The low
voltage used avoids all danger to the operator, this pressure not being
sufficient to be felt even with the hands resting on the copper contacts.
Current is supplied to the welding machine at a higher voltage and lower
amperage than is actually used between the dies, the low voltage and high
amperage being produced by a transformer incorporated in the machine
itself. By means of windings of suitable size wire, the outside current may
be received at voltages ranging from 110 to 550 and converted to the low
pressure needed.
The source of current for the resistance welder must be alternating, that
is, the current must first be negative in value and then positive, passing
from one extreme to the other at rates varying from 25 to 133 times a
second. This form is known as alternating current, as opposed to direct
current, in which there is no changing of positive and negative.
The current must also be what is known as single phase, that is, a current
which rises from zero in value to the highest point as a positive current
and then recedes to zero before rising to the highest point of negative
value. Two-phase of three-phase currents would give two or three positive
impulses during this time.
As long as the current is single phase alternating, the voltage and cycles
(number of alternations per second) may be anything convenient. Various
voltages and cycles are taken care of by specifying all these points when
designing the transformer which is to handle the current.
Pages:
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143