FIELD OF THE INVENTION
This invention relates generally to a device for machining magnetic hub of a large size
exciters used in various applications and industries such as thermal power plants. More
particularly, this invention pertains to a device for machining magnetic hub assembly
mounted on an exciter with attached magnets adaptable to a Turbogenerator stator
and rotor.
BACKGROUND OF THE INVENTION
Electric motors and generators utilize a rotor which rotates within a confined space
whilst the space having stationary wire windings and comprising a stator. For example,
a generator may have a rotor with permanent magnets mounted along the rotor
circumference. This type of generator rotor produces a rotating magnetic field, and the
generated current is taken directly from the stator.
Similarly, large electrical generators (as used in thermal power plants or similar
applications) require external excitation which is provided by the exciter. Conventional
systems provide exciter output connected to the generator field through a series of slip
rings and brushes. An alternative system is a brushless exciter which is joined directly
to the generator field winding without the need for brushes and slip rings.

For large machines, Permanent Magnet Generator (PMG) generators are used instead of
the excitation generators, and a controlled field is produced at the generators.
The PMG with its rotating field and stationary armature supplies high frequency AC
power to the voltage regulator.
The voltage regulator receives voltage and reactive current feedback signals provided
by potential and current transformers and transmit voltage and reactive current
feedback signals. Comparing these signals to a reference set point in the voltage
regulator, the voltage regulator provides a controlled variable DC current to the
stationary field fo the rotating exciter. With its stationary field and rotatin armature, the
exciter generates three phase high frequency AC output.
This output is rectified by the rotating rectifiers. This DC current is fed via conductors to
the center of the generator rotor shaft and carried by a special field lead core bar in the
hollow shaft area under the bearing journal which is then applied to the main generator
field winding. The rotating rectifier is a three phase full wave diode bridge.
In gist, the rotor or the field coils in a generator produce the magnetic flux that is
essential for production of electric power. The rotor is a rotating electromagnet that a
DC (Direct Current) electric power source to excite the magnetic field. This power
comes from an exciter.

Thus, excitation systems have a powerful impact on generator dynamic performance
and availability and which ensures quality of generator voltage and reactive power, i.e.
quality of energy delivered to the consumers.
By manufacturing of a high quality magnetic hub that provides excellent excitation to
the rotor, one can leverage on extending the lifetime and improve reliability , availability
and output.
The prior art procedure of machining a magnetic hub or a PMG is as under :
1. Providing raw forged magnetic hub material;
2. primary turning operation of the Magnet Hub on a CNC lathe machine;
3. machining a dovetail groove along the length as well as forming radius
operations on a vertical boring machine;
4. slotting drilling and tapping operations on a single machine tool horizontal Milling
center.
5. final Tapping operation on face and deburring operations on holes;
6. assembling Magnet shoe on outer diameter of slots of the magnet hub to make it
a complete PMG assembly.

As per prior art, Slotting Drilling and Tapping (refereed above in Sl. 4) of the magnet
hub is carried out on a machine that has least count of rotating table in multiple of 0.5
degree.
A fixture is a work holding or supporting device used in the manufacturing industry for
holding and locating the job in a specific position or orientation as well as supporting
the job which in present case is a magnet hub. The raw material for magnetic hub is a
forged cylinder. The finished job (magnetic hub) consists of a plurality of longitudinal
slots on its outer periphery in a circular array and each longitudinal slots is at an angle
of 22.5 degree. Thus, these longitudinal slots basically divides the whole outer
circumference of the magnetic hub into a plurality of segments having at least one slot
between each segment. The thickness of the slots is different than the segments itself.
All the slots are of equal width and all segments are of equal width. Drilling has to be
done on each segment to make a plurality of through holes in a rectangular array in
which corresponding number of non-magnetic screws shall be fixed.
Since the machining is to be done on the outer circumference of the magnetic hub,
clamping is done from the top which tends the holes for non-magnetic screw to distort
and also the magnetic hub ends to buckle under pressure. Due to the existing
arrangement of clamping, the magnetic hub in which the whole of the top part of the
magnetic hub is being pressurized from top making it stable and restrict its degree of
freedom axially and laterally.

OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a device to hold a magnetic hub
forging for large size exciter while performing on it slotting, drilling, and tapping
operations.
Another object of the invention is to propose a device to hold a magnetic hub forging
for large size exciter while performing on it slotting, drilling, and tapping operations.,
which allows machining of the magnetic hub or any critical object on its periphery or
outer diameter.
SUMMARY OF THE INVENTION
Accordingly, there is provided a device to hold a magnetic hub forging for large size
exciter while performing on it slotting, drilling, and tapping operations comprising a
plate forming the base of the device and having a plurality of holes through which the
base can be fixed on any standard machine bed having T-slots on it; a solid cylinder
welded at the center of the base plate through its bottom face and having a plurality of
equally spaced threaded holes configured on its top face; a plate having a counter bore
hole circumscribed and rigidly held on said solid cylinder on the top face using flexible
threaded connection through a plurality of threaded holes and cap screws inserted

through these holes; a collar formed out of the step upon placing the plate on the solid
cylinder, the collar resting on the top face of the solid cylinder and said counter bore
holes on the plate aligning themselves to threaded hole on the solid cylinder; a
magnetic hub disposable on the base plate such that said plate inherently locks the
magnetic hub’s lateral degree of freedom; a C-clamp fixable on top of the magnetic hub
to restrict axial movement of the magnetic hub, the C-clamp being fixed through a
flexible threaded connection through a plurality of studs and axial threaded holes on the
solid cylinder in a circular array, allowing each stud to be tightened using washer and
hexagonal nut; and a plurality of height adjusting pads can be welded when needed to
the base plate to increase the height of the device.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
These together with other objects and advantages which will become subsequently
apparent reside in the details of constructions and operation as more fully hereinafter
described and claimed, reference being had to the accompanying drawings forming a
part hereof, wherein like numerals refer to like parts throughout, and in which
Figure 1 represents excitation system required for a turbogenerator;
Figure 2 represent photographic view of an exciter rotor;
Figure 3 represents the developed device for holding cylindrical jobs;
Figure 4 represents photographic view of finished job (magnetic hub).

DETAILED DESCRIPTION OF THE INVENTION
The inventive device is constructed with the primary purpose of creating an
arrangement that can clamp and provide stability to magnetic hub forgings during
machining operations viz slotting, drilling, tapping on a horizontal milling center which
has least count in multiple of 0.5 degree rotation.
The device comprises of following constituent components:-
Item 1 refers to a thick plate forming the base of the device,
Item 2 refers to the plate as per the size of the magnetic hub,
Item 3 refers to a plurality of Cap Screw,
Item 4 refers to a plurality C Clamp,
Item 5 refers to a plurality of studs,
Item 6 refers to a plurality of Hexagonal Nut,
Item 7 refers to a plurality of washer,
Item 8 refers to height adjustable pads,

Item 9 refers to a plurality of holes in the base of the device,
Item 10 refers to a solid cylinder mounted on plate,
Item 11 refers to a plurality of threaded holes on solid cylinder,
Item 12 refers to top face of the solid cylinder (10),
Item 13 refers to bottom face of the solid cylinder,
Item 14 refers to a counter bore hole in guide plate,
Item 15 refers to a magnetic hub of the exciter, and
Item 16 refers to an axial hole in the solid cylinder.
Plate (1) forms the base of the device, which is a circular plate with large enough
diameter to accommodate all sizes of magnetic hubs on it. Therefore the Base sub
assembly (1) ensures that a single device (F) can be used for different size of magnetic
hub as the size of the magnetic hub depends on the rating of an exciter for example,
250 MW, 500 MW, 660 MW etc. (rating of the power plant in other words). The Base
sub assembly (1) have a plurality of holes (9) through which the base is fixed on any
standard machine bed having T-slots on it.
A plate (2), having a counter bore hole (4), is circumscribed and rigidly held on the
solid cylinder (10) on said top face (12) using flexible threaded connection through a
plurality of threaded holes (11) and plurality of cap screw (3) inserted through these

