Welding and Metal Fabrication, 1st ed.

Welding and Metal Fabrication, 1st ed.
Chapter
8
Shielded Metal Arc Equipment,
Setup, and Operation
OBJECTIVES
After completing this chapter, the student should be able to:
• Describe the shielded metal arc welding process.
• List the three units used to describe an electric current and tell how they
•
•
•
•
•
•
affect SMA welding.
List the three types of welding current used in SMA welding.
Describe how open-circuit and closed-circuit voltage affect SMA welding.
Describe the force that causes arc blow and explain how it can be controlled.
Explain how each type of welding power source produces the welding
current.
Determine the duty cycle for any given welder and amperage setting.
Demonstrate how to set up a welding workstation.
KEY TERMS
amperage
anode
cathode
duty cycle
electrons
inverter
magnetic lines of flux
open circuit voltage
operating voltage
output
rectifier
step-down transformer
voltage
wattage
welding cables
welding leads
INTRODUCTION
Shielded metal arc welding (SMAW) is a welding process that uses a fluxcovered metal electrode to carry an electrical current, Figure 8-1. The current
forms an arc across the gap between the end of the electrode and the work.
148
841
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Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
Shielded Metal Arc Equipment, Setup, and Operation
FUME
CLOUD
WELD
METAL
• All positions—The flat welding position is the easiCOVERED
ELECTRODE
SLAG
ARC DROPLETS OF
MOLTEN METAL
MOLTEN
POOL
FIGURE 8-1
149
BASE METAL
est and most productive because large welds can be
made fast using SMA welding, but the process can
be used to make welds in any position.
SMAW is a very portable process because it is
easy to move the equipment from the shop to the job
site. Engine-driven generator-type SMA welders are
available that can be used almost anywhere. The limited amount of equipment required for the process
makes moving easy.
Shielded metal arc welding.
American Welding Society
WELDING CURRENT
The electric arc creates sufficient heat and temperature to melt both the end of the electrode and the base
metal being welded. Molten metal from the end of the
electrode travels across the arc to the molten pool on
the metal being welded. The metal from the electrode
and the molten base metal are mixed together to form
the weld. The high temperature at the electrode end
causes the flux covering around the electrode to burn
or vaporize into a gaseous cloud. This gaseous cloud
surrounds, purifies, and protects at the end of the
electrode and molten pool of base metal. Some of the
electrode flux forms a molten protective slag on top
of the molten weld pool. As the arc moves away, the
weld metal cools, forming one solid piece of metal.
SMAW is the most widely used welding process
for metal fabrication because of its low cost, flexibility, portability, and versatility. The welding machine
itself can be as simple as a 110-volt, step-down transformer that can be plugged into a normal electrical
outlet. The electrodes are available from a large number of manufacturers in packages ranging from 1 lb
(0.5 kg) to 50 lb (22 kg).
The SMAW process is very versatile because the
same SMA welding machine can be used to make a
wide variety of weld joint designs in a wide variety of
metal types and thicknesses, and in all positions:
• Joint designs—In addition to the standard butt, lap,
tee, and outside corner joints, at SMAW has been
certified to be used to weld every possible joint
design.
• Metal types—Although mild steel is the most common SMA-welded metal; stainless steel, aluminum,
and cast iron are easily SMA welded.
• Metal thickness—Metal as thin as 16 gauge and
approximately 1/16 in. (2 mm) thick up to several
feet thick can be SMA welded.
Welding current is the term used to describe the electricity that jumps across the arc gap between the end
of the electrode and the metal being welded. An electric current is the flow of electrons. The resistance to
the flow of electrons (electricity) produces heat. The
greater the electrical resistance, the greater the heat
and temperature that the arc will produce. Air has
a high resistance to current flow, so there is a lot of
heat and temperature produced by the SMA welding
arc. Electrons flow from negative (-) to positive (+),
Figure 8-2.
Electrical Measurement
Three units are used to describe any electrical current.
The three units are voltage (V), amperage (A), and
wattage (W).
• Voltage, or volts (V), is the measurement of electrical pressure in the same way that pounds per square
inch is a measurement of water pressure. Voltage
controls the maximum gap that the electrons can
jump to form the arc. A higher voltage can jump a
larger gap. Welding voltage is associated with the
welding temperature.
• Amperage, or amps (A), is the measurement of the
total number of electrons flowing, in the same way
that gallons are a measurement of the amount of
water flowing. Amperage controls the size of the
arc. Amperage is associated with the welding heat.
• Wattage, or watts (W), is a measurement of the
amount of electrical energy or power in the arc.
Watts are calculated by multiplying voltage (V)
times amperes (A), Figure 8-3. Watts are associated
with welding power or how much heat and temperature an arc produces, Figure 8-4.
Copyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
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150
CHAPTER 8
+
HEAT PRODUCED BY
RESISTANCE TO
ELECTRON FLOW
–
COPPER WIRE
ATOMS THAT MAKE
UP THE WIRE
ELECTRONS AS PART
OF AN ATOM
PATH OF ELECTRONS
FLOWING THROUGH
THE WIRE
ELECTRONS MOVING
FORM THE CURRENT
FIGURE 8-2
A = AMPERES
V = VOLTS
W = WATTS
W=V×A
A= W
V
W
V
FIGURE 8-3
Electrons traveling along a conductor. © Cengage Learning 2012
Ohm’s Law.
V=W
A
A
© Cengage Learning 2012
2000 WATTS PRODUCE A
MOLTEN WELD POOL THIS SIZE
4000 WATTS PRODUCE A
MOLTEN WELD POOL THIS SIZE
FIGURE 8-4 The molten weld pool size depends upon the
energy (watts), the metal mass, and thermal conductivity.
© Cengage Learning 2012
SMA Welding Arc Temperature
and Heat
The terms temperature and heat are explained in
Chapter 1.
The temperature of a welding arc is dependent
on the voltage, arc length, and atmosphere. The arc
temperature can range from around 5500°F to above
36,000°F, but most SMA welding arcs have effective
temperatures around 11,000°F. The voltage and arc
length are closely related. The shorter the arc, the
lower the arc voltage and the lower the temperature
produced, and as the arc lengthens, the resistance
increases, thus causing a rise in the arc voltage and
temperature.
Most shielded metal arc welding electrodes have
chemicals added to their coverings to stabilize the arc.
These arc stabilizers form conductive ions that make
the arc more stable and reduce the arc resistance.
This makes it easier to hold an arc. By lowering the
resistance, the arc stabilizers also lower the arc temperature. Other chemicals within the gaseous cloud
around the arc may raise or lower the resistance.
The amount of heat produced by the arc is determined by the amperage. The higher the amperage setting, the higher the heat produced by the welding arc,
and the lower the amperage setting, the lower the heat
produced. Each diameter of electrode has a recommended minimum and maximum amperage range and
Copyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
Shielded Metal Arc Equipment, Setup, and Operation
151
WELDING
MACHINE
AC
ELECTRODE
ELECTRODE
HOLDER
OFF
DC
-
ON
-
-
HEAT AND
LIGHT
WORK
FIGURE 8-5
Energy is lost from the weld in
the forms of radiation and convection.
WORK
CABLE
© Cengage Learning 2012
WELD
FIGURE 8-6
Electrode negative (DCEN), straight polarity
(DCSP). © Cengage Learning 2012
The electrons are leaving the surface of the metal being
welded and traveling across the arc to the electrode.
This results in approximately two-thirds of the welding heat on the electrode and one-third on the metal
being welded. The former term for DCEP was DCRP,
which meant direct-current reverse polarity. DCEP
current produces the best welding arc characteristics.
AC: In alternating current, the electrons change
direction every 1/120 of a second so that the electrode
and work alternate from anode to cathode, Figure 8-8.
The rapid reversal of the current flow causes the
TYPES OF CURRENTS
The three different types of currents used for welding
are direct-current electrode negative (DCEN), directcurrent electrode positive (DCEP), and alternating
current (AC).
DCEN: In direct-current electrode negative, the electrode is negative, and the work is positive, Figure 8-6.
The electrons are leaving the electrode and traveling
across the arc to the surface of the metal being welded.
This results in approximately one-third of the welding
heat on the electrode and two-thirds on the metal being
welded. The former term for DCEN was DCSP, which
meant direct-current straight polarity. DCEN welding
current produces a high electrode melting rate.
DCEP: In direct-current electrode positive, the electrode is positive, and the work is negative, Figure 8-7.
WORK
WELDING
MACHINE
AC
ELECTRODE
ELECTRODE
HOLDER
WORK
WORK
CLAMP
OFF
DC
-
ON
-
-
therefore a recommended heat range. If you were to
try to put too many amps through a small diameter
electrode, it would overheat and could even melt. If the
amperage setting is too low for an electrode diameter,
the end of the electrode may not melt evenly, if at all.
Not all of the heat produced by an arc reaches the
weld. Some of the heat is radiated away in the form of
light and heat waves, Figure 8-5. Some additional heat
is carried away with the hot gases formed by the electrode covering. Heat is also lost through conduction
in the work. In total, about 50% of all heat produced
by an arc is missing from the weld.
The 50% of the remaining heat produced by the
arc is not distributed evenly between both ends of the
arc. This distribution depends on the composition of
the electrode’s coating and type of welding current.
FLOW OF
ELECTRONS
WORK
CLAMP
FLOW OF
ELECTRONS
WORK
CABLE
WELD
WORK
FIGURE 8-7
Electrode positive (DCEP), reverse polarity
(DCRP). © Cengage Learning 2012
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152
CHAPTER 8
1
SEC
120
1
60 SEC
with the weld shape and weld penetration as the type
of current selected.
