FIELD OF THE INVENTION
The present invention relates to preparation of stator frame for insertion and alignment of core to the stator frame of a turbo-generator.
More particularly, the present invention relates to a centering device for preparation of stator frame for core insertion and alignment and a method for the same.
BACKGROUND OF THE INVENTION
A turbo-generator is an essential component of thermal power plant. Essentially, the main components of a generator are the stator frame, rotor, stator core, stator windings and exciter (fig. 3).
Stator Frame:This is the cover of generator.It is a hollow cylinder made up of MS material which supports the stator core and rotor bearings including the cooling medium.
Rotor:Made from a steel forging, the rotor’s drive end is coupled to the turbine, with the other end carrying the slip rings or a brushless exciter. Axial slots on the outside carry the rotor windings. The windings are made of hard silver-alloyed copper and are arranged in multiple layers that are insulated from each other and from the rotor body. The windings are secured in the slots by wedges that can be made of

steel, stainless-steel, aluminum or bronze. Copper leads in the bore of the rotor connect the rotor windings to the slip rings or brushless exciter that supplies the excitation current.
Stator core: The stator core is composed of lakhs of thin laminations of 0.5mm. These laminations are arranged in a circle and stacked on top of each other until the full length of the core is built up. The laminations are insulated from each other by a thin layer of insulating varnish. The stacked laminations (core) are then clamped axially and connected by spring supports to the frame. The laminations incorporate slots on the inside diameter for mounting the stator windings. The windings are secured in the slots by wedges.
Stator core (fig. 3) is one of the major components of Stator and is a part of Turbo-generator, with main function as providing path to the rotating flux produced by the rotation of rotor in turbo generator.
Stator windings: The stator windings are composed of two conductors stacked on top of each other in each of the stator slots. Each conductor or bar, is connected to other bars at the ends of the stator to form the complete stator winding. The bars are connected to form three independent circuits or phases. Each end of each phase is connected to one of the six terminals of the generator. Each bar is typically built up from dozens of individually insulated copper strands that are arrayed in multiple columns. The columns of conductors are twisted, or roebelled through the length

of the bar in such a way that each strand will occupy each position in the strand columns at least once. This minimizes eddy current losses and circulating current losses in the windings.
The rotating magnetic field (also known as revolving-field)of synchronous machine has the field-winding wound on the rotating member (the rotor), and the armature wound on the stationary member (the stator). A DC current, creating a magnetic field that must be rotated at synchronous speed, energizes the rotating field-winding. The rotating field winding can be energized through a set of slip rings and brushes (external excitation), or from a diode-bridge mounted on the rotor. The stator core is made of insulated steel laminations. The thickness of the laminations and the type of steel are chosen to minimize eddy current and hysteresis losses, while maintaining required effective core length and minimizing costs.
The stator core is mounted directly into the stator frame in vertical position.
In generators, the rotor is at the middle of the assembly and the stator is the outer stationary part. The outer body of a generator i.e. stator frame is hollow cylindrical in shape and 16 springs are assembled on the inside of this stator frame. These springs support the core bar.
Longitudinally, a generator has a Turbine End (TE) and an Exciter End (EE) (fig.4).

The need for centering :–
Stator frame is a hollow cylinder of approx 3.8 meter diameter and 10 meter long. 8 concentric circular rings of 30 mm thick and 2.98 meter ID is welded to the stator frame wall along the length of stator at some distance. Assembly of core with stator frame is done by inserting the core into the stator frame. Length of core is approx 6.7 meter and OD is approx 2.96 meter.
There is only 10 mm radial gap between ribs and core outer diameter (OD). Centering is done to make these 8 ribs concentric so that during the core insertion, the core should not foul with any ribs. After the insertion of core, alignment of core is done with respect to the point measured in two concentric ribs. This ensures that the center line of core matches with the center line of stator frame which ultimately results in maintaining equal air gap between outside diameter(OD) of rotor and the inner diameter (ID) of core.
The different issues that may occur if the centering is not proper are:¬- Core cannot be inserted without performing above said operation. If tried, core can foul with stator frame and may get damaged.
- Alignment of core with respect to stator frame cannot be done.
- Assembly cannot be done so long as air gap between rotor and core maintained
equal.

