Most welding applies
heat to join two pieces of metal. Gas welding produces molten metal through the
use of a gas flame. When the molten metal cools, the pieces are joined. Arc
welding uses an electric arc between the electrode and the work to melt the
metal.
Resistance welding
joins two pieces of sheet metal without adding anything. When an electrical
current is applied to the work pieces, for a predetermined time a nugget of
molten metal is created between the two pieces.
The electrical current is transmitted through two electrodes and two copper
alloy electrodes then create the nugget of molten metal
alloy electrodes then create the nugget of molten metal
The nugget is created
at the point where the two pieces interface because this is the point of
maximum heat generation. The heat producing the melting process is caused by
the metals’ electrical resistance to the current flow.
Resistance
welding equipment consists of :
1. A welding transformer to
convert high primary voltage and low primary amperage to low secondary voltage
and high secondary amperage needed for welding.
2. The electrodes for
passing the current through the work pieces.
3. The conductors
which connect these electrodes to the welding transformer.
4. A device for
exerting electrode force on the work.
5. A method of
regulating the electrical current.
6. A contactor to
interrupt the power to the welder transformer.
7. A timer which is capable
of controlling the action of the contactor within the limits of accuracy
required to produce the desired weld characteristics.
8. A method of cooling
the electrodes with water
schematic drawing of these components:
ALTERNATING
CURRENT
The majority of resistance welding machines operate on
single phase, with frequency 50 or 60 Hz (cyles persecond),incorporate with a
single phase welding transformer to convert the high power line voltage to a
low secondary voltage, normally in the range of 1.0 to 25.0 volts.
Depending on the
thickness and type of material to be welded, on secondary of trafo, current may be from 1,000 to as muchas 100,000
amperes or more.
DIRECT
CURRENT
Some resistance
welders are designed to operate on a 3-Phase, and will rectify the current from
alternating current (AC) to direct current (DC). A 3-Phase DC resistance welder is electrically more
efficient than a single phase AC resistance welder, because it usually produces more welding
amperageper KVA and requires less demand on the primary electrical service.
ELECTRICAL
CHARACTERISTICS
Most equipment is
rated at a certain KVA (such as 30 KVA). The KVA (kilo-volt-amp) rating is a
measure of theamount of power the equipment can handle without excessive
internal heating. The duty cycle of a transformer is defined as the percentage
of time on in each one minute period the transformer is actually carrying
current. It is expressed in this formula:
WELD TIME x #
WELDS PER MINUTE
%
DUTY CYCLE = ------------------------------------------------------ x 100
3600
In most resistance
welding applications, actual duty cycle is much less than the 50% duty cycle
used Most equipment is rated at a certain KVA (such as 30 KVA). The KVA
(kilo-volt-amp) rating is a measure of theamount of power the
equipment can handle without excessive internal heating. In most cases, the
welding transformer is the
limiting factor in the equipment and determines the KVA rating. It is standard
practice to rate a welding transformer at
a 50% duty cycle. The duty cycle of a
transformer is defined as the percentage of time onin each one minute
period the transformer is actually carrying current. formula:
WELD TIME x # WELDS PER MINUTE
%
DUTY CYCLE = ------------------------------------------------------ x 100
3600
In most resistance
welding applications, actual duty cycle is much less than the 50% duty cycle
used for rating the machine. Transformers made by different manufacturers, but
with the same KVA rating, may produce widely different amounts of welding
current. It is important to compare not only KVA rating, but welding
current when comparing resistance welding machines
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