Novel modular type Slipring Hub assembly suitable for mounting
Sliprings in Brushless exciter of AC Generating system
FIELD OF INVENTION
This invention relates to Brushless type AC electric power generating system
generally used as a generator, which has a common shaft for an Brushless
exciter unit and a main generator unit where the rotors are electrically
interconnected by a rectifier assembly. Sliprings for earth fault protection is also
provided on main shaft for Rotor Earth fault detection. More particularly, this
invention relates to a novel arrangement for mounting of sliprings externally to the
spider assembly of brushless exciter rotor which forms part of above System. This
can also be extended to Synchronous Motor with brushless exciter and sliprings
employed earth fault detection scheme.
BACKGROUND OF THE INVENTION
In a AC Brushless Power Generation system, produces electric power from
mechanical power. This comprises of Main generator along with Brushless exciter
mounted on same shaft .Brushless Exciter Stator receives controlled field excitation
from Regulator/AC Excitation system.
Brushless excitation system comprises of 3 phase rotating armature of
the exciter rotor which produces alternating voltages, which is rectified to DC with
the help of rotating rectifier bridge mounted with exciter armature on a common
shaft. The rectified voltage is then applied to the main generator rotating field
winding which is also mounted on the common shaft. The rotating rectifier usually
comprises a set of antiparallel semiconductor diodes mounted on a shaft supported
assembly with DC rings and electrically interconnected between the exciter
armature and the main field winding. In additionCapacitors and resistors (also
referred as Varistors) are also employed so as to suppress semiconductor diode
voltage spikes.

Additionally (PMG) pilot exciter having a permanent magnet field driven by the
same common shaft through stub shaft arrangement or suitable can also be
used for pilot excitation or source of reliable supply to Regulator/AC Excitation
system during black start& Transient period. A permanent magnet generator (PMG)
is a device that converts mechanical energy to electrical energy in which rotor is
with permanent magnets. These devices do not require a separate DC supply for
the excitation circuit.
There is also a need for detection of earth fault in field circuit of generating system
without a shutdown of generator in the brushless excitation concept. Earth Fault
may lead to downtime/shutdown of operating set resulting from overheating,
arching and damage to field circuit components. Such faults, although usually
infrequent, should if possible be detected before major damage.
In such above described earth fault detection scheme, a pair of shaft mounted
collector rings with brushes connected electrically to the two ends of main field
winding is mounted on the shaft for detection. Low frequency square wave pulse is
injected to ascertain the healthiness of rotor insulation. Other methods also exist
which depends upon user.
Normally disengaged brushes are provided for contacting the collector rings when it
is desired to test for a ground fault. Square wave generator/Relay provided in the
ground detection arrangement external to generator to indicate the presence or
absence of a ground fault. Since the ground tests can be made relatively
infrequently and since each test when made require brush and ring contact for a
short period of time, the concept of brushless can be claimed.
Further, rotor earth fault detection scheme in the interest of operating facility, is
left to the choice of project consultant/user for small generating sets. This
requirement may also crop up in retrofitting jobs wherein the generating sets
initially is not provided with such a detection provision.

In retrofitting Jobs, provision of sliprings shall necessitates increase in shaft length ,
base frame and complete shaft line which in certain cases may not be feasible due
to limitations imposed by critical speed.
OBJECT OF INVENTION
It is, therefore, an object of the invention to provide a novel arrangement
for mounting & support of sliprings in brushless excitation concept wherein DC
rings are electrically connected to collector assembly of sliprings employed for Earth
fault detection externally to the shaft after exciter assembly. Modular type slipring
hub is employed to support the components in the earth fault detection
circuit in such a manner as to provide convenient interconnection between various
components.
An additional object of the invention is to provide a novel arrangement for further
connection to Pilot exciter (PMG) to be mounted on the same shaft line external to
the slipring assembly.
Another object of the invention is to provide a novel arrangement wherein the
mounting support & sliprings for earth fault and its interconnection arrangement is
sturdy enough to withstand against forces of rotation.
SUMMARY OF INVENTION
More specifically, to illustrate the principles of the invention, our main consideration
is to mount the slipring outside the exciter on slipring hub. Inventive art specifically
targets novel modular arrangement for mounting of slipring hub inside spider hub
externally so that sliprings for earth fault detection can be mounted and adequately
supported.
The annular space for assemblies about the shaft and within the brushless exciter is
limited by design considerations. Space is more critical with critical components
and electrical connections to be supported for long term reliability against
centrifugal forces of rotation.

