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Section II. INSULATOR REPAIR
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TM-5-6120-250-34 Substation Trailer Mounted 500 KVA AC 4160-416Y/240V 208Y/120V 3 Phase 50/60 Hertz Avionics Model 950-2200A Manual
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Tap Changer Drive Mechanism Installation
and  wire brush the area where the fusion weld is to be
space inside the tank with dry nitrogen or
made.
carbon dioxide gas and maintain a small
c.  Loosen locknut 1(i, fig. 4-2) and remove it and
flow of gas during operation. If the bushing
contact nut (6).  Remove conductor (5 I and second
is to be reused, exercise care in the removal
contact nut (6).  Grind off the edge welds at both the
operation to avoid damaging the glass or the
mounting and cap flanges (2 and 41 using a rotary hand
remaining portion of the adapter ring and
grinder or similar tool. Keep the depth of grinding to a
stud spinning.
minimum and as even as possible. If the bushing is to be
b. Grind off the edge welds at both the mounting
reused, exercise care in the removal operation to avoid
and cap flanges (2, and 4, fig. 4-2) using a rotary hand
damaging the glass body (31 or the remaining portion of
grinder or similar tool. Keep the depth of grinding to a
the adapter ring (11 and the stud spinning. With these
minimum and as even as possible. With these two joints
two joints broken the bushing shell can then be with-
broken the bushing shell can then be withdrawn from the
drawn from the transformer, leaving its stud (81 in place
transformer, leaving its stud (8) in place, connected to the
connected to the internal winding lead.
internal winding lead.
d. Slip the bushing shell (31 over the stud (8) and
c.  An alternate method of insulator removal when
insert it through the mounting hole until the stud spacer is
the bushing shell is being replaced by a new one is to cut
seated at the internal end of the bushing and the stud
away the mounting and cap flanges with a chisel. After
spinning is even with the cap flange.
removal of the shell, prize out the remainder of the old
e. Tack weld the assembly in four places and then
bushing flanges and smooth up the adapter ring and stud
use a screwdriver or similar tool to carefully expand the
spinning for rewelding.
stud spinning to bring it in contact with the entire
4-8. Replacing Insulator Bushing
perimeter of the cap flange.
a. The edge welds at the mounting and cap flanges
f.  It is recommended that the fusion weld be
are specifically designed for Tungsten Inert- Gas ('I'IG)
completed in several segments with a cooling period
welding.  To successfully reweld these joints using this
allowed between each section. Welding should be done
fusion method, it is important to keep the radial gap
at 8 to 12 amps dc using straight polarity (electrode
between mating parts as small as possible.
The
negative} and argon gas flow of about 200 cu. ft. / hr.
clearance should not be allowed to exceed 0.005 inches
Similar techniques should be used in  making  the
at any point.  If a new bushing is being installed, its
mounting-flange-to-adapter-ring weld except for a slightly
flanges may have to be ground down slightly to match the
higher current of 32 to 40 amps.
ground-off edges of the adapter ring and stud spinning.
g. When the welding has been completed, pressure
b. Clean the joints of all oil, dirt, grease, paint, etc.
test the joints as outlined under "Leak Test" paragraph 4-
1.
h. Refill transformer per paragraph 4-4.
Section III. TAP CHANGER
applied to the spring and the wedge is forced into position
4-9. General
with a wiping action, insuring positive contact.
The tap changer is the wedge-type with tap leads from the
transformer windings connected to a circular group of
nickel-plated copper rods which are held together
4-10. Tap Changer Drive Mechanism Removal
between two insulating heads. A wedge in the middle can
a. Changer must be on position "ONE" before
be moved by a crankshaft to wedge between any two
removal in order to facilitate reassembly.
adjacent rods. A spring between the wedge and
CAUTION
crankshaft maintains a high-pressure line contact between
Tap changer drive mechanisms are not
current carrying components. When the crankshaft is
interchangeable.  When two or more are
turned to move the wedge from one operating position to
removed at the same time each one must be
another, pressure is gradually reduced on the spring and
identified with the transformer from which it
the wedge is withdrawn from between rods. As shaped
was removed for purposes of reassembly.
guide on the opposite side then pivots the wedge around
b. Unbolt the flange and lift out as an assembly.
to the next set of rods. As the crankshaft continues to
turn, pressure is again
4-4

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