Chapter 3: Arc welding machine and equipments

Introduction

In arc welding process, an electric arc between an electrode and a workpiece or between two electrodes is used to weld base metals. Majority of arc welding processes uses shielding gas while rest uses coating or fluxes to prevent weld joint from surrounding environment.

Figure of welding machine with different parts

1) Power source:

AC and DC both can be used for electric arc welding. Power source is required to maintain the arc between electrode and the base metal. The factors that influence on the selection of a power source are efficiency, duty cycle, floor space, initial and running costs, versatility of equipment, type of electrode to be used, availability of power source (AC or DC) and required output. Both AC and DC are used in electric arc welding according to demand of welding process being used.

I) DC Generator:

• It is run either by an electric motor or diesel engine.

• It supplied voltage in range of 15 to 50 volts and output current in range 200 to 600 ampere.

• Polarities will be fixed

II) AC Transformer:

It is static piece of equipment used either for raising or lowering the voltage of an AC supply with a corresponding decrease in the current.

• It is low cost and ease of operation

• Noiseless in nature

• It has variable polarity

The following factor influences the selection of a power source:

• Available power source (AC or DC)

• Types of electrodes and base metals to be used

• Required output

• Duty cycle

• Efficiency

• Initial costs and running costs

• Available floor space

• Versatility of equipment

2) Welding cables:

It is required for conduction of current from the power source through the electrode holder, the arc, the workpiece and finally back to the welding power source. These cables are insulated copper or aluminum cables.

3) Welding electrodes:

An electrode is a piece of wire or a rode of a metal or alloy, with or without coatings. An arc is setup between workpiece and electrode. Welding electrodes can be classified as:

1. Consumable Electrodes

   1. Bare Electrodes

   2. Coated Electrodes

2. Non-consumable Electrodes

   1. Carbon or Graphite Electrodes

   2. Tungsten Electrodes

Consumable electrode is made of different metals and their alloys. The end of this electrode starts melting when arc is struck between the electrode and workpiece resulting electrode itself as a filler metal.

Non-consumable electrodes do not melt during welding. These electrodes are made up of high melting point materials like carbon, pure tungsten or alloy tungsten.

4) Googles:

Protective googles are used to protect the eyes of the welder from the light sparks produced during welding.

5) Wire brush:

Before the welding is performed, the surface of workpiece is cleaned using wire brush.

6) Protective clothing

Welder or an operator should wear protective clothes with long sleeves like apron to secure body from direct exposure of heat.

7) Earth clamp:

The earth clamp is clamped to the welding table or the job itself so that electrical circuit is complete. The reason to do so is to minimize the chances of shock due to insulation or other failure in machine.

8) File:

It is a hand tool used to remove fine amounts of materials from workpiece.

9) Chipping hammer:

A chipping hammer is used to remove slags from workpiece after welding is done.

10) Safety Goggle:

It is for the protection of eyes and the supervision of weld bead.

11) Electrode holder:

It is used for holding the electrode manually and to conduct the current to it. It varies in size from 150 to 500 Amps.

12) Hand screen:

A face shield is also used to protect eyes from the light sparks produced during welding.

13) Hand gloves:

It is for the protection of hand while operating welding manually.

14) Welding gauges:

It allows us to determine the size of joints, joint width and height, depth and width of preparation, angle of bevel, included angle, root gap, dept of root face, convexity, smoothness of transition weld to the base metal and leg length.

15) Current regulating hand wheel:

Current regulation is achieved by movement of a lever or wheel, which varies the strength of the magnetic field between primary and secondary windings. This is the better way of achieving smooth control of current in compare to other method which uses reactor or a choke.

16) Electric oven:

Electric oven is used for storing electrodes according to the electrode supplier recommendation because temperature requirements varies for different electrodes types. For good and sound weld electrodes should be heated properly.

17) Center punch:

It is the tool to mark the pattern of welding. The depth of the punch should not exceed 0.3 mm because the arc cannot ignite on a center punch struck too deeply.