The centering process done earlier was having many drawbacks.
There was a chance of misjudgment of reading in earlier centering process.
The process had a centering device mounted on the outer faces of stator frame. A piano wire is passed through the centering device. Inside micrometer was used for a centering the piano wire. One end of the micrometer is touched with the stator frame where as other with the piano wire. Centering of piano wire was done by maintaining equal distance at 4 places with 90 degree apart. The main problem involving this method was the judgment of task performer which varies upto some mm depending upon the reading of micrometer. There was always a chance of error.
OBJECTS OF THE INVENTION
The object of the present invention is to develop a centering device for insertion and alignment of core in the stator frame.
Another object of the present invention is to develop a centering device for insertion and alignment of core in the stator that reduces the cycle time of the operation as well as reducing the rework.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig. 1 Photographic view of the apparatus being used for centering.
Fig. 2 Photographic view of the arrangement while taking reading by micrometer.
Fig. 3 Important components of a turbo-generator.

Fig. 4 3D model of hollow stator assembly.
Fig.5 Schematic diagram of the device.
Fig.6 Schematic diagram showing passage of piano wire through the notch hole of
adjustable arm.
Fig. 7 The circuit connection of micrometer with light and sound arrangement and
piano wire.
Fig. 8 Alignment arrangement.
DETAILED DESCRIPTION OF THE INVENTION
For the purpose of centering and aligning core insertion in the stator frame, a fixture has been designed. The fixture comprises:
1. Channel (1) – Two MS channel (1) are used whose length is approximately 3800. These channels (1) are bolted at both ends of stator frame (7).
2. Adjustable arms (3) – A plurality of adjustable arms (3) are attached one each on channel(1) on either side of stator frame.
The adjustable arm (3) is provided with a notch (4) having a hole (5) through which the piano wire (6) is inserted. The adjustable arm centers the piano wire with the help of 3 screws.
3. Stick inside micrometer (9) – The device is provided a stick inside micrometer (9)
with light and sound arrangement (10). The bottom end (2) of the stick inside
micrometer (9) is connected to the piano wire (6) via metallic wire coming out

of light and sound device (10). During centering as soon as the other end of the micrometer touches the piano wire, light glows along with a long beep sound which ensures proper contact.
4. The piano wire (6) passes through the holes (5) provided in the notch (4) of each adjustable arm (3) on either side of stator frame (7) and provided with hanging weight (11) to keep the wire (6) straight. The adjustable arm (3) can be adjusted to and fro as well as top and bottom with the help of three bolts (8).
The channel (1) is having two holes (2) at both ends. The stator frame (7) is provided with holes arranged equidistantly on the circumference at either end namely turbine end and exciter end. The fixture is held on at the middle region on stator end through these holes.
The piano wire (6) is made to pass through the holes (5) provided on the notch (4) of each adjustable arm (3) on either side of the stator end. The piano wire (6) is having weights(11) at both ends for tension.
The piano wire which is passing through the holes in the notch of the adjustable arm is centered at both the ends by using stick inside micrometer. 4 points are marked at 90 degree apart at 12 o’clock, 3 o’clock,6 o’clockand 9 o’clock positions. The distance of these points from the piano wire are measured with the help of stick inside micrometer (Range 100 mm to 2100 mm), all readings are made equal by adjusting the

adjustable arm. Thus centering of piano wire is carried out at both the ends. Now piano wire is at the center line of the hollow stator frame. This wire is the reference for further operations.
The stator frame has eight concentric circular ribs to create chamber aftercore insertion. This centering device is also used to check the concentricity of ribs so that during core insertion, it does not foul.
These eight concentric ribs are placed along the length of stator frame. These ribs form six chambers. Each chamber has three springs at 3 o’clock, 6 o’clock and 9 o’clock positions.
Spring basket (matching part) is mounted to the outside diameter (OD) of the core and welded to the spring through bush.
The centering device is used to check the concentricity of ribs.
Once the piano wire is centered, it becomes the reference for further activity. Here eight points are marked on the rib of stator frame along the circumference.
The distance between marked points and piano wire is measured and compared with the value required for the gap for core insertion.
If variation is observed in measured values, marking is done on the job for rectification. This is done by gas cutting and grinding.

