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Electric Block Machine

 

BLOCK -02 ELECTRICAL MACHINE BLOCK:-

   Block-02 is designed to manufacture turbo generater and medium size AC electrical machines .

   02 Electrical machines block consists of the following work centres-

 1. 3211 Press Shop Manufacturing of lamination and core assembly.

 2. 3212 Light Machine Shop :-

                       All  hardware items are processed in this shop.

 3. 3213 Heavy Machine Shop :-

                      a. Rotor Sloting

                      b. Finish Machining

                      c. Drilling

 4. 3214 Copper Shop :-

                       Manufacturing  of rotor coils.

 5. 3215 Coil Shop:-

                       Manufacturing of exciter coils and core coils.

 6. 3216 Stator Winding Shop :-

                        Manufactured winding of stators layed down in stater here.

 7. 3217 Assembely Shop :-

                        Total assembly of Turbo generator

 8. 3218 Rotor Winding Shop :-

                         Rotor assembly is done here.

 9.3219 Insulation Shop :-

                          Manufacturing of insulation items.

 10. CDC:- Centre Dispatch Cell .

              For handling and transporting the various components over-head crane facilities are available ,depending upon the product manufactured here.There are also a number of self-propelled electrically driven transfer trolleys for inter movement of components/assemblies.

 

 ELECTRICAL MACHINES :-

            The departments involved in manufactue of the Electrical Machines are:

            *  Electrical Machines Engineering .

            *  Technology Department .

            *  Planning Section

            *  Production Department                                                                                                        

            *  Commercial Section

            *  Purchase Section

            Production department is the Block-02 which I am describing here.

 

PHYSICAL ASPECTS TO ELECTROMECHANICAL ENERGY CONVERSION:

       In general terms, an electromechanical energy-conversion device is a link between an electrical and mechanical system.By approximately coupling the two systems,it makes possible conversion of energy from electrical to mechanical form or from mechanical to electrical form.In a device acting as a generator ,the mechanical system delivers energy through the device to the electrical system;in a device acting as motor the electrical system delivers energy through the device to the mechanical system.The process is essentialy reversible,although part of energy is irreversibly converted to heat;any motor can be made to run as a generator,and any can be made to run as a generator and any generator can be made to deliver mechanical power as a motor.

       The coupling between the electrical and mechanical systems is magnetic fields are in general present,and energy stored in these fields is inevitably associated with the energy conversion.The energy in the coupling field of the device may change during the energy conversion process.Infact it may be said that the tendency for the energy in the coupling field to release itself and to do work is the reason for the coupling between the electrical and mechanical systems.

       Electromechanical energy conversion accordingly depends on the existence of the nature in the phenomena interrelating mechanical and electrical fields on the one hand and mechanical force and motion on the other.The principle phenomena utilized practically are the following:

1. A mechanical force is exerted on the current carrying conductor in a magnetic field,and between current carrying circuit by means of their magnetic fields.The energy conversion process is reversible because a voltage is induce in a circuit undergoing motion in a magnetic field.

2. A mechanical force is exerted on a ferromagnetic material tending to align it with or bring it into the position of the densest part of the magnetic field.When a current carrying coil creates a magnetic field,the energy conversion process is reversible because motion of the material will cause a change in the flux linking the coil,and the change in the flux linkages will induce a voltage in the coil.

3. A mechanical force is exerted on the plates of a charged capacitor and on the dielectric material in an electric field;conversely relative motion of the plates and dielectric results in a change of  either the charge or the voltage between the plates or  both.

4. Certain crystals are slightly deformed when voltage gradients are applied in particular directions ,and conversely,when they are deformed ,an electric charge appears.This phenomenon is known as the piezo-electric effect.Although the deformation of the crystal when voltage is applied is small,the associated mechanical force may be very large.