1
SEC
120
–
–
–
FIGURE 8-8
Alternating current (AC). © Cengage
Learning 2012
welding heat to be evenly distributed on both the work
and the electrode, that is, half on the work and half on
the electrode, Figure 8-9. The even heating gives the
weld bead a balance between penetration and buildup.
The amount of heat on the electrode and work
are a factor in the weld buildup and penetration.
However, the actual or effective heat input to the
metal being welded is affected by the type of electrode being used. In some cases, the higher heat on
the electrode end causes it to produce a more forceful
arc that digs into the base metal, resulting in a deeper
weld. So, the electrode type can have as much to do
WELDING POWER
The shielded metal arc welding process (SMAW)
requires a constant current arc voltage characteristic,
illustrated by the red line in Figure 8-10. Gas tungsten arc welding (GTAW) also uses this same type of
welding power, but gas metal arc welding (GMAW)
and flux cored arc welding (FCAW) both use a different type of welding power called constant voltage.
The SMA welding machines’ voltage output
decreases as current increases. This output power
supply provides a reasonably high open circuit voltage
before the arc is struck. The high open circuit voltage quickly stabilizes the arc. The arc voltage rapidly
drops to the lower closed circuit level after the arc is
struck. Following this short starting surge, the power
(watts) remains almost constant despite the changes
in arc length. With a constant voltage output, small
changes in arc length would cause the power (watts)
to make large swings. The welder would lose control
of the weld.
OPEN CIRCUIT VOLTAGE
Open circuit voltage is the voltage at the electrode
before striking an arc (with no current being drawn).
The open circuit voltage is much like the higher surge
WELDING
MACHINE
32
AC
WORK
WORK
CLAMP
DC
31
ON
ELECTRONS
FLOW IN BOTH
DIRECTIONS
30
29
28
CONSTANT VOLTAGE LINE
FOR GMAW AND FCAW
27
WORK
CABLE
WELD
CONSTANT CURRENT LINE
FOR SMAW AND GTAW
OFF
VOLTAGE
ELECTRODE
ELECTRODE
HOLDER
26
WORK
25
96
98
100 102 104 106 108 110 112 114 116
AMPERAGE
FIGURE 8-9
In an alternating current, electrons flow
back and forth. © Cengage Learning 2012
FIGURE 8-10
current (CC).
Constant voltage (CV) and constant
© Cengage Learning 2012
Copyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
Shielded Metal Arc Equipment, Setup, and Operation
153
CAUTION
The maximum safe open circuit voltage for welders
is 80 V. Higher voltages increase the chance of electrical shock.
80
(A)
OPERATING VOLTAGE
Operating, welding, or closed circuit voltage, is the
voltage at the arc during welding. Operating voltage
is much like the water pressure observed as the water
hose is being used, Figure 8-11C. The operating voltage will vary with arc length, type of electrode being
used, type of current, and polarity. The welding voltage will be between 17 and 40 V.
75
(B)
20
ARC BLOW
(C)
FIGURE 8-11
Electricity can have an initial surge much
like the surge of water when a garden hose nozzle is first
opened. © Cengage Learning 2012
of pressure you might observe when a water hose
nozzle is first opened, Figure 8-11A and B. It is easy
to see that the initial pressure from the garden hose
was higher than the pressure of the continuous flow
of water. The open circuit voltage is usually between
50 and 80 V. The higher the open circuit voltage, the
easier it is to strike an arc because of the initial higher
voltage pressure.
When electrons flow, they create lines of magnetic force
that circle around the path of flow, Figure 8-12. These
lines of magnetic force are referred to as magnetic
lines of flux. They space themselves evenly along
a current-carrying wire. If the wire is bent, the flux
lines on one side are compressed together, and those
on the other side are stretched out, Figure 8-13. The
unevenly spaced flux lines try to straighten the wire so
that the lines can be evenly spaced once again. The
force that they place on the wire is usually small, so
the wire does not move. However, when welding with
very high amperages, 600 amperes or more, the force
may actually cause the wire to move.
The welding current flowing through a plate or
any residual magnetic fields in the plate results in
unevenly spaced flux lines. These uneven flux lines
can, in turn, cause the arc between the electrode and
the work to move during welding. The term arc blow
LINES OF MAGNETIC FORCE
–
–
–
–
–
ELECTRON FLOW
FLUX LINES OR POINTS OF EQUAL STRENGTH
AT UNIFORM INTERVALS FROM THE WIRE
FIGURE 8-12
Magnetic force around a wire. © Cengage Learning 2012
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154
CHAPTER 8
SPACING DECREASES, THUS
INCREASING MAGNETIC FORCE
TIP OF ELECTRODE
WELD
DIREC
TION
SPACE
SPACING INCREASES, THUS
DECREASING MAGNETIC
FORCE
FIGURE 8-13
bends in wires.
