Power supply for welding processes

1. POWER SUPPLY
Unit -3
Foundry and welding technology-PR5401
-2021507031

2. What power supply do I need for welding?
The primary functions of the power source are to produce
sufficient heat to melt the joint and to generate a stable arc and metal
transfer. As the welding processes require high current (50-300A) at
relatively low voltage (10-50V),the high voltage mains supply (230 or
400V) must be reduced by a transformer.

3. Welding power supply
A welding power supply is a device that provides or modulates an electric current to
perform arc welding. There are multiple arc welding processes in common use ranging from
relatively simple Shielded Metal Arc Welding (SMAW) to more complicated welding processes using
inert shielding gas like Gas metal arc welding (GMAW) or Gas tungsten arc welding (GTAW). Welding
power supplies primarily serve as devices that allow a welder to exercise control over whether
current is alternating current (AC) or direct current (DC), as well as the amount of current and
voltage. Power supplies for welding processes that use shielding gas also offer connections for gas
and methods to control gas flow. The operator can set these factors to within the parameters as
needed by the metal type, thickness, and technique to be used. The majority of welding power
supplies do not generate power, instead functioning as controllable transformers that allow the
operator to adjust electrical properties as needed. However, in some welding applications, notably
SMAW, used in areas isolated from power grids, welding power supplies are used that combine the
functions of electrical generation and current modulation into a single mobile unit mounted on a
vehicle or towed trailer.

4. Classification
• Welding machines are usually classified as constant current (CC) or constant voltage (CV); a constant current
machine varies its output voltage to maintain a steady current while a constant voltage machine will
fluctuate its output current to maintain a set voltage. Shielded metal arc welding and gas tungsten arc
welding will use a constant current source and gas metal arc welding and flux-cored arc welding typically use
constant voltage sources but constant current is also possible with a voltage sensing wire feeder.
• Constant current sources are used for welding operation which are performed manually like Shielded Metal
Arc Welding or Gas Tungsten Arc Welding. Being manual processes, the arc length is not constant throughout
the operation. This is attributed to the fact that it requires very high amount of skill to keep the hand at
exactly same position above the workpiece throughout the welding. Using a constant current source makes
sure that even if the arc length changes, which causes a change in arc voltage, the welding current is not
changed by much and the heat input into the weld zone remains more or less constant throughout
operation.
• The nature of the CV machine is required by gas metal arc welding and flux-cored arc welding because the
welder is not able to control the arc length manually. If a welder were to attempt to use a CV machine for a
shielded metal arc welding (SMAW) task, the small fluctuations in the arc distance would cause significant
fluctuations in the machine's current output. With a CC machine the welder can count on a fixed number of
amps reaching the material, regardless of how short or long the electric arc gets.

5. Power supply designs
The welding power supplies most commonly seen can be categorized within the following types:
Transformer
• A transformer-style welding power supply converts the moderate voltage and moderate current
electricity from the utility mains (typically 230 or 115 VAC) into a high current and low voltage
supply, typically between 17 and 45 (open-circuit) volts and 55 to 590 amperes. A rectifier converts
the AC into DC on more expensive machines.
• This design typically allows the welder to select the output current by variously moving a primary
winding closer or farther from a secondary winding, moving a magnetic shunt in and out of the core
of the transformer, using a series saturating reactor with a variable saturating technique in series
with the secondary current output, or by simply permitting the welder to select the output voltage
from a set of taps on the transformer's secondary winding. These transformer style machines are
typically the least expensive.
• The trade off for the reduced expense is that pure transformer designs are often bulky and massive
because they operate at the utility mains frequency of 50 or 60 Hz. Such low frequency
transformers must have a high magnetizing inductance to avoid wasteful shunt currents. The
transformer may also have significant leakage inductance for short circuit protection in the event of
a welding rod becoming stuck to the workpiece. The leakage inductance may be variable so the
operator can set the output current.

6. Generator and alternator
Welding power supplies may also use generators or alternators to convert mechanical energy
into electrical energy. Modern designs are usually driven by an internal combustion engine but older
machines may use an electric motor to drive an alternator or generator. In this configuration the utility
power is converted first into mechanical energy then back into electrical energy to achieve the step-
down effect similar to a transformer. Because the output of the generator can be direct current, or even a
higher frequency AC, these older machines can produce DC from AC without any need for rectifiers of
any type, or can also be used for implementing formerly-used variations on so-called heliarc (most often
now called TIG) welders, where the need for a higher frequency add-on module box is avoided by the
alternator simply producing higher frequency ac current directly.

