Step selector for a stepped transformer

A step selector for a stepped transformer has an electrically insulating cylindrical wall provided with fixed contacts, e.g. at different levels and, for each contact level, a radial arm composed wholly of electrically insulating material is affixed to a cylindrical shaft rotatable about the axis of the cylinder and within the latter. The radial arm has a flange lying against the shaft and extending over substantially 120.degree. of an arc thereof. An inward projection on the flange engages a recess in the shaft and the flange is held against the shaft by at least one band of fibrous material impregnated with a self-hardening synthetic resin. The outer end of the arm carries a contact bridge slidable on a contact ring which nonrotatably surrounds the shaft and lies in the plane of the respective fixed contact which is engageable by the bridge.

CROSS-REFERENCE TO RELATED APPLICATION 
This application is related to the concurrently filed U.S. application Ser. 
No. 07/389,429 and U.S. application Ser. No. 07/389,422, respectively 
based upon German Patent Applications No. P 38 27 386.1 of Aug. 12, 1988 
and P 38 29 489.3 of Aug. 31, 1988. 
FIELD OF THE INVENTION 
My present invention relates to a cylindrical step selector for a stepped 
transformer and, more particularly, to a step selector of the type in 
which a rotatable shaft carries generally radial arms, each of which 
entrains a bridging piece in sliding contact with a stationary contact 
ring into engagement with a fixed contact in a wall of a cylinder. 
BACKGROUND OF THE INVENTION 
In the Austrian Patent Document No. 199,759, a step selector for a stepped 
transformer is described which comprises a plurality of fixed contacts 
respectively associated with the steps of a transformer in at least one 
plane in a wall of an electrically insulating cylinder. For each of these 
planes, a respective contact bridge is provided. The inner end of each 
contact bridge slidably engages a contact ring which is also connected in 
the electric circuit. The contact bridge is entrained by an arm extending 
generally radially from and connected to a switching shaft which is 
rotatable about the axis of the cylinder. The outer end of each contact 
bridge is engageable with the or each fixed contact lying in the 
aforementioned plane. 
In the past, it has been necessary to attach the support arm for the 
contact bridge with metallic parts so that there was always the 
possibility of voltage jumping, excessive arc extension and the like which 
could be detrimental to the breakdown voltage of the selector and could 
lead to premature deterioration thereof. 
OBJECTS OF THE INVENTION 
It is, therefore, the principal object of the present invention to provide 
an improved step selector for the purposes described which will be free 
from the last mentioned drawback. 
A more specific object of this invention is to provide a step selector of 
the type described which is free from metallic parts in the connection of 
the support arm with the selector shaft. 
Still another object of the invention is to provide a step selector with a 
minimum number of metallic parts and which, especially, is free from 
metallic parts in the connection of the support shaft for the bridge 
contact with the selector shaft. 
SUMMARY OF THE INVENTION 
These objects and others which will become more apparent hereinafter are 
attained, in accordance with the present invention, by providing a support 
arm having a flange at its inner end which conforms to and extends over 
about 120.degree. of the cylindrical outer surface of the support shaft, 
the support arm being composed of an electrically insulating material. The 
step selector shaft, which is also composed of an insulating material, can 
be formed with a recess into which a projection on said flange can extend, 
this projection being unitary with the flange. 
According to an important feature of the invention, the flange is held 
against the support arm by at least one band of thermosetting or other 
hardening resin impregnated fibrous material which can be wound in at 
least one turn around the shaft and the flange so as to attach the two 
together. 
Thus the cylindrical step selector of the invention distinguishes over the 
arrangement shown in the Austrian patent in that the support arm is formed 
entirely cut of the insulating material and is molded unitarily with the 
radial portion of the arm and with the flange which lies against the 
electrically insulating preferably tubular, cylindrical shaft. The recess 
can be a throughgoing opening formed in the wall of the support shaft and 
the resin can be a self-hardening resin, preferably an epoxy resin or some 
other thermosetting resin capable of bonding to the synthetic resin 
material of the shaft and of the flange. 
A cylindrical step selector for a stepped transformer thus can comprise: 
a cylinder having an electrically insulating wall; 
a fixed contact for switching a respective transformer step mounted in the 
wall in a respective plane perpendicular to an axis of the cylindrical 
wall, whereby each step can have a respective such contact lying in a 
respective such plane; 
a generally cylindrical rotatable switching shaft of electrically 
insulating material disposed along the axis inwardly of the cylinder wall; 
an electrically conductive contact ring lying generally in the plane and 
surrounding the shaft without rotatable entrainment by the shaft; 
a bridge support affixed to the shaft, the support being formed with: 
a generally radial arm, 
a flange at one end of the arm shaped to conform to the shaft and fitting 
around the shaft over substantially 120.degree. thereof, 
a projection formed on the flange and engaged in a recess formed in the 
shaft, and 
at least one self-hardening insulating band of a resin impregnated material 
wrapped around the shaft and the flange and securing the support to the 
bridge shaft; 
an electrically conductive contact bridge carried by the other end of the 
arm and slidably engaging the contact ring and engageable with the fixed 
contact; and 
means forming an electrical connection with the contact ring. 
In this system, preferably, the shaft is tubular. The contact bridge can 
comprise a pair of contact members straddling a portion of the arm and 
extending radially in opposite directions beyond this portion of the arm 
into slidable engagement with the contact ring and the fixed contact. 
Means can be provided for resiliently pressing these members toward one 
another, this means advantageously including pins projecting through the 
aforementioned portion of the arm and the members and surrounded by 
openings braced against heads of the pins and the members. 
Advantageously, two of the self-hardening insulating bands of 
resin-impregnated material are provided on axially opposite sides of the 
arm. 
The contact ring can surround the shaft with all-around clearance and can 
be supported within the cylindrical wall by the means forming the 
electrical connection with the contact ring. The latter means may be 
provided with bracing strips of the type described in the copending U.S. 
application Ser. No. 07/389,429. 
The step selector of the invention has the important advantage in that all 
of the metallic parts are avoided except where these parts must 
participate in the current flow path. In spite of the absence of metallic 
parts, there is a reliable attachment of the support arm for the contact 
bridge on the switching shaft because of the manner in which the flange is 
attached to the shaft. 
The invention makes it possible for three such support arms, each having a 
flange extending over 120.degree., to be mounted in the same plane 
perpendicular to the axis of the switching shaft as is the case where, for 
example, where a three-phase step selector is required with identical 
support arms and contact bridges for certain transformers. Generally, 
however, the support arm will be located at axially spaced rings of the 
shaft, noting the commonly assigned copending U.S. application Ser. No. 
07/389,429.