holes. The threaded hole (11) and cap screw (3) arrangement to rigidly fix plate (2) is
selected in such a way that the head of the cap screw gets inside the counter bore hole
(14) on the plate (2).
The inside diameter of the plate (2) is stepped. The plate (21) is placed on the solid
cylinder (10) such that the collar made out of the step (due to internal diameter
difference) rests on the top face of the solid cylinder (10) and counter bore holes (14)
on the plate (2) align themselves to said threaded hole (11) on the solid cylinder (10).
It is to be noted that the size of the magnetic hub (15) i.e. its internal and external
diameter depends on the rating of the exciter vis-à-vis the rating of the power plant.
The bigger the power power plant rating, the bigger will be the turbo generator and
hence, more will be the excitation requirements namely, the amount of excitation
controls, the size of the PMG or the magnetic hub. Thus the internal diameter as well as
the external diameter, slots and segments sizes may vary depending upon the size of
the magnetic hub or rating of exciter or excitation requirements. Thus, to accommodate
variable sizes of magnetic hub, different size of plate (2) can be used.
Magnet hub (15) is placed on the base plate (1) and is inherently locked in its lateral
degree of freedom through the matching plate (2). The axial movement of the magnetic
hub is retrained through fixing a C-clamp (4) on the top of the magnetic hub (15). The

C-clamp is fixed through a flexible threaded connection through plurality of studs (5)
and axial threaded holes (16) on the solid cylinder (10) in a circular array. Each studs
are tightened using washer (7) and hexagonal nut (6). Plurality of height adjusting
pads (8) can also be welded to the base plate (1) of the also provided to increase the
height of the fixture as needed for the machining.
TANGIBLE GAINS
(A) Saving for 250 MW Magnet Hub
Magnet Hub Machining time as per prior art = 96 Hours
Magnet Hub Machining time using invented fixture = 40 Hours
Time savings per job = 96 – 40 = 56 hours
Machining cost per hour = 3000 INR
Machining cost per job = 3000*56=168000 INR
Rate of Man-hour cost = 800 INR Per Hour
Saving of man hours cost = 800*56= 44800 INR
Total Saving per job =168000 + 44800=212800 INR
Total saving in one financial year = 212800*3 = 638400 INR

(B) Saving for 500 MW Magnet Hub
Magnet Hub Machining time as per prior art = 100 Hours
Magnet Hub Machining time using invented fixture = 56 Hours
Time saving per job = 100-56=44 Hours
Machining cost per hour = 3000 INR
Machining cost per Job = 3000*44=132000 INR
Man hour cost = 800 INR Per Hour
Saving of man hours cost = 800*44 = 35200 INR
Total Saving per job = 13200 + 35200 = 1627200 INR
T
Total saving in one financial year = 167200*3= 501600 INR
(Assuming 3 sets of 500 MW Exciters are manufactured in a year)

WE CLAIM :
1. A device to hold a magnetic hub forging for large size exciter while performing
on it slotting, drilling, and tapping operations comprising a plate (1) forming the
base of the device and having a plurality of holes (9) through which the base can
be fixed on any standard machine bed having T-slots on it; a solid cylinder (10)
welded at the center of the base plate (1) through its bottom face (13) and
having a plurality of equally spaced threaded holes (11) configured on its top
face (12); a plate (2) having a counter bore hole (14) circumscribed and rigidly
held on said solid cylinder (10) on the top face (12) using flexible threaded
connection through a plurality of threaded holes (11) and cap screws (3) inserted
through these holes; a collar formed out of the step upon placing the plate (2)
on the solid cylinder (10), the collar resting on the top face of the solid cylinder
and said counter bore holes (14) on the plate (2) aligning themselves to
threaded hole (11) on the solid cylinder (10); a magnetic hub disposable on the
base plate (1) such that said plate (2) inherently locks the magnetic hub’s lateral
degree of freedom; a C-clamp (4) fixable on top of the magnetic hub to restrict
axial movement of the magnetic hub, the C-clamp being fixed through a flexible
threaded connection through a plurality of studs (5) and axial threaded holes
(16) on the solid cylinder (10) in a circular array, allowing each stud to be
tightened using washer (7) and hexagonal nut (6); and a plurality of height
adjusting pads (8) can be welded when needed to the base plate (1) to increase
the height of the device.

2. The device as claimed in claim 1, wherein the plate (1) is a circular plate with
large enough diameter to accommodate all sizes of magnetic hubs or any other
shaped objects to ensure adaptation flexibility of the device for different sizes
magnetic hub.
3. The device as claimed in claim 1, wherein the threaded holes (11) of Solid
Cylinder (10) are arranged in a circular array covering whole 360 degree on
circumference, and wherein the axis of these threaded holes (11) is
perpendicular to longitudinal axis of the solid cylinder (10).
4. The device as claimed in claim 1, wherein the inside diameter of the plate (2) is
stepped to accommodate variable sizes of magnetic hub.