7. Inverter
Since the advent of high-power semiconductors such as the insulated gate bipolar transistor
(IGBT), it is now possible to build a switched-mode power supply capable of coping with the high loads
of arc welding. These designs are known as inverter welding units. They generally first rectify the utility
AC power to DC; then they switch (invert) the DC power into a stepdown transformer to produce the
desired welding voltage or current. The switching frequency is typically 10 kHz or higher. Although the
high switching frequency requires sophisticated components and circuits, it drastically reduces the bulk
of the step down transformer, as the mass of magnetic components (transformers and inductors) that is
required for achieving a given power level goes down rapidly as the operating (switching) frequency is
increased. The inverter circuitry can also provide features such as power control and overload
protection. The high frequency inverter-based welding machines are typically more efficient and provide
better control of variable functional parameters than non-inverter welding machines.
The IGBTs in an inverter based machine are controlled by a microcontroller, so the electrical
characteristics of the welding power can be changed by software in real time, even on a cycle by cycle
basis, rather than making changes slowly over hundreds if not thousands of cycles. Typically, the
controller software will implement features such as pulsing the welding current, providing variable ratios
and current densities through a welding cycle, enabling swept or stepped variable frequencies, and
providing timing as needed for implementing automatic spot-welding; all of these features would be
prohibitively expensive to design into a transformer-based machine, but require only program memory
space in a software-controlled inverter machine. Similarly, it is possible to add new features to a
software-controlled inverter machine if needed, through a software update, rather than through having to
buy a more modern welder.

8. Other types
Additional types of welders also exist, besides the types using
transformers, motor/generator, and inverters. For example, laser welders also
exist, and they require an entirely different type of welding power supply design
that does not fall into any of the types of welding power supplies discussed
previously. Likewise, spot welders require a different type of welding power
supply, typically containing elaborate timing circuits and large capacitor banks
that are not commonly found with any other types of welding power supplies.

9. Basic power source designs

10. Five types of power source exist: AC transformer; DC rectifier; AC/DC transformer rectifier, DC
generator and inverter.
The type of control, e.g. primary tapped, saturable reactor, thyristor and inverter is an important factor
in the choice of power source. A simple primary tapped machine may be the ideal and robust choice
for many MIG (GMA) welding jobs but it has its limitations. If there are insufficient steps, it may be
impossible to tune the optimum condition and supply fluctuations will affect the output. Thyristor
control allows continuously variable adjustment of the output, is independent of supply voltage
variations and can be controlled remotely. Thyristor power sources may be used for most welding
processes, i.e. can have either a flat (MIG [GMA]) or drooping (MMA [SMA] and TIG [GTA]) output
characteristic.
Inverter power sources offer all the advantages of thyristor control, but with additional performance,
weight savings and efficiency. Transistors are used to convert mains AC (50Hz) to high frequency AC
(>500Hz) before transforming down to a suitable voltage for welding and then rectifying to DC. Thus,
the inverter is essentially a power block which may be controlled, often by software, to give the static
and dynamic characteristics required for the selected welding process. Hence, most inverters offer
multi process capability. Also, the response of modern inverters opens up the possibilities of high
frequency pulsing as required for pulsed MIG (GMA) and dynamic feedback to control metal transfer
as in dip transfer MIG.

12. The primary function of a welding power source is to convert electrical power into a
current type that is suitable for the welding application being performed. There’s a
lot to consider when selecting the best power source for your welding systems.
Whether you’re looking to invest in new manual welding machines or ready to
upgrade an outdated welder to a more modern piece of automatic welding
equipment, we’re here to simplify the process. Read on to learn more about welding
power sources and what key factors to consider before making a purchase.
Types of welding processes and power supplies:

13. There are a variety of power sources to choose from. To make the best
choice, you need to pair your material type with the welding process and the best
method of supplying power to the arc.
Choose your Welding Process
Both the welding process and material type play a big role when selecting a
power source because they are not always compatible with each other.
Gas Metal Arc Welding (GMAW) or Flux-Core Arc Welding (FCAW)
Most commonly referred to as MIG or Flux-Core Welding; this process can be
used on all of the major commercial metals, a wide range of thicknesses, and requires
less operator skill than TIG or stick welding. Welding speeds are higher because of
the continuously fed electrode, absence of slag, and higher metal deposition
rates. Whenever possible, GMAW and FCAW are the chosen welding processes
utilized in production shops.

14. Shielded Metal Arc Welding or Stick Welding (SMAW)
This is the most common form or ARC Welding. A stick or electrode is
placed at the end of a holder and an arc is struck between the tip of the
electrode and the metal welding surface. SMAW power supplies are generally
the least expensive but are only capable of being used in manual operations.
Gas Tungsten Arc Welding (GTAW)TIG welding process
In GTAW—or TIG welding—an arc is established between a non-
consumable tungsten electrode and the base metal. A shielding gas protects
the tungsten and molten metal from oxidation. GTAW produces high-quality
welds on almost all metals and alloys. It can be controlled down to very low
amperages making it ideal for thinner materials. GTAW can be done with or
without filler material, it also has very little spatter and no slag. Its biggest
disadvantage is speed—GTAW is by far the slowest welding process.

