Unit 212 AC/DC power, CC & CV, arc blow

Principles of Fabrication and

Welding

Learning outcomes/Canlyniadau Dysgu

1. Sketch a diagram of an MMA welding transformer and

regulator

2. Describe AC & DC currents used in welding
3. Explain the terms Constant Current and Constant Voltage

Knowledge check

What do the following abbreviations mean?

MMA …………………………………………
TIG ……………………………………………
MIG ……………………………………………

Name the two types of gases used in Oxy-gas welding / cutting;

Name another gas combination that could be used for oxy-gas cutting

Oxygen and Acetylene

Oxygen and Propane

Manual Metal Arc
Tungsten Inert Gas

Metal Inert Gas

Open Ended

What are the two gases that can be used for gas welding and gas cutting?

Label the Diagram of an AC MMA Welding

Transformer and Regulator

Multiple Choice

A transformer is an iron core with what type of metal wire wrapped around it?

• Aluminium

• Iron

• Steel

• Copper

Alternating current
means the flow of
current changes
direction – between
positive and
negative

Three phase AC supply

Moving coil or tapped

Primary input to secondary output

Multiple Choice

A moving coil or tapped reactor are methods of controlling what, during welding?

• Voltage

• Wire speed

• Current

• Shielding gas

Rectification – one way valve.

AC current used to produce DC output.

Transformer AC voltage rectified
to DC output power for welding.

Filter smoothes out rectified DC
for stabilized welding arc.

Solid State (no valves)
rectification by PCB

Inverter control 1 – Power rectified to
DC, filtered back for smoothness
then converted to HF AC

Inverter control 2 – HF AC stepped
down and rectified to DC.

Advantage; smaller parts, multi
process capability, good arc
characteristics.

Knowledge Check

Welding transformers convert high input voltage

low amperage to low voltage high amperage?
True or False

Filtered DC power is much smoother than

standard rectified DC power? True or False

Inverter power sources give the smoothest arc

characteristics? True or False

Basic Transformer

Transformer rectifier

Inverter control

Multiple Choice

A step down welding transformer converts high voltage, low amperage to low voltage high amperage, True or False?

• False
• True

Drooping (constant current)

MMA & TIG

Flat (constant voltage) MIG

Change in arc length gives small
change arc voltage, but large change
in current

Change arc length gives large
change arc voltage but small
change in welding current.

Arc blow or arc wander

Learning outcomes

1.Explain what is welding “arc blow”

2.Explain two causes of “arc blow”

3.Suggest one method of preventing “arc blow”

ARC BLOW

No magnetic attraction / With magnetic attraction

Arc blow or arc wander

• What is ‘arc blow’?

Arc blow occurs in DC welding when the arc stream does not follow the shortest path from the electrode to the workpiece.

It is deflected to one side or the other, forwards or backwards while welding is taking place.

Arc blow or arc wander

• Causes of arc blow include:-

1. Welding into a corner
2. Using a DC current
3. The return lead/clamp is placed too close to

the weld.

4. High welding current
5. The welding cable is coiled up, causing an

increase in the magnetic field.

Arc blow or arc wander

• Preventing arc blow:-
1. Move the return lead/clamp further away

from the weld.

2. Unravel the cable to reduce the effect of the

magnetic field.

3. Hold the shortest arc possible.
4. Wrap the return cable around the table,

bench or workpiece.

5. Reduce the current.

Multiple Select

Arc blow in welding is due to which of the following? Tick 3

• Welding into a corner

• Welding vertically down

• Welding using a DC current

• Coiled up cables creating an increased magnetic field

• Welding using an AC current