Welding Transformer

- Always deliver AC output only.
- Simple construction, low cost, less maintenance.
- Single Phase: 230V, Two Phase: 415V (Two lines of Three phase), Three Phase: 415V.

It has generally two coils namely Primary (high voltage and low current) and Secondary (low voltage and high current) coils. Both are electrically isolated. Primary and secondary windings are wound with copper or aluminum. Aluminum is used for lowering weight and economy. Since the current rating is less aluminum conductor size will be heavier than cupper. Both the windings are placed on magnetic core made up of silicon lamination.

The relation between current, voltage and no of turns are as follows.

Primary voltage (V1) = Secondary current (I2) = Primary no of turns (N1)
Secondary Voltage (V2) Primary current (I1) Secondary no of turns (N2)

Cooling systems are Oil Cooling, air cooling and forced air cooling.

Current control mechanisms are Tapped choke, Moving coil, Magnetic shunt, moving core and saturable reactor type.

Tapped choke
Tapped reactor is connected in series with secondary winding. Output current is not continuous variation, only step variation. This machine is used for general fabrication.
Moving coil type
By changing the position of primary or secondary coil the magnetic coupling will change. Lead screw is used to change the position of coils. Current is high when both the coils are near and less if far. Continuous current variations but require regular maintenance.
Magnetic shunt
Changing the magnetic coupling between primary and secondary by putting a movable magnetic shunt. Continuous current variations but require regular maintenance. Magnetic shunt causes the leakage flux to vary and thereby adjusts the output current.
Moving core
Moving a core inside the reactor. A continuous current variation is possible. The moving core changes the air gap which changes the reactance. Larger the air gap, smaller the impedance and higher the current.
Saturable reactor
By putting a saturable reactor in the secondary circuit. Eliminates moving parts but more expensive. Secondary reactor impedance is controlled by regulating the saturation level of the core electrically. DC control coil is used. If DC current is flowing in the coil the impedance is less, more output current and reverse in the case of lesser DC current.
Welding generator
It is rotary type machine driven by electric motor or internal combustion engine (Diesel or petrol). The output of the generator is DC. It has field winding in the stator and armature winding is in rotor. When armature is rotated by motor or engine small AC voltage is produced. It is rectified by means of commutator and the current is collected by carbon brushes. This voltage is connected to the output terminal and also to the magnetic field through regulator to vary the output current.
Welding rectifier
Output of the machine is DC. Earlier Dc machines are come with moving coil, moving core, saturable reactor and Thyristor or SCR type. First three types are as same as principle explained in transformers. The only difference is secondary terminals are connected with output through diode bridges. Diodes are used to change AC to DC. This is called rectification. In the output, DC choke is connected in series to filter the AC components to smooth DC current.
Thyristor type
In the Thyristor type, control PCBs are used to switch on SCR there by controlling the output current. By varying the firing angle of SCR gate the current pass through this will vary. These machines are very rugged construction. Since there is no moving part no maintenance is required. Regular removing of dust will increase the life of electronics components and reduce the failure of machine.
Invertor type
In all the above welding machines incoming voltage is reduced by using transformer and is used for welding directly or using rectifiers. In this case 80% weight of the machine is transformer so size is larger. In the invertor type machines the incoming voltage is rectified (AC to DC) and filtered. This DC voltage is again converted in to AC of high frequency (20KHZ to 100KHZ) by using MOSFETs or IGBTs. This high frequency high voltage is reduced to lower voltage by using ferrite core transformer. The output of this transformer is high frequency low voltage. This output is rectified with special type of diodes to use for welding. The size & weight of transformer is less. In the conventional machines transformer loss is high and efficiency is less. But in invertor machines losses are less and converting efficiency is high.

Energy saving is much. Power factor is high. So power factor improvement capacitors are need not be used.

All the above machines are differed in construction.

Now different types of machines are explained in terms of usage.

