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.
- 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.
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.
MMAW/ARC
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.
Advantages:
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 |
Example:
Machine current rating is 600Amps in 60% duty cycle.
To calculate current rating for 100% duty cycle:
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.
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 |
| 100 |
| Hence current at 100% duty cycle | = | 600 x 0.78 |
| = | 468Amps |
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.