In addition to above, 2nd and 7th ribs are having additional three flat points at 3 o’clock, 6 o’clock and 9 o’clock positions. These positions are marked ‘X’ readings. The radial gap between these points and piano wire is measured and recorded.The radial distance thus obtained is used for the core alignment purpose after core insertion.
The centered piano wire is now the reference for the spring alignment. Spring is aligned and welded to the stator frame. Spring is the part of generator which joins the stator core with the stator frame. Spring helps in minimizing the vibrations when the generator is in working condition.
The center of the whole system is the center of the stator frame. Stator frame is positioned vertically and in vertical position, core is inserted into the stator frame. After placement of core, the readings of ‘X’ points marked on the stator frame are taken for reference as below:
Reading of Point X on the right side – outer radius of core = distance (Z) between the core and the body/stator frame.
During the alignment of core with stator frame, once this value ‘Z’ is achieved bushes are inserted to the spring eye and one end of the bush is welded to the spring and other to the spring basket pad mounted on the core outer periphery.
After complete assembly of the generator, testing of the generator is being done to check for any possible defect before shipping to site. For testing also, the cooler

housing, end shield (TE and EE) are being assembled on the generator. The fixture (F) is now again assembled on the end shield on both side of the generator and the piano wire is put into centre of the generator through the fixture (F) taking centre of the core as reference so that the bearing in the end shield can be placed using this piano wire as the center. For this, again the stick inside micrometer is being used. After this centering process, the end shield is being locked and end shield comes in correct position as per assembly requirements wrt frame.

We claim:
1. A centering device (F) for preparation of stator frame for core insertion and
alignment comprising:
a base channel (1) having two holes (2) at the ends for fixing with the corresponding holes of stator frame body (7) on either side;
an adjustable arm (3) disposed in the middle of the base channel (1), the said adjustable arm (3) provided with a notch portion (4) having a hole (5) for passing a piano wire (6), the piano wire (6) passing through the holes (5) of the notch (4) of the corresponding adjusting arm (3) on the other side of statorbody (7) such that the piano wire (6) remain horizontal wherein the piano wire (6) is provided with hanging weight (11) at both ends for keeping the wire (6) in stretched condition characterized in that the adjustable arm (3) is provided with three screws (8) for moving the piano wire (6) to and fro as well as top and bottom after each measurement of reading of the position of piano wire (6) wherein an inside micrometer (9) disposed for taking measurement for the distance of the piano wire (6) from the inside surface of stator (7) followed by further adjustment of arm (3) till the piano wire (6) is at the centre of stator frame (7).
2. The centering device (F) as claimed in claim 1, wherein one end of the inside
micrometer (9) is permanently connected to the piano wire (6) through a light and
sound arrangement circuit (10) such that when one of the micrometer leaf

touches the piano wire, it makes a beeping sound, while the other leaf of the micrometer butt with interior-surface of stator (7) from which the reading for the position of piano wire (6) is taken.
3. A method for centering and aligning core insertion in stator frame comprising.
positioning the stator frame (7) in horizontal direction;
marking at 4 positions in the periphery of stator frame (7) at both ends at 90o apart, the positions marked as 12 o’clock, 3 o’clock, 6 o’clock and 9 o’clock;
fixing the fixture base channel (1) with the two holes (2) on the stator frame periphery at 3 o’clock, 9 o’clock positions on either side;
allowing the piano wire (6) to pass through the holes (5) in the notch portion (4) of adjustable arm (3) on either side such that the piano wire (6) remain horizontal wherein the piano wire is kept under stretching condition by two hanging weights (11);
measuring the reading of the four positions (12 o’clock, 3 o’clock, 6 o’clock and 9 o’clock) with respect to the piano wire (6) with the help of an inside micrometer (9) wherein the end (12) of the micrometer is connected to the piano wire (6) through a light and sound circuitry (10) when the leaf edges of the micrometer are abut with the marked positions and the piano wire (6) respectively;
truing the variations by adjusting the adjustable arm (3) with the help of adjusting screws (8) and repeating the previous operation till such time the piano wire (6) is at the central axis of stator frame (7);

checking the concentricity of the inner diameter of the 8 ribs that are already welded with stator frame with respect to the previously centered piano wire (6) by marking at eight points;
calculating the distance between the marked points and the piano wire and comparing the values with actual gap required for core correction;
removing the observed variation by gas cutting the rib;
removing the centering attachment for executing the core insertion.