18) Welding booth:

It is the room required for welding which helps to prevent the coworkers from sparks, gases and noise. It should have ventilation with good flow of air. Walls should be painted with black, brown or yellow to prevent welding light from reflecting.

19) Welding table:

Table provides great comfort while welding in small jobs. Its legs are made from angle iron or pipe and a fixed thick plate is placed over the legs. The place on table can be constructed in such way it can be moved up and down according to our requirement. On one of is leg, a box is made to keep electrodes and a plate is made on another leg to hold earthing clamp.

20) Tong:

It is a hand tool which helps to take workpiece from one place to another place. It is made from low carbon steel.

Classification of welding machine

On the basis of AC and DC current, machine set can be categorized in:

1. Motor Generator Set

2. Engine Driven Generator Set

3. AC or Transformer Set

4. Rectifier Set

Motor Generator Set

This set is used to convert voltage, phase and frequency of power. It is a set or collection of motor and a generator mechanically coupled through the common shaft. It mainly converts electric power to any other type of power. Its arc voltage ranges from 15 to 45 volts but open circuit voltage ranges from 60 to 1000. It has the efficiency of 60%. We can even change the polarity according to our need which is helpful while welding thin and thick metals. Similarly, Motor generator set has capacity of 20 to 1000 amperes. The diagram of a motor generator set is shown below,

Motor Generator are made targeting single or multiple operator. We are provided with following tools to keep machine under our control.

1. Startor

2. Polarity Switch

3. Volt/Ampere Switch

4. Current Control

Startor

The main work of startor is to start and stop the Machine. After connecting to the power supply, we should push green button to start the machine and push red button to stop the machine.

Polarity Switch

We need to change the polarity to weld the metals considering the qualities of metal and electrode. So, to change that polarity we use this polarity switch.

Direct Current (DC) always flows from positive pole to negative pole whereas Alternating Current (AC) flow in both direction (positive to negative pole and vice versa for 50 to 60 times a second). Therefore, we can only perform polarity change in DC but not in AC.

Volt/Ampere Switch

The main function of this switch is to show the supplied electric current in volt and ampere. Some machine has two different meters to show voltage and current reading while other have single meter to show both readings. Some machines even use this switch to show the polarity.

Current Control

Its main function is to supply necessary current according to electrode used and thickness of workpiece or base metal.

Engine Driven Generator Set

Engine Driven Generator Set can perform welding with the help of diesel or petrol. Since it is not depended in electric supply, we can take it to anywhere necessary. However, it is noisy and expensive to buy and maintain. It can be expensive for working and maintenance.

Necessity of D.C. Generator

• To convert AC supply into DC

• To perform welding in places with no electric power supply

• To select the polarity and to weld non-ferrous metals easily

Parts of D.C. Generator

• Main Poles

• Body or Yoke

• Armature

• Commutator

• Carbon Brushes

• Fan

• Prime Mover

AC or Transformer Set

This machine is special type of Step-Down Transformer which increases or decreases the line voltage or ampere to required voltage or ampere as needed in arc welding.

1. It is cheaper in cost

2. This machine is used widely because of its light weight, smaller size, less noisy, with no movable parts, less power consumption and higher functionalities.

3. It has low arc blow.

4. It can perform task in both Single Phase and Three Phase whereas Single phase is used for light task while latter is used for heavy tasks.

While performing welding it is important to control and set current. There are different techniques to achieve control the current and they are as follows:

i. Movable Coil Method

ii. Variable Resistance Method

iii. Variable Choke Method

AC Transformer:

AC Transformer transforms the supplied main AC supply into required AC supply for welding. It is of two types:

1. Air Cooled

2. Oil Cooled

Air Cooled (no oil requirement):

1) It mainly converts from low voltage to high ampere and high voltage to low ampere

2) Lots of thin cores made up of thin Silicon Silicate are kept inside the Transformer. Those cores are further insulated to prevent from electric shorts. Those cores are wrapped from left and right side by coil made up of Aluminum or Brass. The coil from where electricity is passed is considered as Primary Winding (PW) and other is Secondary Winding (SW). Electrode (-) is kept in SW while Earthing clamp (+) is kept in PW. No wire should touch the area of both winding.

