CHAPTER ONE
HISTORY/INTRODUCTION
1.0
The machine is specially designed for industrial and
agricultural welding. It is only powered from A.C power source for all welding.
The unit is
designed to operate on single phase 240v A.C. supply and it is mainly a
transformer unit. It is capable of operating from 25 Amps socket for most of
its output range. The current drawn form the mains supply may exceed the 25
Amps, hence the unit must be provided with 30A switch fuse input as those
provided for electric cookers, heaters etc. this connection must be made to the
dead side of a 30A circuit. The circuit should not be shared by any other
appliance.
The unit consists of the following
main components single phase transformer (with tap changing secondary). The
transformer is a step-down transformer with different tapping at H, M and L;
welding tung cables.
The transformer is a
conventional-cooled, single phase transformer which has shell type of
lamination core and insulated windings of copper cols. In addition, the
windings are given double insulation with the use of varnish solution which
helps to prevent short-circuiting in the windings.
With the
help of Angle iron and lamination core, the core loss, iron loss in the machine
is reduced to minimum. They also help to reduce humming in the machine. Hence,
efficiency of the machine is greatly improved.
1.1
AIMS AND OBJECTIVES
Sequel to
the vast need and use of iron rods, metal bars and pipes, right from the
domestic level up to industrial extent, the arc welding machine was built in
order to ensure that the ulterior motive of the manufacturers of these products
(iron rods, metal bars and pipes) by their manufacturers is being achieved as
part f technological advancement. With the electric Arc welding machine, the
difficulty of using iron rods, bars and pipes in construction works has been
removed.
Moreso, the Arc welding machine has
also contributed a lot towards the economy of life in such a way that when
products like machine tools, agricultural tools, machine parts, motor parts etc
get broken, they could be restored to usage, or made functional once again
through welding, by the use of the electric Arc welding machine. In that
respect, the problem has been solved without going back to purchase a new one.
1.2
SCOPE
The machine
is specially designed for industrial and agricultural welding. It is powered
from single phase, 240v 50Hz power supply. It is operated from 30A socket.
During use;
when tap changing, the entire machine should be off before tap changing the
load terminal when welding.
When there is low welding current and
due to variation of power supply, a higher tap should be
used to increase the welding current.
This electric Arc welding machine is
a heavy duty welding machine which is
capable of welding all or 12 gauges of electrodes, while in operation.
For material cutting, the machine
should be tapped at the highest tapping, although cutting is not very effective
at low voltage supply.
1.3
PRINCIPLE OF TRANSFORMER OPERATION
An
alternating voltage applied to P circulates an alternating current through P
and this current produces an alternating flux in the iron core, the mean path
of this flux being represented by the dotted line D. if the whole of the flux
produced by P pass through S, the emf induced in each turn is the same for P
and S. hence if N1 and N2 be the number of turns on P and
S respectively (refer to fig. 2.0).
Total emf
induced in S = N2
x emf per turn = N2
Total emf
induced in P = N1
x emf per turn = N1
When the
secondary S is open circuit, its terminal voltage is the same as the induced
emf. The primary current is then very small, so that the applied voltage V1Â
is practically equal and opposite to emf induced in P, hence
V2 =
N2
V1 N1
Since the
full load efficiency of a transformer is nearly 100 percent, I1V1
x
primary power factor = I2V2 x
secondary power factor. But, the primary and secondary power factor at
full load are nearly equal
I1 =
V2
I2 V1
When a load
is connected across secondary terminals (short circuited), the secondary
current - by Lenz’s law produces a
demagnetizing effect. Consequently, the flux and the emf induced in the primary
are reduce slightly. But this small change may increase the difference between
the applied voltage and the e.m.f induced in the primary from say 0.05% to say
1%, in which case the new primary current would be 20 times the no-load
current. The demagnetizing ampere-turns of the secondary is thus nearly
neutralized by the increase in the primary ampere-turns and since the primary
ampere-turns on no load are very small compared with the full load
ampere-turns.
-
full load primary ampere-turns = full
load secondary ampere-turns.
i.e
I1N2 = I2N2
so that I1 = N2 = V2
I2 N1 V1
It
will be seen that the magnetic flux forms the connecting link between the
primary and secondary currents and that any variation of the secondary current
is compared by a small variation of the flux and therefore of the e.m.f induced
in the primary, thereby enabling the primary current to vary approximately
proportional to the secondary current. This balance of primary and secondary
ampere-turns is an important relationship whenever transformer action
occurs.
1.4
EQUIVALENT CIRCUIT OF THE MACHINE
Fig. 1.4
Fig. 1.4
RP and RS are resistances equal to
resistance of the primary and secondary windings of the machine.
Similarly, inductive reactance XP
and XS represent the
reactance of the windings due to leakage flux in the machine. The inductance
reactor X is such that it takes a reactive current equal to the magnetizing
current (IM) of the transformer. The core losses due to hysteresis
and eddy currents are allowed for by a resistor R of such value that it takes a
current IC equal to the core-loss component at the primary current,
i.e IC2R is equal to the core loss of the actual
transformer. The resultant of (IM) and IC is IO.
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