5. Most ferromagnetic material shows a very small deformation under the influence of a magnetic field and,conversly ,the magnetic properties are affected when the materials are strained mechanically .This phenomenon is called magnetostriction.As in the piezoelectric effect ,the full elastic force of the material is available even though the changes in dimensions are small

ROTATING  MACHINES:                                                                                             BASIS OF OPERATION :

   In discussing and analyzing the operation of physical devices, more than one view point can be adopted. The simplest view point leads to quick ,earthy understanding of certain specific features,but it is frequently a restricted one,which does not fully reveal the general properties common to many similar devices.Most sophisticated and generalized usually evolved only after some experience with similar approaches,highlight the general common properties and lead ultimately both to greater in sight and to valuable criterial for optimum operation .All electric machines operate on the same basic principles and only a few fundamental laws govern the behaviour of these machines.A through understanding of these fundamental laws is essential for the study of electrical machines.These  laws are listed below:

 

THE FUNDAMENTAL LAWS:

 

1. FARADAY'S LAW OF INDUCTION

2. KIRCHOFF'S LAW OF ELECTRIC CIRCUIT

3. CIRCUITAL LAW OF THE ELECTRIC FIELD (AMPERE'S LAW)

4. LAW OF FORCE ON THE CONDUCTOR IN A MAGNETIC FIELD (BIOT SAVART'S LAW)

 1. FARADAY'S LAW OF INDUCTION:

            If a magnetic flux linking a closed conducting circuit is changing an electromotive force is induced in the circuit.

           If   represents the flux linking the circuit and d   the change in flux during the time dt, then the magnitude of the induced emf is proportional to the rate of change of flux,d /dt

                                       e  = n d /dt

           The direction of the induced emf is determined by Lenz's law,which states that the induced emf opposes the change in flux.

 2. KIRCHOFF'S LAW OF ELECTRIC C IRCUIT:

 A. KIRCHOFF'S MESH LAW:

       Around any closed loop(or mesh) in a network ,the algebraic sum of all emf's and potential drops is equal to zero.

 B. KIRCHOFF'S CURRENT LAW:

      At any node (or junction),the total current entering the node is equal to the total current leaving the node.Alternatively the algebraic sum of all the currents at a node (or junction) is zero.

 3. CIRCUITAL LAW OF THE ELECTRIC FIELD (AMPERE'S LAW):

      The line integral of the magnetic field strength along a closed path is equal to the sum of the ampere-turns with which this path is linked

 

TURBO GENERATORS

INTRODUCTION TO TURBO GENERATORS :

    The growing use of electrical in the world leads to new  problems concerning the generation of electrical energy. The most frequently used machines, for the generation of electrical power are the synchronous machines.

                                             Faraday's law of electromagnetic induction states that the induced    voltage is proportional to the rate of change of the linkage of the magnetic lines of force.In the case of electrical rotating machinery the part of the machine,in which the voltage is induced is called the armature.The component part of the electrical machine which carries the excited or field winding for the production of the required magnetic lines of force is called the field system.If the field is so provided that the of force after a particular distance change their direction,the field system is said to posses alternating poles.Alternating poles mean that the poles are alternatively north and south poles.The change of flux is caused when the armature moves in a flux set by alternating poles.It is naturally just the same whether the poles moves and the armature winding remains stationary or the poles remains stationary and the armature moves.Almost all alternating current generators are now a days designed with rotating field system.This arrangement is the natural order of things for a few very good reasons:

 

1. The high voltage,high current and therefore high-power handling element is the armature on any AC or DC rotating electrical machine.Armature coils are therefore larger than field coils.

 

2. Since no alternate switching of coil polarities is needed on an AC machine, no commutator function is needed.Thus,the high power windings may be made stationary for direct connection.The universal motor is an exception to this condition.

 

3. The field structure and coils are not ordinarily required to handle more than a fraction of the total power.Thus,their rotating electrical connection may be made smaller.Since no polarity switching is required,collector are usually used.