In accordance with broad principles of the invention, Spider plate assembly of
existing Brushless exciter rotor is modified for making suitable mounting provision
of specially designed slipring hub on it. Spider plate assembly is mounted on main
shaft of generator. Specially designed slipring hub with extended length is mounted
on this spider plate. Seating flange of slipring hub is designed in two halves with
suitable openings on two sides for taking out rotor lead connection and cable
thimble to be connected to DC connection ring of exciter rotor. Mounting provision
of slipring hub on spider plate of Brushless exciter is critical due to space constraint
posed by ID of connection ring, rotor lead connection, cable thimble and related
assembly.
This construction allows easy assembly and dismantling of sliprings on generators
with Brushless exciter. Provision for sliprings for Earth fault detection can be made
without affecting the generator rotor shaft design. Increase in Shaft length for
providing slipring will be eliminated. Shaft length for alternator with sliprings and
without slipring will remain same. Sliprings can be added to generator at any time
after assembly also if not envisaged earlier. Inventive art shall also allow the
generator to be started on black start for initial voltage buildup.
The inventive art shall allow the user to flexibly select and incorporate Earth fault
detection scheme at any stage in Generating equipment(specially of smaller size).
This shall also eliminate the dependency of rotor shaft design for provisions of
sliprings. The modular design adopted to suit brushless excitation system also
plays a important role in optimization of generating sets wherein the set could be
designed for higher output with increased shaft length which was designated to be
allocated for provisions of sliprings in prior art.
BRIEF DESCRIPTION OF DRAWINGS
FIGURE 1: is a schematic view of a A.C.Power generating system for various
components wherein there is incorporated a rotating rectifier exciter with sliprings
for earth fault detection located on shaft in between exciter and Main Field in prior
art.

FIG. 2 shows drawing of prior art with sliprings mounted on main shaft (LHS side)
before Brushless exciter.
FIG. 3 shows drawing of Brushless Exciter with broad principles of the invention
with slipring hub and slipring arrangement mounted outside exciter spider.
FIG. 3A : 3D model of the inventive art with slipring hub mounted on Exciter spider
plate.
FIG. 4 & 4A shows drawing and 3D model of slipring hub developed in novel
arrangement.
Fig 5 : Assembly Drawing of slipring.
Fig. 6 : Assembly of Slipring and Slipring hub.
BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE
INVENTION
Figure 1: shows schematic of AC Generating power system consisting of Main
generating field windings 10 electrically connected to rotating rectifier bridge with 3
Phase AC exciter armature 15 mounted on the same shaft. Slipring assembly 12 for
rotor earth fault detection is also provided on main shaft. Capacitors 14 and
resistors 13 are also employed so as to suppress semiconductor diode voltage
spikes. Exciter Stator 16 is fed by controlled excitation for Excitation/Regulation
Panels 17. Pilot exciter 18 with permanent field is often provided for small
generating sets mounted with help of stub shaft external to exciter which powers
the excitation panels 17 during initial start/black start or during faults.
Figure 2: electrically connected Prior art described above is illustrated in cross
sectional longitudinal view in which there is a rotating rectifier bridge consisting of

antiparallel diodes 18 mounted on diode assembly 19 with heat sink 20 electrically
connected exciter spider 22 supported DC rings 21 with insulated studs 33 34 and
arrangements. The DC rings are electrically connected to main field windings
through rotor leads brazed to thimble 31, 32.
In prior art, sliprings assembly consists of sliprings 26 supported and insulated from
slipring hub 25 which is shrink fitted on shaft 27. Sliprings are electrically connected
to DC rings 21 through insulated Copper studs 28. The provision of slipring
assembly on main shaft requires additional length of approx. 250 mm on main shaft
27 in between main field and exciter in prior art.
Figure 3 : Describes the inventive art to illustrate the principles of the invention,
wherein a new arrangement sliprings for earth fault detection is mounted on hub
which is supported on exciter spider 22 instead of main shaft 27 in prior art.
Figure 3A : 3D model of new developed slipring hub assembled on Brushless Exciter
rotor is shown.
Figure 4 : A novel modular type slipring hub 29 is shown in Fig. 04. This slipring
hub is specially designed for constrained annular space inside DC rings 21, which
are supported on exciter spider plate 22. In the novel arrangement, Seating flange
30 of slipring hub is made in two halves so that it does not foul with rotor leads,
cable thimble 31,32 (Fig.03) and studs (33,34) for mounting DC connection ring.
Seating flange 30 is 16 mm thick plate made as per Detail-‘X’ so that spigot seating
of new slipring hub 29 onto the Spider plate assembly 22 of Brushless exciter is
ensured. As axial gap between spider plate of exciter and inner (1st) DC connection
ring is only 20 mm. Hence, counter bore holes as shown in Detail-‘X’ are made on
seating flange 30 of slipring hub and allen bolts are used for fixing it on the spider
plate assembly 22 of Brushless exciter so that bolt head does not foul axially with
inner DC connection ring 21. Hub cylinder 36 is 20mm thick plate rolled in a
cylinder having two cutouts as shown. Two cutouts are made along the length in