15. Plasma Arc Welding (PAW)welding power sources
Plasma Arc Welding is essentially an extension of GTAW. Both GTAW and PAW
use constant-current power sources and a high-frequency source for arc starting. The
primary difference is that the electrode is recessed in a nozzle to constrict the arc. PAW is
generally more expensive than GTAW but it is more tolerant of joint misalignment and
can give better penetration.

16. Select the Proper Power Supply
Welding power source types are defined by how they modulate electrical currents and
what arc welding process is best supported by this modulation:
Direct Current (DC)
A DC power source is a flow of electrons in a single direction through a circuit. In
welding, it creates a steadier arc and smoother output. It can be used to weld with a negative
ground, or the flow of electrons can be reversed to flow toward a positive ground in reverse
polarity.
Alternating Current (AC)
The AC power source is the bidirectional flow of electrons in which the polarity shifts a
hundred or more times per second from a negative to a positive ground. Arcs tend to be less
stable and welding is harder to control. However, AC welding can break apart oxide formation
and allow for purer welding in some processes.

17. Pulsed Current
This is a form of DC welding in which the current goes from a high peak current to a lower
background current at a frequency determined by the operator. This narrows the arc, allowing greater
penetration while reducing the effect on surrounding materials. As a result, pulsed current welding is
an excellent choice for welding thin metal or performing deep welds on thicker materials.
Pulsed Voltage and Heat
Pulsing GMAW power supplies focus on controlling pulsed voltage and heat applied to the
consumable electrode. Controlling the pulsed voltage (heat) and wire feed speed allows greater
control over how the wire melts and the rate of deposition. Adaptive pulse GMAW carefully monitors
feedback and automatically compensates to keep the arc consistent despite variation by the welder
and differences in height and joint location.
Additional Items to Consider
Once you have your welding process and the type of power source selected you should
consider a few more key items to determine the size including:
What is your Input Power?
Your power source needs to match the type of input power available. The amount of
electricity your welding system needs will ultimately depend on the type of power supply you select.
•Single-phase: 115, 200 or 230 VAC
•Three-phase: 230, 460 or 575 VAC

18. Material Thickness
Simply put, the thicker the material the more power
required.
Duty Cycle
Duty cycle is the percentage of arc on-time a welding
power source can operate in a given period. One of the most
common mistakes welders make is under-sizing their power
source. It’s important to understand how much amperage
your power source can generate at any given duty cycle and
ensure it’s MORE than enough to meet your demands.
Understanding the types of welding processes and power
supply types is a large undertaking and can be overwhelming
but a reliable power source will serve you for many years.

19. Advantages:
1.Very low initial investment
2.Simple to use and service.
Disadvantages:
1.Very high no load current.
2. There is no control of current. Current is fixed, will also depends on the
electrode and input voltage.
3.Very inefficient.
4.Very low power factor.
5.Due to 1 and 2 draws very large current from the electricity establishment.
(see the table).
6.Due to 3 running cost is high.
7.Poor quality of weld.
8.Brute force of current.
9.Welding at low currents is not at all possible.
10.Bulky equipment, thus occupies large floor space.
11.Poor portability.
12.TIG / Argon welding not possible.
13.Welding of non- ferrous metals not possible.
14.Lower deposition rate and deposition efficiency.
.
Fixed current welding transformer

20. Advantages:
1.Very low initial investment
2.Simple to use and service
Disadvantages:
1.Very high no load current.
2.Very inefficient.
3.Very low power factor.
4.Due to 1 and 2 draws very large current from the electricity establishment. (see
the table).
5.Due to 3 running cost is high.
6.Poor quality of weld.
7.Better control of current compared to the previous type but not satisfactory.
8.Bulky equipment, thus occupies larger floor space.
9.TIG / Argon welding not possible.
10.Welding at low currents is not possible.
11.Poor deposition rate and efficiency
Variable current welding transformer (magnetic shunt
type).

21. Advantages:
1.Moderate initial investment
2.Simple to use.
3.Moderate skill required to service the equipment.
Disadvantages:
1.High no load current.
2.Efficiency is better than the earlier cases but not high.
3.Low power factor.
4.Due to 1 and 2 draws large current from the electricity establishment.
5.Due to 3 running cost is high.
6.Low speed of control.
7.Better quality of weld compared to the previous types.
8.Better control of current compared to previous types.
9.Bulky equipment, hence occupies large floor space.
10.Poor portability.
11.Averaage deposition rate and efficiency.
Thyristorised welding rectifier.