 

4. The armature and field coils are both placed in slots in the punched magnetic structure,but the stationary armature can be conveniently made to with deeper to handle the required larger coils.

 

5. It is easier to cool the stator than the rotor which is an advantage of the normal AC construction.

 

6. The main reason for designing the alternating generator with rotating field system is that AC generators are designed for high voltages(4-33kV),which are difficult to be tapped from slip rings incase of rotating armature type.For smaller voltages the rotating armature type may be cheaper but in order to achieve uniformity all AC generators are designed with rotating fields.

 

7. It is economical to have armature winding on the stator and field winding on the rotor.In order to illustrate this,consider a 3-phase ,star-connected,200MVA,11kV,synchronous machine.Its line current is 10,500 A.If the armature winding is placed on the rotor,three slip rings each capable of handling10,500 A would be required.Further each slip ring must be properly insulated from the shaft for a voltage of 11/1.732,i.e.6.35 kV.The star-point of the 3 phase winding must also be brought out through fourth slip-ring,in order to connect it to a grounded metal plate through a resistance.

 Assume now that low-power required for the field winding is 1 MW at 500 volts.Then the excitng or field current is 1000/0.5=2000A.Only two slip rings,each capable of handling 2000A,are required.Also each slip ring should be insulated from the shaft for a voltage of 500 volts only.This shows that it is cheaper to have field winding on the rotor rather than the armature winding.Thus the construction of synchronous machine with armature winding on the rotor is much more economical.In addition,a synchronous machine with armature winding on the stator and field winding on the rotor has reduced slip ring losses and is therefore more efficient.

8. Stationary armature windings can be insulated satisfactorily for higher voltages,allowing the construction of high-voltage,say 33kV,synchronous machines.

9. Low-power field winding on the rotor gives a lighter rotor and therefore,low centrifugal forces.In view of this ,higher rotor speeds are permissible,thus increasing the synchronous machine output for given dimensions.

The type of construction used for a synchronous generator depends upon the type of prime mover.Three different types of prime movers very commonly used these days.These are:

1. STEAM TURBINES

2. HYDRAULIC TURBINES

3. DIESEL TURBINES

1. STEAM TURBINES: Steam turbines have efficiency at large speeds and hence synchronous machines driven by steam turbines are high speed.

           We know that

                NS =120f/P

   For a standard frequency 50 cycles/second and with the minimum possible number of poles to,the maximum operational speed of a synchronous machine by turbine alternators is 3000 revolution/minute.In America,however,the maximum operational speecd of such generators is 3600 revolution/minute,as the frequency is 60 cycles/second.Such high speeds call for horizontal shaft of the machine and have to be designed with rather lower values of the diameter of the rotor.The peripheral speed of machine is given by:

                                vp DN/60

    where D is the diameter of the rotor in meters and N is the speed of the rotor in revolutions per minute.Since the peripheral speed increases proportionally with the diameter and  with the increase in peripheral speed the much forces acting on the rotor increase,the diameters are kept low and the rotors have cylindrical shapes.

 2. HYDRAULIC TURBINES:

                  Hydraulic turbines have different forms. The type of hydraulic turbine used as  a prime mover for the generator, depends upon the water head available . If the water head available is high, a pelton wheel is used as a hydraulic turbine . In such a case the hydraulic turbine has a high speed and both the turbine and generator may either be vertical or horizontal, the horizontal type being more frequent. Pelton wheels are used for water heads 400 m and above. Upto a water head of 380 m Francis turbines are used and  in water heads upto 50 m Kaplan turbines , the turbine and the generatorare of the vertical type. Since the water head is not high, the speed of such a prime mover varies from 50 rpm . The type of synchronous machine used with the hydraulic turbines are the salient pole synchronous machine, i.e. synchronous machines with projected poles.