cylindrical plate of slipring hub to facilitate rotor lead coming out and its termination
at DC connection ring 21. Diameter of hub cylinder is kept less than the ID of DC
connection rings 21 so that slipring hub can be moved along the axis of shaft
towards exciter during its assembly on exciter rotor spider plate 22. Hub cyclinder
36 is welded to seating flange 30 on LHS. Hub cylinder 36 is welded to Hub flange
37 on RHS. Slipring assembly(Fig.05) is mounted on Hub flange 37 of slipring hub
with the help of 4 nos. M12 hex screws and spring washers. Hub Flange 37 of
slipring hub is 16thkplate with provision made for mounting of slipring.
Figure 4A : 3D model of new slipring hub developed is shown in Fig. 4A for
reference.
Figure 5 : Slipring is manufactured as per Fig.05. Slipring is made of round bar 38
with mounting provision made at DE(or LHS) side. Glass tape, hardener and resin
are applied on OD of round bar 38 along the length. Two nos. sliprings 40 are
mounted on this round bar 38 along with insulation provision 41 as shown in
Fig.05. Both sliprings 40 are insulated with each other designed for rated current
and speed with insulation provisions 43 shown in fig.05. On the NDE side, stub
shaft 42 is also made available to couple PMG on it in case where PMG is required.
Thus both slipring as well as PMG can be mounted in Generating system in the
inventive art.
Figure 6 :Slipring is assembled on new slipring hub as shown in Fig.06.
In Figure 3, Brushes 43 is mounted on Brush holder 44 which are placed centrally
on the sliprings 40. Brush holder 44 rests on supporting arm 45 and this brushgear
arrangement is supported by bracket 46. Bracket 46 is mounted on base frame.
The above description has been presented only to describe the broad principles of
the invention and is not limited to the embodiments described above.

The novel concept of mounting slipring on Brushless exciter shall eliminate the
dependency of rotor design for provisions of sliprings required for earth fault
protection. The slipring can be mounted externally on exciter rotor with the help of
new developed modular type slipring hub, even after assembly of complete
Generator. There will be no change in Generator shaft line due to addition/ deletion
of slipring. Such protective feature can be added to the AC generating sets anytime
even if the same is not envisaged during preliminary design stage.
With the aid of this novel arrangement and sliprings and brushes mounted as per
assembly, generating sets will be capable of taking black start (emergency start)
for initial voltage buildup. The word ‘black start’ in technical terms means the
process of restoring an electric power station to operation without relying on the
external electric power (for field excitation).

We claim
1. A novel modular type slipring hub assembly (29) suitable for mounting
sliprings in brushless exciter comprising:
- a pair of seating flanges (30) made up of two halves;
- the said flanges (30) welded on diametrically opposite sides on one face
of a hub cylinder (36);
- a circular shaped hub flange (37) welded with the other face of the said
hub cylinder (36);
- the hub cylinder (36) comprising seating flanges (30) made in two halves
and having two cutouts along the length for avoiding fouling with rotor
leads, cable thimble (31, 32) and studs (33, 34) for mounting D.C
connection ring (21);
- a plurality of holes provided on hub flange (37) for mounting with the
flange of slipring assembly (39) by nuts and bolts on one side and on the
other side with the face of exciter rotor spider plate (22) in an
arrangement such that the slipring assembly (39) is configured to be
connected with the rotor via slipring hub assembly (29) and exciter rotor
spider plate (22) without increasing the length of rotor shaft (27)
avoiding a major space constraint,wherein DC rings are electrically
connected to collector assembly of sliprings employed for earth fault
detection externally to the shaft after exciter assembly.