3) From PW current supply according to capacity of welding machine is passed. Then, current flows towards coil and eventually magnetic force is generated in iron core. After that, magnetic force changes direction and passes to SW. As a result, electro motive force (emf) is induced which is used for our welding purposes. (+) and (-) both gets heat of 50%.

4) To cool the Transformer from getting heated up, a fan run by electric supply is there.

5) It has meter to show current and handle to increase/decrease the current supply.

Oil Cooled (oil required):

1) Here all internal components are winded and placed inside iron box. Transformer inside of that box is immersed in oil because of that there is no necessity of fan to cool down the machine.

2) Oil maintains current flow and also temperature. There is a handle to increase or decrease the current according to requirement.

Care and Maintenance of AC Transformer:

1) Transformer body should have proper earthing

2) Change the oil after certain period of time

3) Always follow the instruction provided by manufacturer

4) Never run the machine for long period in full capacity

5) Switch off the machine before repairing the machine

6) Machine should be installed in safe place from sun and rain

7) Cover the machine with clothes during idle time to safe from dust

Rectifier Set

This type of set provides both AC and DC current. Normally, switch or lever are provided to start the machine. It is a type of AC Transformer but has extra Rectifier stack coated with Selenium which converts AC to DC. Rectifier set has better efficiency than motor generator set and can weld all types of metal. It can also be used as Tig machine and is noiseless as well as controllable.

Care and Maintenance of Rectifier:

1) All connections must be well tightening.

2) Apply oil or lubricant every three month in Shaft where fan is attached

3) Do not switch on/off the Switch while machine is running

4) Run machine only when fan is present

Power source

Necessity of Power Source:

1) To supply AC and DC current required for arc welding.

2) To generate High voltage (OCV) and Low voltage (AV) to perform Arc Striking and Maintaining Arc respectively while welding.

3) To convert into required current and volt while performing Arc Welding

4) To maintain proper relationship between Arc Voltage and current.

5) To work with all types of electrodes for welding

Selection of Power Source:

To select the proper Power Source, we need to follow the following points carefully:

1) Know beforehand whether you need AC or DC or both current supply for your work

2) Be careful while choosing the ampere capacity of the machine.

3) Look for machine that can maintain the constant Voltage and Current

4) If you need to perform welding away from place where machine is kept, then you need to select machine with remote controllable functionality

5) Machine should be able to use both small and big electrodes

6) Machine must be able to supply same rate of current without fluctuation despite supply voltage varies

7) If there is no electricity available, then Engine Driven Generator must be selected.

8) Maintenance cost should be cheaper of that machine

9) Be careful on initial investment on machine

Comparison of arc welding machine set

comparison

	Motor Generator set	Rectifier set	Transformer set
Initial cost	It is expensive.	It is cheaper than motor generator and costlier than transformer.	It is cheaper.
Weight	It is heavier.	It is lighter than motor generator and costlier than transformer.	It is lighter.
Floor space	It requires large space.	It does not require large space.	It too does not require large space.
Moving parts	It has movable parts.	It has no movable parts.	It too does not have movable parts.
Maintenance	It is expensive for maintenance.	It is easier for maintenance.	It is easier for maintenance.
Power Input	It needs more electric power.	It needs less electric power.	It needs less electric power.
Noise	It is noisy.	It is less noisy.	It is less noisy.
Arc blow	It has arc blow problem.	Arc blow problems appears in DC but not in AC current.	It does not have arc blow problem.
Life	It works for long span of time.	It has life span of about 7 years.	It has the longest lifespan among all.
Stability of Arc	It does not have arc stability.	It has arc stability.	It is difficult to maintain arc stability.
Metal	Both ferrous and non-ferrous metals can be welded.	Both ferrous and non-ferrous metals can be welded.	Only ferrous metals can be welded.
Efficiency	It has efficiency of 60%.	It has efficiency of 84%.	It has efficiency of 67%.

Duty cycle

Duty cycle is the ratio of arcing time to the weld cycle time multiplied by 100. Welding cycle time is either 5 minutes as per European standard or 10 minutes as per American standard. If arcing time is 5 minutes continuously then as per European and American standards duty cycle is 100% and 50 % respectively. At 100% duty cycle minimum current is drawn.

