IL 44-751-1
Instruction Leaflet
Page 6 November, 2001
Tap Deck
The tap deck is a flat insulating plate. This plate serves as the base to which all of the other tap
changer components are bolted. The main deck contains six through holes, which are used to
bolt the tap changer to the transformer framework. The flat insulating plate is machined from high
strength, electrical grade, low power factor Micarta
. This structure carries all of the mechanical
loads created by the weight of the tap leads and the thrust and weight of the drive shafts.
Worm Gear Set
This gear set is mounted on the main deck’s outboard face. Its function is to convert the rotary
action of the driving shaft 90 degrees in order to turn the moving contacts from position to
position. This gear set is contained within a cast metal housing. The function of the housing is to
position and hold the gear set and to provide a smooth, rounded corner enclosure in order to
minimize electrical stress concentration. The nature of the gearing is such that one full turn of the
drive shaft rotates the moving contacts from one set of stationary contacts to the next.
Contacts
The stationary contacts are machined from copper flat stock and are 0.25 inches (6.4 mm) thick.
The leading edges are chamfered to permit the moving contacts to easily slide up into position.
These contacts are tin-plated. The stationary contacts are bolted to copper mounting posts,
which in turn
,
are bolted to the phase deck. Two bolts are used for each contact. The bolt heads
and nuts are shielded with special, toriodially-shaped, washers to minimized electrical stress
concentration.
The copper mounting posts serve as the connection point for the tap leads. The tap leads from
the coil should be terminated with a two-hole flat connector. The flat connector is bolted to the
mounting post with two 0.375 (9.5 mm) bolt sets. The mounting posts “capture” the bolt head so
that only the nut has to be turned to tighten the joint.
The moving contact set is a “pincer” type of contact; the spring loaded contact plates slide over
the stationary contacts. See Figure 5. Each contact plate is made from copper flat stock. The
copper plates are tipped with semi-spherical, silver-copper alloy contact buttons. This alloy
contact button assures a positive, low friction, low resistance current path at all times, even when
taps are changed very infrequently. The sliding action of the contact button over the chamfered
stationary contact plate creates a good wiping action during the tap change which further
guarantees a solid, low resistance electrical contact. Contact pressure is precisely controlled by
two coil springs, which are set and calibrated at the factory to provide uniform and consistent
contact pressure. The combination of contoured interface surfaces between the moving and
stationary contacts and the accurately calibrated spring loaded pincer contacts minimizes the
force needed to drive the tap changer from one position to another. The pincer type of contact
takes advantage of the magnetic attractive forces created during through faults or current surges.
These additional compression forces yield high fault current withstand capability by preventing
contact bounce and arcing during faults. These features permit the tap changer to have a
relatively low driving torque and, yet maintain a high through fault current withstand capability.