Magnetic forces concentrate around
FIGURE 8-15
Correct current connections to control
arc blow. © Cengage Learning 2012
© Cengage Learning 2012
refers to this movement of the arc. Arc blow makes
the arc drift like a string would drift in the wind. Arc
blow can be more of a problem when the magnetic
fields are the most uneven such as when they are concentrated in corners, at the ends of plates, and when
the work lead is connected to only one side of a plate,
Figure 8-14.
The more complex a weldment becomes, the
more likely arc blow will become a problem. Complex
weldments can distort the magnetic lines of flux in
unexpected ways. If you encounter severe arc blow
during a weld, stop welding and take corrective measures to control or reduce the arc blow.
Arc blow can be controlled or reduced by connecting the work lead to the end of the weld joint and
then welding away from the work lead, Figure 8-15.
Another way of controlling arc blow is to use two
work leads, one on each side of the weld. The best way
to eliminate arc blow is to use an alternating current.
ARC
WORK LEAD CABLE
TIP OF ELECTRODE
WORK LEAD
CABLE
Because an alternating current changes directions,
the flux lines do not become strong enough to bend
the arc before the current changes direction. If it is
impossible to move the work connection or to change
to AC, a very short arc length can help control the arc
blow. A large tack weld or a change in the electrode
angle can also help control arc blow.
Arc blow may not be a problem as you are learning
to weld in the shop because most welding tables are
all steel. However, if you are using a pipe stand to hold
your welding practice plates, arc blow can become a
problem. Try reclamping your practice plates.
TYPES OF POWER
SOURCES
Two types of electrical devices can be used to produce the low-voltage, high-amperage current combination that arc welding requires. One type uses
step-down transformers. Because transformer-type
welding machines are quieter, more energy efficient,
require less maintenance, and are less expensive,
they are now the industry standard. The other type
uses an engine to drive an alternator or generator.
Engine-powered generators are widely used for portable welding.
Transformer-Type Welding
Machines
FIGURE 8-14
Arc blow. © Cengage Learning 2012
A transformer welding machine is connected to a highvoltage alternating current (AC) that is supplied to the
welding shop by the utility company. It converts it to
the low-voltage welding current. The heart of these
Copyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
Shielded Metal Arc Equipment, Setup, and Operation
PRIMARY COIL
SECONDARY COIL
CORE
MAGNETIC FIELD
FIGURE 8-16
Parts of a step-down transformer.
© Cengage Learning 2012
welders is the step-down transformer. All transformers have the following three major components:
• Primary coil—The winding that is attached to the
incoming electrical power.
• Secondary coil—The winding that has the electrical
current induced and is connected to the welding
lead and work leads.
• Core—Made of laminated sheets of steel and is
used to concentrate the magnetic field produced in
the primary winding into the secondary winding,
Figure 8-16.
PRIMARY
COIL
155
As electrons flow through a wire that is wound
into the primary coil, each wire’s weak magnetic
field is combined to produce a much stronger central magnetic field. The magnetic field is constantly
building and collapsing as the alternating current
cycles back and forth 60 times a second. As the magnetic field passes back and forth over the secondary
coil, an electric current is induced in the wires of
this winding. The iron core in the center of these
coils increases the concentration of the magnetic
field, Figure 8-17.
A welding transformer has more turns of wire
in the primary winding than in the secondary winding and is known as a step-down transformer. A
step-down transformer takes a high-voltage, lowamperage current and changes it into a low-voltage,
high-amperage current. Except for some power lost
by heat within a transformer, the power (watts) into
a transformer equals the power (watts) out because
the volts and amperes are mutually increased and
decreased.
A transformer welder is a step-down transformer.
It takes the high line voltage (110 V, 220 V, 440 V, etc.)
and low-amperage current (30 A, 50 A, 60 A, etc.) and
changes it into 17 to 45 V at 190 to 590 A.
Welding machines can be classified by the method
by which they control or adjust the welding current.
The major classifications are multiple coil, called
taps; movable coil or movable core, Figure 8-18; and
inverter type.
Multiple Coil Welders
The multiple-coil machine, or tap-type machine,
allows the selection of different current settings by
tapping into the secondary coil at a different turn
value. The greater the number of turns, the higher the
IRON
CORE
SECONDARY
COIL
120 VOLTS
× 30 AMPERES
3600 WATTS
FIGURE 8-17
INPUT
OUTPUT
30 VOLTS
× 120 AMPERES
3600 WATTS
Diagram of a step-down transformer. © Cengage Learning 2012
Copyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
156
CHAPTER 8
AC POWER
INPUT
TAP TYPE
FIGURE 8-18
MOVABLE COIL
MOVABLE CORE
Three basic types of transformer welding machines. © Cengage Learning 2012
amperage induced in the turns. These machines may
have a large number of fixed amperes, Figure 8-19,
or they may have two or more amperages that can be
adjusted further with a fine adjusting knob. The fine
adjusting knob may be marked in amperes, or it may
be marked in tenths, hundredths, or in any other unit.