3.DIESEL ENGINES:

                                    Diesel engines, as prime movers are these days used in low rating synchronous machines.They are universally manufactured as horizontal type and therefore both the prime mover and the generator are of horizontal construction . Because of the slow speeds of the diesel engines, the synchronous generator used this type of prime movers are salient pole synchronous machines when driven by a diesel engine. Synchronous machines are very sensitive to the torque variations or swings of the diesel engines.

                                    The report describes the operation and and maintenance aspects of a turbo generators and its auxilasies  along with their constructional features. The machine being developed on the proven line skodal design under collaboration with Czechoslovakia,the operation and the maintenance  procedures are developed based on the manufactures and his collaborators experienced in the field o Turbo Machines

 

SYNOPSIS OF THE FUNCTION OF TURBO GENERATORS:

1. The generator rotor is driven by a prime mover and on driver side gas/diesel/steam /hydro ,depending on the equipment to which it is meant for.  

 2. The non-drive side of the rotor is equipped with a rotating side armature,which produces AC voltage.This is rectified to DC by using a DC commutator/rotating diode wheel depending upon the type of exciter.

 3.The rear end of above exciter armature is mounted with a permanent magnet generator rotor.

 4. As the above rotating system is put into operation,the PMC stator produces AC voltage.

 5. This voltage is rectified by thyristor circuit to DC. 

 6. This supply is given to exciter field.the field is also controlled by taking feedback from main generator terminal voltage,to control external field variations required by automatic voltage regulator.

              The rectfied DC supply out of exciter is supplied to turbo generator rotor winding either through brushes or central lead which will be directly connected to torbo generator rotor winding.This depends on type of the exciter viz.,DC commutator machine or brushless exciter.

 7. The main AC voltage of  generator is finally available at turbo generator stator as indicated in the figure.

              Necessary protection equipment is also however which is not covered in this scope  

 

 

LARGE SIZE TURBO-GENERATORS (LSTG):

 These type of generators are those which has steam turbines as their prime mover and current is supplied by exciter system.Their main types are:

 

1. T  H R I

2. T A R I

3. T H D I

4. T H D D

5. T H D F

6. T H F F

   Here basic terms are;

   

  T =  Turbo generator

  A = Air cooled

  H = Hydrogen cooled

                                                                                                  

  R = Direct radial cooling with gas

  D = Direct axial cooling with gas

  F = Direct axial cooling with water

  I  = Indirect cooling

 

 MAIN PARTS OF LSTG:

  The main parts of Turbo generator are:

   STATOR:

        It includes stator frame,stator core,stator winding, generator coolers,stators end covers,temperature measuring device.

   ROTOR:

        It includes rotor winding ,rotor shaft,field connections,rotor retaining rings,fans and bearings.

   COOLING SYSTEM:

         It includes the cooling circuits required for the effective cooling of the generator.There are mainly 4 types of cooling system depending upon the rating of the generator

    

    *Air cooling system(60MW)

    *Hydrogen cooling(100MW)

    *Water cooling(500MW)

    * Water and hydrogen cooling(1000MW)

 

  EXCITERS:

        Small generators which are coupled to main generators.The purpose of this is to provide the necessary excitation currents.

                                                                                                                                                                

  STATOR

 CONSTRUCTIONAL FEATURE OF STATOR BODY:

 1. STATOR FRAME:                                   

     The stator frame consists of a casting of welded plate construction reinforced internally in the radial and axial direction by web plates making the entire frame perfectly rigid to minimize core vibrations and suitably designed to ensure efficient cooling. Wedge shaped steel guide bars welded to the frame directly support the stator core.

                   End covers, which are made of Aluminium alloy, are bolted to the ends of the frame. In the end covers, are located suction ducts with specially designed guide vanes, to ensure uniformity in fanning suction all around. Ingress of oil vapours into the machine is minimized, by sealing around the rotor and at the dividing planes of the end covers.