MMAW/ARC machine, TIG, MIG, Submerged Arc, Plasma cutting, Spot, Seem, Butt, Capacitor Discharge.

Manual Metal Arc Welding is performed with flux coated electrodes. Different types of electrodes are used for various applications.
Submerged Arc
It is an arc welding process but arc is completely submerged under a blanket of granular, fusible flux, which adequately shields the arc from atmospheric contamination. In the welding process flux is fed mechanically to the joint head of the arc by gravitational force, the wire is fed with wire feeder to the welding head, the length of arc is controlled, the traverse of arc or the work piece can also be controlled. The welding tip and the welding zone are always surrounded and protected by molten flux.
  • Absence of smoke and arc flash, hence minimum need for protective clothing.
  • High weld metal quality.
  • Smooth and uniform weld finish with no spatter
  • Extremely high deposition rate and welding speed.
  • TIG (Tungsten Inert Gas Arc) Welding
    TIG is a process in which the source of heat is an arc formed between a non-consumable tungsten electrode and the work piece. The arc and molten puddle are protected from atmospheric contamination (ie oxygen and nitrogen) with a gaseous shield of inert gas such as argon, helium or argon –helium mixture.
    DC and AC TIG machines are available. Tungsten electrode torch is available with gas cooled and water cooled. DC or AC power source is connected in series with High Frequency (HF) unit. In the case of DC TIG once the arc is struck. The HF superimposition can be cutoff, but in case of AC tig, the hf superimposition is required continuously. In AC tig filter capacitor is used to suppress DC components. But in modern AC Inverter based TIG welding this capacitor is eliminated by using electronics control.
    MIG (Metal Inert Gas ) / Co2 welding
    The arc and molten puddle are protected from contamination like oxygen and nitrogen. It is also called MAG (Metal Activate Gas) process. Shielded gas in this process is 100% pure CO2, argon-CO2 mixture. The American Welding Society refers to the process as Gas Metal-Arc Welding (GMAW).
    MIG process is basically semi automatic operation, in which the arc length and the feeding of the wire into the arc are automatically controlled. The welding operator’s job is reduced to positioning the gun at a correct angle and moving it along the seam at a controlled travel speed.
    MIG machine consists of constant voltage welding rectifier, wire feeder, shielding gas supply, controls for governing wire drive, current, gas flow, and a welding gun or torch.
    The arc can be started by simply feeding the electrode to the work. It is necessary, however to have the wire moving as soon as it comes in contact with the work, since the large current pulse at the short circuit can burn off the wire faster than it is fed, causing it to fuse with the contact tube. This is called the burn back phenomenon.
    Recommended Machine current ratings
    Current range in Amps Wire size in mm for steel Wire size in mm for Aluminium
    150 0.8 to 1.00
    250 0.8 to 1.2 1.2
    400 0.8 to 1.6 1.2 to 1.6
    600 1.8 to 2.00 1.2 to 1.6
    Cable size recommended
    Welding current in Amps Copper cable size in sq.mm Aluminum cable size in sq.mm
    100 16 27
    150 25 42
    230 35 58
    400 50 82
    600 70 112
    600 (Heavy duty) 95 153
    Electrical Calculations
    Duty Cycle of machine in % = (Rated current)2 x rated duty cycle in %
    (Desired Current)2
    Machine current rating is 600Amps in 60% duty cycle.
    To calculate current rating for 100% duty cycle:
    100 = (600)2 x 60
    (Desired Current)2
    (Desired Current)2 =3600 X 60
    Hence current at 100% duty cycle = 600 x 0.78
    = 468Amps
    Power Factor: pf = KW/KVA

    KW = Actual power drawn by a machine to produce rated load
    KVA = Apparent power or product of voltage and current

    For single phase machine KVA = Volt x Current
    For three phase machine KVA = 1.732 x Volt x Current

    The pf of welding transformer will be around 0.45,
    for Thyristor type around 0.8 and
    for invertor type pf will be 0.95 at 75% welding current.