Movable Coil or Core Welders
Movable coil or movable core machines are adjusted
by turning a hand wheel that moves the internal parts
closer together or farther apart. The adjustment may
also be made by moving a lever, Figure 8-20. These
machines may have a high and low range, but they
do not have a fine adjusting knob. The closer the primary and secondary coils are, the greater the induced
current; the greater the distance between the coils,
the smaller the induced current, Figure 8-21. Moving
the core in, concentrates more of the magnetic force
on the secondary coil, thus increasing the current.
Moving the core out, allows the field to disperse, and
the current is reduced, Figure 8-22.
Inverter Welders
Inverter welding machines are much smaller than
other types of machines of the same amperage range.
This smaller size makes the welder much more portable as well as increases the energy efficiency. In a
standard welding transformer, the iron core used to
ADJUSTING CRANK
AMPERAGE SCALE
FIGURE 8-20
A movable core–type welding machine.
The Lincoln Electric Company
PRIMARY COIL
HIGH-CURRENT
POSITION
LOW-CURRENT
POSITION
ADJUSTING
CRANK
MOVABLE COIL
AMPERAGE SCALE
FIGURE 8-19
Tap-type transformer welding machine.
The Lincoln Electric Company
FIGURE 8-21
Movable coil.
© Cengage Learning 2012
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Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
Shielded Metal Arc Equipment, Setup, and Operation
157
LOW-CURRENT
POSITION
HIGH-CURRENT
POSITION
FIGURE 8-22 Moving the core out allows the
field to disperse, and the current is reduced.
© Cengage Learning 2012
FIGURE 8-23
Typical 300-ampere inverter-type power
supply weighing only 70 lb. Arcon Welding, LLC
concentrate the magnetic field in the coils must be a
specific size. The size of the iron core is determined
by the length of time it takes for the magnetic field
to build and collapse. By using solid-state electronic
parts, the incoming power in an inverter welder is
changed from 60 cycles a second to several thousand
cycles a second. This higher frequency allows a transformer that may be as light as 7 lb to do the work of
a standard transformer weighing 100 lb. Additional
electronic parts remove the high frequency for the
output welding power.
The use of electronics in the inverter-type welder
allows it to produce any desired type of welding
power. Before the invention of this machine, each
type of welding required a separate machine. Now
a single welding machine can produce the specific
type of current needed for shielded metal arc welding, gas tungsten arc welding, gas metal arc welding,
and plasma arc cutting. Because the machine can be
light enough to be carried closer to work, shorter
welding cables can be used. The welder does not
have to walk as far to adjust the machine. Welding
machine power wire is cheaper than welding cables.
Some manufacturers produce machines that can be
stacked so that when you need a larger machine, all
you have to do is add another unit to your existing
welder, Figure 8-23.
GENERATOR AND
ALTERNATOR WELDERS
Generators and alternators both produce welding
electricity from a mechanical power source. Both
devices have an armature that rotates and a stator
that is stationary. As a wire moves through a magnetic force field, electrons in the wire are made to
move, producing electricity.
In an alternator, magnetic lines of force rotate
inside a coil of wire, Figure 8-24. An alternator can
produce AC only. In a generator, a coil of wire rotates
inside a magnetic force. A generator can produce AC
or DC. It is possible for alternators and generators to
both use diodes to change the AC to DC for welding.
In generators, the welding current is produced on
the armature and is picked up with brushes, Figure 8-25.
In alternators, the welding current is produced on the
stator, and only the small current for the electromagnetic force field goes across the brushes. Therefore,
the brushes in an alternator are smaller and last longer. Alternators can be smaller in size and lighter in
weight than generators and still produce the same
amount of power.
Copyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
158
CHAPTER 8
COIL
N
S
ROTOR
COIL
FIGURE 8-24
Schematic diagram of an alternator.
© Cengage Learning 2012
(A)
N
PERMANENT OR
ELECTROMAGNET
ARMATURE
BRUSH
PICKUP
S
FIGURE 8-25
Diagram of a generator. © Cengage
Learning 2012
Older engine-driven generators and alternators
may run at the welding speed all the time. Newer
engine-driven machines have a controller that reduces
their speed to an idle when welding stops. This controller saves fuel and reduces wear on the welding
machine. To strike an arc when using this type of
welder, stick the electrode to the work for a second.