 

2.PIPE CONNECTION:

      To attain a good aesthetic look the water connection to gas coolers is done by routing stainless steel pipes inside the stator body which emanate from bottom and emerge out of wall.

            From sidewalls these are connected to gas coolers by means of  U-tubes outside the stator body.For filling the generator with hydrogen ,a perforated manifold is provided at the top inside the stator body.Before filling hydrogen,carbondioxide is filled as hydrogen react to air result in explosive mixture.Thus carbondioxide displace displace air and later it is replaced by hydrogen.Same procedure is adopted when it is required to shut T.G.

3.TERMINAL BOX:

      The bearing and ends of three phases of stator winding are brought out to the slip-ring end of the stator body through terminal bushing in a terminal box.The terminal box is welded construction of (non-magnetic)austenltic steel plates.This material eliminates chances of stray losses due to eddy current,which may result into excessive heating.

  

CONSTRUCTIONAL FEATURE OF STATOR CORE:

 

 CORE:

         The main features of core are:

   *To provide the mechanical support (holding the windings).

   *To carry effeectively the electromagnetic flux generated by the rotor winding.

    *To ensure the perfect linkage between core and rotor.

 

   In order to minimize the hysteresis and eddy current losses of the rotating magnetic flux,which interacts with the core,the entire stator core is made up of segmental,annealed insulated punching of hot rolled high quality silicon steel.These punchings are assembled in an interleaved manner on the machine guide bars and separate into packets of approximately 50mm thick between which are the ventilation ducts.Between two packets one layer of ventilation is provided.Steel spacer are spot welded on stampings.

   The spacers from ventilating ducts where the cold hydrogen from gas coolers enter the core radially inwards there by taking away the heat generated due to eddy current losses.

   The punchings as shown in the figure are stamped from thin sheets of 0.5mm thick and contain open-end slots for the stator bars with dovetail slots for wedges to hold the stator bars in place.Other dovetail slots at the back of the punching are for assembly and locking of the segments on the guide bars.The assembled punchings are clamped into a stiff cylindrical core by pressure applied through steel and clamping plate.Pressure is applied to the teeth by non-magnetic pressuring  fingers,which are bolted to the end clamping plates.In order to reduce end heating from end leakage flux and its associated electrical losses that are occuring at the ends of the stator core,the packets of punchings are stepped back to increase the gap between the punchings and rotor.The insulation is a thermo-setting varnish,which maintains its insulating value at temperatures above the normal temperature range.

  

 OPERATIONS:

A.The Cold Rolled Non Grain Oriented (C.R.N.G.O) steel sheets in the required shapes according to the size of the laminations and then fed into the shearing press for punching and dovetail shaped stator laminations segments.

 B.BLANKING AND NOTCHING:

     The holes and slots are made on segments.Nearly 500 tonnes crank press is used for this purpose.

 

 C.DEBURRING OPERATIONS:

    In this operation the burns in the sheet due to punching are deburred.There are chances of short circuit with in the laminations if the burns are not removed.The permissible limit is about 5 micrometers.For deburring punched sheets are passed under rollers (with required clearance) to remove the sharp burns or edges.

 

D.VARNISH:

    The laminations are now varnished to provide insulation.

 E. Alkyl phenolic varnish is used to provide the insulation between the laminations then it is passed through furnace so that it may get consolated.

 

 

CONSTUCTIONAL FEATURE OF STATOR WINDINGS:

The double layer stator windingis of the involute type and is manufactured from electrolyic copper.It is located in open slots.The coils are pre formed from copper strips as half coils.The winding is composed of insulated half coil bars assembled in the stator slots in two layers and are brazed together at each end to form coils.They are in turn suitably grouped into proper phases,using bus type connector rings supported on end brackets.The stator bars are composed of insulated copper strands transposed by rouble method so that the eddy current  losses are reduced and to ensure proper distribution of flux.A futher partial transposition is carried out in the end windings.These arrangement s avoid circulating current losses which would otherwise be present under load conditions due to distribution of magnetic flux in the slot.

