Arc Welding Power Source (7 things to know!)

What is Arc Welding Power Source?
The electric machines which are designed and developed to produce a desired electric power required for the welding purpose are known as the welding power source of arc welding.

The Arc welding process requires the electric power of low voltage and high current to produce the arc.

There are various types of power source available for arc welding process.

You can see the classification/types of power source mentioned below.

Classification of Welding Power Sources

Based on types of design

  • Rotating type engine driven generator
  • Rotating type motor driven generator
  • Static type transformer
  • Rectifier / inverter

Based on method of current control

  • Variable coupling
  • Tab changer switch
  • Field control
  • Securable reactor
  • Electronic control with feedback (Thyristor control, transistor control, IGBT control, etc.)

Based on output current

  • Direct current (DC)
  • Alternative current (AC)
  • Combination of AC and DC current

Based on types of static characteristics

  • Constant current type
  • Constant voltage type
  • Combination of both constant current and constant voltage

Based on types of cooling

  • Nature air cooled
  • Forces air cooled
  • Natural oil cooled
  • Forced oil cooled

Based on class of insulation

  • Class A
  • Class B
  • Class E
  • Class F
  • Class H

Arc Welding Power Source

There are two power sources used in the arc welding process.

  1. AC power source
  2. DC power source

AC power source

The AC power source includes a transformer and motor or engine driven alternator.

There are no moving parts in a transformer and hence it has less maintenance cost, lower noise, plus it has higher efficiency of around 80% to 85%.

The other advantage is that it has a compact design and so it is light weight.

Also, low magnetic arc blow is obtained by using an AC rectifier.

For 1 kg of deposited metal, the energy consumption in AC power source is less as compared to that of DC power source.

The main disadvantage of AC power source is its lower power factor, possibility of unstable arc, and only coated electrode is suitable for welding using AC power source.

In order to increase the stability of arc, either frequency or voltage of the AC source is increased.

The other disadvantage of using an AC power source is that you cannot use bare electrodes (without coating) for welding.

DC power source

DC power source used in arc welding process can have either straight or reverse polarity.

When the DC power source is set on straight polarity, the electrons flow from electrode to the plate.

And when it is set on reverse polarity, the electrons flow from plate to the electrode, which creates greater concentration of heat at the tip of the electrode.

Because of this heat, the electrode tends to melt off quickly.

Hence, while using reverse polarity in DC current, the electrode of greater diameter is used.

If straight polarity is used in DC current, then it will produce a narrow and deep weld.

Characteristics of welding power source

The following are the characteristics of welding power source that you should know.

  1. Static characteristics (VI characteristics)
  2. Open circuit voltage (OCV)
  3. Static and dynamic characteristics
  4. Current rating and duty cycle
  5. Class of insulation
  6. Power factor

Let me tell you few important things about each of these characteristics one by one.

#1) Static characteristics curves (VI characteristics)

VI characteristics means voltage-current characteristics and its curve (i.e VI characteristics curve) shows the relation between load voltage and load current under various load conditions.

For obtaining a VI characteristics curve, a pure resistive load is first connected across the output terminals of the power source.

The load is changed gradually from no load condition, and the voltage across the load and current passing through the load is measured.

The curves obtained in this way are plotted on the Voltage-Current graph as shown in the graph below.

welding power source characteristics

If a machine gives a dropping curve, then such a machine is said to be a constant current (CC) type machine.

If the machine gives relatively flat curves, then it is described as a constant voltage (CV) machine.

Normally, the CV machines are designed for DC welding.

#2) Open circuit voltage (OCV)

Open circuit voltage is that voltage across the output terminals, when there are no load conditions.

OCV is also termed as no load voltage.

In constant voltage type power source, OCV is not very important. But in case of constant current type power source, the OCV plays a very significant role in ensuring good arc stability.

If the OCV value is higher, then it will give good arc stability, but a very high OCV value will give shock to the welder. Hence, its value is restricted to 100 V max.

The welding rectifiers available in the market have the OCV value between 65 V to 80 V.

If the OCV value is very low, then it is difficult to maintain the arc.

General purpose electrodes operate perfectly at the OCV value of 80 V.

#3) Static and dynamic characteristics

The static characteristics are those characteristics which describe the relation between voltage and current under various fixed load conditions. While dynamic characteristics are those characteristics which describe the relation between voltage and current under variable load conditions.

If a power source is having good dynamic characteristics, then it will give a very stable and smooth arc during the welding operation.

#4) Current rating and duty cycle

Power sources are of various current rating and duty cycles and they are rated by the manufacturer on the basis of current output at specific duty cycles.

Duty cycle is defined as a percentage of five minutes interval that it operates at a given current setting.

For example: 60% duty cycle indicates that the arc is in action for 3 minutes out of the 5 minutes time period.

#5) Class of insulation

The duty cycle of the power source is based on the maximum allowable temperature of components used in the power source.

This value of temperature depends on the type of insulation used in the manufacturing of components of the power source.

#6) Power factor

For any power source, the power factor is calculated using the formula given below;

Power factor = kW / kVa

Where, 
kVa = power drawn from the supply line,
kW = actual power used to produce the rated load.

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