

The SCR is non-conducting until a proper signal is applied to the gate and the conduction

stops only if the anode voltage is less than the cathode voltage.

The output current is controlled by adjusting the gate-on timing of SCR and is smoothened

by using the reactor to reduce the fluctuation of welding current.

DDDD........C/smoothedC/smoothedC/smoothedC/smoothed

A.C.A.C.A.C.A.C.

ReactorSCR


8

Inverter controlled welding machineThe inverter controlled welding machine is based on the fact that transformer size is significantly

reduced if its operating frequency is increased.

As shown in the figure, the primary AC input is rectified first and the DC voltage is converted to

a high frequency AC voltage by using a inverter. And the high frequency AC voltage is adjusted to

suit the welding by using a high frequency transformer. Then the transformer output is rectified

again to supply the DC power for arc welding.

The welding machine of inverter type, where the frequency of operation is between 5000 and

50000Hz, has the following advantage, compared with the conventional machine of SCR type,

(1) Mechanical: compact and light.

(2) Electrical: high response and accurate control.

D.C.D.C.D.C.D.C.（（（（smoothedsmoothedsmoothedsmoothed））））

RectifierRectifierRectifierRectifier TransformerTransformerTransformerTransformer

WorkWorkWorkWork----piecepiecepiecepiece

D.C.D.C.D.C.D.C.A.C.A.C.A.C.A.C. High Frequency A.C.High Frequency A.C.High Frequency A.C.High Frequency A.C.

InverterInverterInverterInverter

TorchTorchTorchTorchMains power lineMains power lineMains power lineMains power line


© OK 0806

Inverter controlled power sourceInverter controlled power source

------ high timehigh time--response and compact size response and compact size ------


100100100100100100100100

200200200200200200200200

00000000

Current

Current

（（AA

））

10101010msmsmsms

Inverter controlled power sourceInverter controlled power sourceInverter controlled power sourceInverter controlled power sourceInverter controlled power sourceInverter controlled power sourceInverter controlled power sourceInverter controlled power source

22222222 kgkgkgkgkgkgkgkg

msms

100100100100100100100100

200200200200200200200200

00000000

Current

Current（（AA））

10101010msmsmsms

Time (10ms/div)

ower source of CRower source of CRower source of CRower source of CRower source of CRower source of CRower source of CRower source of CR--------typetypetypetypetypetypetypetype

10 10 10 10 10 10 10 10 kgkgkgkgkgkgkgkg

Time (10ms/div)

Courtesy of DAIHEN Corporation.

9

Ex. Wire feeding system in MAG weldingEx. Wire feeding system in MAG weldingEx. Wire feeding system in MAG weldingEx. Wire feeding system in MAG welding

In MAG／MIG welding, three methods of wire feeding system have been used.

Explain the methods and their features in brief.


Torch

Feeding motor

Wire reel

Feed roller Flexible conduit

(a) Push type

Torch

Feed roller

Flexible conduit

Motor

Wire reel(b) Pull type

Wire feeding system in MAG welding ①

The wire feeding systems for MAG welding machines are classified into three types, the push type,

pull type and push-pull type.

The push type in figure (a) is equipped with a feeding motor near the wire reel. This type is suitable for

steel wires which are hard to buckle and has excellent operability with a light welding torch.

The pull type in (b) is quipped with a feeding motor near the torch. This type is suitable for thin wires

and soft wires, but the conduit cable can not be very long.

© OK 0911


10

Wire feeding system in MAG welding ② JWES_JWES_JWES_JWES_OHJI

The push-pull type in (c) is a combined system of the push type and the pull one, which makes it

possible to feed a soft wire stably and smoothly.

Torch

Feed roller

Flexible conduit

Feed roller

Wire reel

Motor

Motor

(c) Push-pull type

Courtesy of DAIHEN Corporation.

Push type

Torch

(a) Push type

Pull type

Push typeTorch

Robot

Wire reel

(c) Push-pull type

Ex. Explain briefly the following technical terms.

① Duty cycle

② Voltage reducing device


11

controller

transformer for voltage reducing

welding transformer

200ＶS1

S2

current

transformer

primary secondary

controller

transformer for voltage reducing

welding transformer

200ＶS1

S2

current

transformer

primary secondary

electrode contact

starting time delay time

time

（30Ｖ～40Ｖ）

25Ｖreduced

voltage

(with in 0.06s)

25Ｖ

80Ｖ

arc voltage

no load voltage

reduced

voltage ( 1.0s )≈

output voltage electrode contact

starting time delay time

time

（30Ｖ～40Ｖ）

25Ｖreduced

voltage

(with in 0.06s)

25Ｖ

80Ｖ

arc voltage

no load voltage

reduced

voltage ( 1.0s )≈

output voltage

When an AC arc welding machine is no load, the voltage between the electrode and the base plate is high.

The voltage is dangerous from the viewpoint of the electric shock. To avoid the electric shock, “Safety and

Health Administrative Regulations in Japan” requires a voltage reducing device for the welding in narrow

spaces or in locations more than 2m from the ground. The device reduces the voltage between the electrode

and the base plate to less than 25 volts when the welding arc is off, as shown in the figures bellow.

JIS C 9311 requires : time to reduce no load voltage : 1.0±0.3 sec.

Voltage reducing deviceVoltage reducing deviceVoltage reducing deviceVoltage reducing device JWES_JWES_JWES_JWES_OHJI

© OK 0912

Up & Down

Swing

Twist

Back & Forth

Bend

Back & Forth

Up and Down

Twist

Bend

Right & Left

Arc welding robotArc welding robotArc welding robotArc welding robot

The industrial robot plays an important role in the flexible manufacturing system. As shown in the figure, the welding robot is classified into two types, articulated (jointed) and rectangular coordinates types.

(a) Articulated type (b) Rectangular coordinates type

Recently, the articulated type of robot is preferred to the rectangular coordinates type because the articulated robot is capable of flexible and quick motion, and the work envelope is wide for the installation space.


12

Ex. Arc sensor (through-arc sensor））））

In the robotic welding, the “arc sensor” technique is applied to the seam tracking in

process. Explain the principle of the arc sensor in robotic welding.


Arc sensor techniques utilize the change of the arc current or voltage during weaving of the welding

torch in the groove. If the arc is oscillated laterally across the seam, the stand off (torch-head to

work-piece distance) will change. In MAG arc welding with a constant voltage power source, the arc

current will change corresponding to the torch position as shown in the figure.

Accordingly, in the arc sensor system, the torch position can be estimated from the pattern of current

or voltage fluctuation during the oscillation. The arc sensor seam tracking system is commonly

available on welding robots and applied to the fillet and heavy section butt welding.

arc current

weaving position

Seam tracking by arc-sensor

Arc sensorArc sensorArc sensorArc sensor JWES_JWES_JWES_JWES_OHJI

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Welding wire

Welding torch Power source for detecting

circuit

Detecting circuit

Principles of wire-touching sensor(Typical sensing process for fillet welding)

Wire-touching sensor utilize the change in voltage or current when the welding wire

tip touches the base metal to detect the position of the groove or the base metal.

In the typical sensing process as shown in the figure, the touching procedure is

repeated several times before welding to detect a particular position such as the groove

and the edge of the work-piece.

Wire touching sensorWire touching sensorWire touching sensorWire touching sensorJWES_JWES_JWES_JWES_OHJI

exercises in welding process and equipment ---part 3

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