IL
44-751-1
Instruction
Leaflet
Page 7 November, 2001
Figure 5. Moving contact
Driving System
The drive system consists of several parts: 1) the worm gear drive, 2) the inter-phase shafts, 3)
the main drive shaft and 4) the external operating mechanism. See Figure 6.
The moving contact’s steel shaft is fixed to the worm gear assembly. The gear is meshed with
the worm driver, which is attached to the drive shaft through ball joints. The entire worm driver
assembly is designed to provide positive drive shaft engagement with a minimum of backlash and
maximum forgiveness of shaft misalignment. The worm driver terminates in a ball-shaped, shaft
end.
The inter-phase shafts are non-metallic Micarta tubes. These shafts slide over the ball-ends of
the worm drive shaft creating a ball and socket type of joint. The ends of each inter-phase shaft
have slip joints to make assembly easy and also to allow for some minor miss-alignment and to
accommodate movement due to thermal expansion of the transformer.
The main drive shaft connects the operating mechanism to the first tap changer deck. This shaft
is also a non-metallic Micarta tube. A telescoping connecting tube connects the main drive shaft
to the external operating mechanism
The combination of ball joint connectors, telescoping and slip joint shaft tubes allows the drive
mechanism to accommodate misalignments between tap changer decks or the operating
mechanism and permits the shaft and drive mechanism to adjust to dimensional changes that
occur as the transformer responds to temperature variations.
The shaft seal of the external operating mechanism consists of a packing gland backed up by a
secondary O-ring to form a redundant shaft seal between the operating shaft and the
atmosphere. The operating mechanism is sealed to the mounting boss by a flat gasket, which is
retained by a machined groove in the flange of the mounting boss. The operating mechanism is
bolted to the mounting boss.
The external operating mechanism drives the tap changer’s operating shaft such that one
complete revolution of the external handle indexes each tap deck one position. This one-turn-
per-tap action allows the driving mechanism system to absorb any backlash in the entire tap
changer structure and still provide precise positioning of the tap changer contacts.
To make a tap change, the operator must perform two separate actions: 1) pull the fixing pin
which frees the mechanism shaft and 2) turn the operating handle 360 degrees. The position
number is indicated by a number on a Geneva dial and is visible through the view port. A position
number is only fully visible when the tap changer is in position. The fixing pin will not re-seat itself