When you hear the welding machine (welder) pick
up speed, remove the electrode from the work, and
strike an arc. In general, the voltage and amperage are
too low to start a weld, so shorting the electrode to
the work should not cause the electrode to stick. A
timer can be set to control the length of time that the
welder maintains speed after the arc is broken. The
time should be set long enough to change electrodes
without losing speed.
Portable welders often have 110- or 220-volt plug
outlets, which can be used to run grinders, drills,
lights, and other equipment. The power provided
(B)
FIGURE 8-26
A portable engine generator welder.
The Lincoln Electric Company
may be AC or DC. If DC is provided, only equipment
with brush-type motors or tungsten light bulbs can
be used. If the plug is not specifically labeled 110 volts
AC, check the owner’s manual before using it for
such devices as radios or other electronic equipment.
A typical portable welder is shown in Figure 8-26.
It is recommended that a routine maintenance
schedule for portable welders be set up and followed.
By checking the oil, coolant, battery, filters, fuel, and
other parts, the life of the equipment can be extended.
A checklist can be posted on the welder, Table 8-1.
Copyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
Shielded Metal Arc Equipment, Setup, and Operation
Check Each Day before Starting
Oil level
Water level
Fuel level
159
+
–
Check Each Monday
FIGURE 8-28 One rectifier in a welding power supply
results in pulsing power. © Cengage Learning 2012
Battery level
Cables
Fuel line filter
+
Check at Beginning of Month
OUTPUT DC
Air filter
Belts and hoses
Change oil and filter
–
INPUT AC
Check Each Fall
Antifreeze
Test battery
Pack wheel bearings
Change gas filter
FIGURE 8-29
Bridge rectifier.
© Cengage Learning 2012
The owner’s manual should be checked for any additional items that might be needed.
CONVERTING AC TO DC
60
50
30
10
10
–
AC
DC
FIGURE 8-27
Rectifier. © Cengage Learning 2012
11
0
+
M
A
X
100
20
+
90
30 40
20
50
70 80
60
90
110 120
100
–
70
80
An alternating welding current can be converted to direct
current by using a series of rectifiers. A rectifier allows
a current to flow in one direction only, Figure 8-27.
If one rectifier is added, the welding power appears
as shown in Figure 8-28. It would be difficult to weld
with pulsating power such as this. A series of rectifiers,
known as a bridge rectifier, can modify the alternating
current so that it appears as shown in Figure 8-29.
Rectifiers become hot as they change AC to DC.
They must be attached to a heat sink and cooled by
having air blown over them. The heat produced by a
rectifier reduces the power efficiency of the welding
machine. Figure 8-30 shows the amperage dial of a typical machine. Notice that at the same dial settings for
AC and DC, the DC is at a lower amperage. The difference in amperage (power) is due to heat lost in the
rectifiers. The loss in power makes operation with AC
more efficient and less expensive compared to DC.
A DC adapter for small AC machines is available
from manufacturers. For some types of welding, AC
does not work properly.
40
(ESAB Welding & Cutting Products)
0
13
Table 8-1 Portable Welder Checklist
M
A
X
FIGURE 8-30
Typical dial on an AC-DC transformer
rectifier welder.
© Cengage Learning 2012
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160
CHAPTER 8
DUTY CYCLE
The duty cycle is the percentage of time a welding machine can be used continuously. Most SMA
welding machines cannot be used 100% of the time
because they produce some internal heat at the same
time that they produce the welding current. SMA
welders are rarely used every minute for long periods
of time. The welder must take time to change electrodes, change positions, or change parts.
The duty cycle of a welding machine increases
as the amperage is reduced and decreases as the
amperage is raised, Figure 8-31. Most SMA welding machines weld at a 60% rate or less. Therefore,
most manufacturers list the amperage rating for a
60% duty cycle on the nameplate that is attached to
the machine. Other duty cycles are given on a graph
in the owner’s manual. A 60% duty cycle means that
out of every 10 minutes, the machine can be used for
6 minutes at the maximum rated current. When providing power at this level, it must be cooled off for
4 minutes out of every 10 minutes.
The manufacturing cost of power supplies
increases in proportion to their rated output and
duty cycle. To reduce their price, it is necessary to
reduce either their rating or their duty cycle. For this
reason, some home-hobby welding machines may
have duty cycles as low as 20% even at a low welding setting of 90 to 100 amperes. The duty cycle on
these machines should never be exceeded because
500
AMPERES
400
300
WELDER 1
400 A 60%
DUTY CYCLE
200
WELDER 2
300 A 60%
DUTY CYCLE
WELDER 3
200 A 60%
DUTY CYCLE
150
100
90
80
70
20
a buildup of the internal temperature can cause the
transformer insulation to break down, damaging the
power supply.
WELDER ACCESSORIES
A number of items must be used with a welding
machine to complete the setup. The major items are
the welding cables, the electrode holders, and the
work clamps.