  The bars are insulated with mica based insulation.This insulation has excellent electrical properties,and does not require impregnation.Its moisture observation is extremely low.And outer covering of asbestos tape is applied to protect the abrasion in the slot.In order to minimize the effect of Corona slightly conductive coating (containing graphite) is then applied to the bar in the slot portion and semi conductive coating is applied to a certain length on the overhang portion of bars.An advantage of the involute coil winding is that end winding is at an angle to the stator core endthus minimizing the stray losses in the stator core supporters.

 

END WINDING:

   The end winding i.e.the overhang portion has being throughly reinforce against the short circuit forces with epoxy glass laminated spacers and by further binding on to the fastener blocks-which in turn are screwed to the non-magnetic damper ring bolted to the stator frame.This solid copper damper ring one on either side considerably reduces reduces heating on the end iron

 

RESISTANCE TEMPERATURE DETECTORS:

     The stator slots are provided with platinum resistance thermometers to record and watch the temperature of the stator core,tooth region and between coil sides of machine in operation.According to VDE 05 30/7,55 all AC machines rated for more than 5 MVA or with armature cores longer longer than 1 metre is to be provided with at least 6 resistance thermometers or thermocouples,which shall be built inside the stator, suitably distributed around the circumference at the likely hottest points.In the case of longer lengths it is to recommended by the specification to distribute the resistance thermometers along the length.The thermometer should be fitted in the slot but outside the coil insulation.When the winding has more than one coil side per slot the thermometer is to be placed between tw insulated coil sides.According to the same specifications the length of the resistance thermometers depends upon the length of the core of the armature.

 

             LENGTH  OF  THE                      LENGTH  OF  THE

           CORE METER                              RESISTANCE

             (IN METER)                                 THERMOMETER               

                                                                          (IN CM)

                      Upto 0.5                                                           12.5

                    Over 0.5 to 1.0                                                     25.0

                     Over 1.0                                                            50.0

 

The leads from the detectors are brought out and connectes to the terminal board for connection to temperature meters or relays.

The resistance temperature(RTD) is a resistanc element.Operation of the RTD is based on             the principle that the electrical resistance of a metallic conductor varies linearly with its temperature.The resistance wire of these detectors are moulded into a fiber glass strip approximately 2 mm thick and trimmed to slot width.All moulded strip detectors are non-inductively wound to cancel the extraneous voltage induced in them.Thus,by measuring resistance(usually incorporating quick and fairly accurate Wheatstone's bridge)temperature can be computed from calibration curve supplied with the test report.

Thermometers for the remote measurement of temperatures of air at inlet and outlet(i.e. cold and hot) have also being provided with in the stator frame.

 

DESIGN FEATURE OF STATOR:

1.Overhang portion genuinely shifts to support rings,where it rests.

 2.End packet of core stampings are put in steps.

 Simultaneously a cut is provided on these stamping.The main motto behind this is to reduce eddy current losses as at ends leakage flux is maximum.

3.Forming angles of all stator bars are different.Thus it is possible to connect them in given particular fashion.End bars have got different lug to take phase connection out it.

4.One design factor is chording of the pole windings.If in the 36-slot,4-pole machine,an individual coil enters slot 1 and comes in slot 10,it will have spanned 90 mechanical degrees of the stator circular structure.In this case 90 mechanical degrees is 180 electrical.Thus the two sides of the coil are in the same relative position on the adjacent north and south pole positions.This is a full-pitch coil constuction.

The more usual AC machine coil will cover less of the periphery of the machine and is then said to be fractional pitch.A typical situation might have a coil enter slot 1 and leave slot 7.This then covers six out of a possible nine slot pitches,and is a 6/9 or 66.7% pitch.The majority of AC machines are of fractional pitch type,for which there are a few important advantages:

*The ends of the coils are shorter,which means less copper loss due to less total length.