The welding current which can be drawn at a duty cycle can be calculated from given equation:

DR x IR2 = I2 x D100



Where, DR   = Required duty cycle
       IR     = Current at required duty cycle
       I    = Current at 100% duty cycle
       D100 = 100% duty cycle

Polarity

Polarity helps to understand how much electricity is passed in which pole and in which direction on arc welding. It results from the fact that electrical circuit has both negative and positive pole. DC flows in one direction resulting in constant or fixed polarity. Similarly, AC flows in both direction (half time one direction and other half in other direction) changing its polarity 120 times per second with 60-hertz current.

Polarity can be changed in two ways:

1. Straight Polarity

2. Reverse Polarity

Importance of Polarity:

The electric circuit is created when we turn on the welder has a negative and a positive pole and this property is called polarity. Polarity is important in welding because selection of right polarity affects the strength and quality of the weld. Selection of wrong polarity will lead to lots of spatter, bad penetration and lack of control of our welding

Choice of Polarity:

We choose polarity based on the quality and size of workpiece and type of electrolyte. We can choose polarity as instructed by the manufacturer.

Identification of Wrong Polarity:

1) If workpiece and electrode does not melt at same time.

2) If electrode gets heated up more than normal case.

3) If penetration is low

4) If we cannot move the electrode around easily

5) If we hear the unusual or abnormal sound

6) Difficulty in keeping arc stable

7) If welded bead has blunder and so on.

Straight polarity

If electrode holder is connected to negative pole and Earthing Clamp is connected to positive pole, then such polarity is called Straight Polarity. Here, current flows from electrode to workpiece. Similarly, 2/3 of the total arc heat is generated in workpiece while rest of the heat energy is focused in electrode.

Advantages Straight polarity:

1) We can use thin or medium coated electrode

2) While welding thick metal plates huge amount of penetration and fusion can be achieved which results in proper fusion of base metal.

3) Suitable for metals with high melting temperatures such as stainless steel, titanium

Reverse polarity

If electrode holder is connected to positive pole and earthing clamp is connected to negative pole, then such polarity is called Reverse Polarity. Here, current flows from workpiece to electrode. Also, (2/3) rd of the total arc hear is generated in electrode holder and rest of the heat energy is generated in workpiece.

Advantages of Reverse Polarity:

1) Arc cleaning action is good.

2) Suitable for welding thin plates

3) Suitable for welding metals with low melting temperature like copper, aluminum.

4) Suitable for positional welding

5) Can use electrode with thick coating

Advantages of AC arc welding machine over DC

1) AC can be supply over longer distances using step up transformers but DC cannot be transmitted by this method.

2) AC is cheaper to generate than DC.

3) AC generators (85%) have higher efficiency than DC (60%).

4) Maintenance cost will be less due to non-movable parts.

5) No chances for Arc blow in AC whereas Arc blow can occur in DC.

Advantages DC arc welding machine over AC

1) Electrodes with thin coating can be used

2) We can operate in any place with no electricity supply since it runs with petrol or diesel.

3) We can increase and decrease the flow of current while wielding since it is remote operable.

4) We can operate in safe manner due to its less open circuit voltage (around 30 volt)

5) We can perform good weld for different metals since both positive and negative pole have different temperature.

Care and maintenance of arc welding machine

If we want to keep our generator’s work performance in same pace and to increase its lifespan as well, we need to perform following statements carefully:

1) Regular checkup of water level in Radiator and oil level in engine.

2) Change the Engine Oil once it works for 250 hours.

3) Apply lubricants once a week in barring near to fan.

4) Check for oil leak in tank and pipe.

5) If motor is generator, then electric motor should be given proper earthing.

6) Do not run machine in Weak Phase.

7) Check the connection of electric supply.

8) Do not use Polarity Switch while machine is running.

9) Check out for Carbon Brushes condition regularly.

10) Clean the Generator every 3 months using air compressor.

What’s Next

chapter4