Welding Cables
The terms welding cables or welding leads are
used to mean the same thing. Cables used for welding must be flexible, well-insulated, and the correct
size for the job. Most welding cables are made from
stranded copper wire. Some manufacturers sell a
newer type of cable made from aluminum wires. The
aluminum wires are lighter and less expensive than
copper. Because aluminum as a conductor is not as
good as copper for a given wire size, the aluminum
wire should be one size larger than would be required
for copper.
The insulation on welding cables will be exposed
to hot sparks, flames, grease, oils, sharp edges, impact,
and other types of wear. To withstand such wear, only
specially manufactured insulation should be used for
welding cable. Several new types of insulation are
available that will give longer service against these
adverse conditions.
As electricity flows through a cable, the resistance to the flow causes the cable to heat up and
increase the voltage drop. To minimize the loss of
power and prevent overheating, the electrode cable
and work cable must be the correct size. Table 8-2
lists the minimum size cable that is required for
each amperage and length. Large welding lead sizes
make electrode manipulation difficult. Smaller cable
can be spliced to the electrode end of a large cable to
make it more flexible. This whip-end cable must not
be over 10 ft (3 m) long.
CAUTION
30
40
50 60 70 80 90 100
% DUTY CYCLE
FIGURE 8-31 Duty cycle of a typical shielded metal
arc welding machine. © Cengage Learning 2012
A splice in a cable should not be within 10 ft (3 m) of
the electrode because of the possibility of electrical
shock.
Copyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
Shielded Metal Arc Equipment, Setup, and Operation
161
Copper Welding Lead Sizes
Length of Cable
Amperes
ft
m
50
75
100
125
150
175
200
250
300
350
400
15
23
30
38
46
53
61
76
91
107
122
100
150
200
250
300
350
400
450
500
2
2
2
2
1
1/0
1/0
2/0
3/0
3/0
4/0
2
2
1
1/0
2/0
3/0
3/0
4/0
2
1
1/0
2/0
3/0
4/0
4/0
2
1/0
2/0
3/0
4/0
1
2/0
3/0
4/0
1/0
2/0
4/0
1/0
3/0
4/0
2/0
3/0
2/0
4/0
450
500
Aluminum Welding Lead Sizes
Length of Cable
Amperes
ft
m
50
75
100
125
150
175
200
225
15
23
30
38
46
53
61
69
100
150
200
250
300
350
400
2
2
1/0
2/0
2/0
3/0
4/0
4/0
2
1/0
2/0
3/0
3/0
1/0
2/0
4/0
2/0
3/0
2/0
4/0
3/0
4/0
Table 8-2 Welding Lead Sizes
Splices and end lugs are available from suppliers.
Be sure that a good electrical connection is made
whenever splices or lugs are used. A poor electrical
connection will result in heat buildup, voltage drop,
and poor service from the cable. Splices and end lugs
must be well-insulated against possible electrical
shorting, Figure 8-32.
A properly sized electrode holder can overheat if
the jaws are dirty or too loose, or if the cable is loose.
If the holder heats up, welding power is being lost. In
addition, a hot electrode holder is uncomfortable to
work with.
Electrode Holders
The electrode holder should be of the proper amperage
rating and in good repair for safe welding. Electrode
holders are designed to be used at their maximum
amperage rating or less. Higher amperage values cause
the holder to overheat and burn up. If the holder is too
large for the amperage range being used, manipulation
is hard, and operator fatigue increases. Make sure that
the correct size holder is chosen, Figure 8-33.
CAUTION
Never dip a hot electrode holder in water to cool
it off. The problem causing the holder to overheat
should be repaired.
FIGURE 8-32
Power lug protection is provided by
insulators. ESAB Welding & Cutting Products
Copyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
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162
CHAPTER 8
FIGURE 8-35
workpiece.
A work clamp may be attached to the
© Cengage Learning 2012
200-AMP CAPACITY
FIGURE 8-33 The amperage capacity of an
electrode holder is often marked on its side.
Thermadyne Industries, Inc.
Replacement springs, jaws, insulators, handles,
screws, and other parts are available to keep the holder
in good working order, Figure 8-34. To prevent excessive damage to the holder, welding electrodes should
not be burned too short. A 2-in. (51-mm) electrode
stub is short enough to minimize electrode waste and
save the holder.
Work Clamps
The work clamp must be the correct size for the current being used, and it must clamp tightly to the material. Heat can build up in the work clamp, reducing
welding efficiency, just as was previously described for
the electrode holder. Power losses in the work clamp
are often overlooked. The clamp should be touched
occasionally to find out if it is getting hot.
In addition to power losses due to poor work
lead clamping, a loose clamp may cause arcing that
can damage a part. If the part is to be moved during
welding, a swivel-type work clamp may be needed,
Figure 8-35. It may be necessary to weld a tab to
thick parts so that the work lead can be clamped to
the tab, Figure 8-36.
