Lincoln Electric SAE-300 HE, Service Manual, Page: 15 / 84

LINCOLN ELECTRIC SAE-300® HE Service Manual
10
WELDING GENERATOR OPERATION (Continued)
Fine Current And O.C.V. Control:
The fine current and O.C.V. control handle manipulates the
64 Ohm rheostat in the weld generator excitation circuit.
This rheostat controls the current passing through the
shunt field coils in the weld generator. Changing the
current in these coils changes the current output of the
weld generator and has a significant affect on the open
circuit voltage (O.C.V.)
Interpole coils:
Armature current from the positive brushes are routed
through four interpole coils, before being connected to the
weld output terminal. These coils are narrower than the
shunt and series coils and are located in the generator
stator between them. Their purpose is to reduce distortion
of the magnetic field. The magnetic field generated by the
shunt coils will become distorted when current is drawn
from the armature. This distortion will increase as the
current flow increases. The interpole coils are connected
and arranged to counteract this magnetic distortion. If not
corrected, the distortion would cause reduced output and
excessive sparking on the commutator.
Current Control:
The Coarse Current Control handle turns a rotor inside the
reactor assembly. This reactor assembly functions together
with the generator’s series coils to regulate the output
current and produce the drooping volt/amp curve that is so
important to a constant current welding source. This
current control has almost no effect on the OCV.
Series coils and reactor:
Current from the negative brushes is routed through the
generator’s series coils and the reactor assembly before
being connected to the negative weld output terminal.
These series coils are wound and arranged in such a way as
to reduce or buck the current flowing from the armature.
Because the series coils do not reduce the weld output until
current is flowing, OCV is not reduced and starting the arc
is easier.
Reactor Assembly:
The reactor assembly functions like a specialized, high
current rheostat and is connected in parallel with the series
coils of the generator. At the very minimum setting the
reactor is electrically open, forcing all of the current flowing
from the armature to pass through the series coils. This
setting will produce the lowest weld current that can be set
with this control.
Moving the current control off of the minimum setting
closes the circuit in the reactor and allows some of the
current to bypass the series coils. Continuing to move the
control to the higher settings reduces the resistance of the
reactor and causes even more current to bypass the series
coils. When the current control is set to maximum, the
reactor resistance is at minimum and nearly all of the
current passes through the reactor. Because the current
passing through the reactor is not reduced by the bucking
action of the series coils, weld current is increased.