SPECIFIC DESCRIPTION 
As will be apparent from the drawing, the step selector comprises an 
electrically insulating cylinder 1 which is the supporting member of the 
step selector and can have the tubular switching shaft 3 journaled thereon 
by the journaling system shown in the copending U.S. application Ser. No. 
07/389,422. 
The cylinder 1 serves as a support for the fixed step contacts 2 which can 
lie in different planes perpendicular to the axis of the shaft 3 and of 
the cylinder 1 which is coaxial with the shaft 3. 
The shaft 3 can be driven in a stepwise manner about the axis to select the 
transformer steps. 
For each contact plane, a contact ring 4 is provided and can surround the 
switching shaft 3 without engagement therewith, i.e. with all-around 
clearance. The ring 4 is fixed on the wall of the cylinder 1 by a 
connecting conductor 5, e.g. a bus bar. 
A contact bridge can continuously slide on the contact ring 4 at its inner 
end and serves to electrically connect this contact ring with a fixed step 
contact 2. To this end, the members 6a and 6b of the bridge 6 can be 
pressed toward one another and against the ring 4 and the contact 2 by 
springs 6.1 surrounding pins 6.2 and braced against the heads 6.3 thereof 
and against the plates 6a and 6b. 
The contact bridge 6 is held on the switching shaft 3 by a support arm 7 
which is composed entirely of electrically insulating material and has a 
portion 7.1 reaching between the plates 6a and 6b and traversed by the 
pins 6.1. The support arm 7 is formed unitarily with the flange 8 having 
an arc-segmental cross section conforming to the tubular configuration of 
the switching shaft 3 and resting against the outer surface thereof over 
an arc of the latter which approximates 120.degree.. 
From the inner face of the flange 8, a projection 9 extends radially 
inwardly through an opening 3a in the wall of the shaft 3. 
In addition, the support arm 7 is held on the shaft by a pair of 
electrically insulating bands or straps 10 of synthetic resin impregnating 
self-hardening plastic material which impregnates a fibrous reinforcing 
strip. The best results have been achieved with glass fiber woven or 
nonwoven bands impregnated with epoxy resin. 
As is shown clearly in FIGS. 1 and 2, the insulating bands are wound in at 
least one turn around the flange and the shaft 3 above and below the arm 
7. 
A single band can be used with light weight arms, the two bands being 
preferred for larger and heavier constructions of the step selector.