FIGURE 8-36
Tack welded ground to part. © Cengage
Learning 2012
EQUIPMENT SETUP
Arc welding machines should be located near the welding site but far enough away so that they are not covered with spark showers. The machines may be stacked
to save space, but there must be room enough between
the machines for proper air to circulate to keep the
machines from overheating. The air that is circulated
through the machine should be as free as possible of
dust, oil, and metal filings. Even in a good location,
the power should be turned off periodically and the
machine blown out with compressed air, Figure 8-37.
UPPER ARM
SPRING
AIR
LOWER ARM
HANDLE
INSULATOR ASSEMBLY
FIGURE 8-34
Replaceable parts of an electrode holder.
© Cengage Learning 2012
FIGURE 8-37
Slag, chips from grinding, and dust must
be blown out occasionally so that they will not start a fire
or cause a short-out or other type of machine failure.
© Cengage Learning 2012
Copyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
Shielded Metal Arc Equipment, Setup, and Operation
The welding machine should be located away
from cleaning tanks and any other sources of corrosive fumes that could be blown through it. Water
leaks must be fixed and puddles cleaned up before a
machine is used.
Power to the machine must be fused, and a power
shutoff switch provided. The switch must be located
so that it can be reached in an emergency without
touching either the machine or the welding station.
The machine case or frame must be grounded.
The welding cables should be sufficiently long to
reach the workstation but not so long that they must
always be coiled. Cables must not be placed on the
floor in aisles or walkways. If workers must cross a
walkway, the cable must be installed overhead, or it
must be protected by a ramp, Figure 8-38. The welding machine and its main power switch should be off
while a person is installing or working on the cables.
The workstation must be free of combustible
materials. Screens should be provided to protect
other workers from the arc light.
The welding cable should never be wrapped
around arms, shoulders, waist, or any other part
of the body. If the cable was caught by any moving
equipment, such as a forklift, crane, or dolly, a welder
could be pulled off balance or more seriously injured.
If it is necessary to hold the weight off the cable so
163
FIGURE 8-38
To prevent people from tripping when
cables must be placed in walkways, lay two blocks of wood
beside the cables. © Cengage Learning 2012
that the welding can be done more easily, a free hand
can be used. The cable should be held so that if it is
pulled, it can be easily released.
CAUTION
The cable should never be tied to scaffolding or ladders. If the cable is caught by moving equipment, the
scaffolding or ladder may be upset, causing serious
personal injury.
Check the surroundings before starting to weld.
If heavy materials are being moved in the area around
you, there should be a safety watch. A safety watch can
warn a person of danger while that person is welding.
SUMMARY
It is important that you understand basic principles
of the SMA welding process. This background will
help you better understand problems that might arise
as you are welding or to more quickly understand the
operation of a new piece of equipment. For example,
failure to control arc blow can result in weld failures.
Also, understanding electricity will help you interpret
information given on manufacturers’ tables, charts,
and equipment specifications.
Before starting any new job or welding operation,
be sure to check with the equipment manufacturer’s
safety guidelines for proper operation and maintenance. Follow all recommended guidelines.
Keeping your work area clean and orderly shows
a sense of pride and craftsmanship as well as helps to
prevent accidents.
REVIEW QUESTIONS
1. What are some advantages of SMA welding?
2. Explain the term welding current.
3. What three units are used to describe any electrical
current?
4. Explain the relationship between voltage and arc
length.
5. Explain the relationship between the amperage setting and the amount of heat produced by the arc.
Copyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
164
CHAPTER 8
6. How is heat produced by an arc lost before it
reaches the weld?
7. What are the three different types of current used
for welding?
8. Compare DCEN to DCEP.
9. How do the electrons flow in AC?
10. What kind of current is required for the SMAW
process?
11. What is open circuit voltage?
12. What is closed circuit voltage?
13. What is arc blow?
14. What should you do if you encounter severe arc
blow during a weld?
15. List three ways to control or reduce arc blow.
16. What are the two types of electrical devices used
to produce a low-voltage, high-amperage current
combination?
17. What are the advantages of transformer-type
welding machines?
18. Describe the three major components of a transformer.
19. How is the current adjusted on a movable coil or
core?
20. What are the advantages of inverter welding
machines?
21. What are the differences between an alternator
and a generator?
22. How can an alternating welding current be converted to direct current?
23. Why can’t most SMA welding machines be used
continuously?
24. What does a 60% duty cycle mean?
25. Why should the electrode cable and work cable
be the correct size?
26. What can cause an electrode holder to overheat?
27. What problems can a loose work clamp cause?
28. Make a list of items to consider when properly
setting up an arc welding machine.
Copyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
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