*The end coils can be formed more compactly.The end bells will need less winding space,resulting in a shorter unit.

 *There is a distinct reduction in machine harmonics due to cancellation of  higher harmonics.Since all AC equipment is designed to operate on a pure sine wave,the generation of harmonics is to be avoided.This is especially so when the factor that achieves it is otherwise desirable.

 

*TRANSPOSITION OF STATOR WINDINGS:

Where the current by conductor necessities a large cross-section to be used,the liability to excessive eddy loss and heating makes it essential to subdivide the conductor into strips.This is due to fact that the conductors or parts of the conductors nearer the top of the slots have lesser self-inductance that those nearer the bottom of the slot.Hence,the currents tends to flow in the top portions of each conductor.To prevent such unequal distribution of current the large conductors in alternator armature are stranded ,and each strand is insulated with enamel.Each conductor is made up so that all strands occupy top,intermediate and bottom positions for equal distances.This is done by twisting or transposition of coils.In a long-cored turbo-alternator,the twisting may be carried out three or four times in a single slot.The effects of transposition are the same as twisting the bundle of strands 180 degree or 360 degree.This is accomplished without substantially increasing the width of strands.For a multi turn coil 180-degree transposition usually suffices.For a single turn coil,because of its greater depth at 360 degree transposition may be necessary.The effect is to equalise the eddy e.m.f. in all the laminations and to allow the layers to be paralled at the ends without producing eddy circulating currents between the layers.The cross-over is obtained by special shaping of conductor.This kind of transposed conductor is called ROEBEL BAR

  Another type of transposition provides 180-degree twist of the strand bundle in the end turn position of the winding where the increase width dimensions can be tolerated.The group of strands can be insulated from adjacent groups through out the coils inclided in transposition.

 

END COVERS:

 End covers of fabricated steel or aluminium alloy castings are employed with guide vanes on inner side for ensuring uniform distribution of cooling air or gas.

 In case 1500 rpm generators,end windings are first enclosed in glass epoxy moulded end covers and an overall steel outer cover is provided over the stator.

 

 

ROTOR

CONSTRUTIONAL FEATURE:

In Turbo Genertator there are two types of  rotor designs:

 

* Forged solid rotor

* Laminated rotor

 

The main function of the rotor is that the rotor acts like Electromagent by taking currents from the excitation systems. It is coupled to Turbine rotor to take the driving torque and thus produce  a flux,which leads to the generation of currents.The changes in the flux produced by the rotor windings induces voltage in the stator winding.It also has top withstand high power torque produced by the by the turbine and the backward torque produced due to the generator current.For large capacity generator is forged from a single piece ingot of steel alloyed with nickel,chromium,maganese and vanadium,heated to obtain the required mechanical and magnetic properties.The forgings must be homogeneous and flawless.The forging undergoes a series of stringent quality control test like metallographic,ultrasonic and heat stability etc.before it is accepted for further processing.

      The rotor forging is planed and milled to form the teeth.About 2/3 of the rotor pole-pitch is slotted,leaving 1/3 unslotted (or slotted to lesser depth) for the pole centre.Longitudinal slots are machined radially in the rotor body to accomdate field winding.The slot pitch is selected in such a way that two solid poles displaced by 180 degree are obtained.

     Additional dummy slots and sub-slots (under the main slots) provide adequate passage for the cooling air for rotor body and winding.

     Rotor shaft is subjected to assembly section in dust proof enclosure.In this section we mount insulation, winding damper,connecting lead and retaining ring on the shaft with great accuracy.They are described here:

 

1. ROTOR SHAFT:

    Details of shaft are given here

                             Length             :-                  9 m (app)

                           Diameter           :-                  1 m (app)

                           Material             :-                 Alloy steel

                         No. of poles          :-                       2

 

2. ROTOR WINDING:

 Rotor coils are of multi-turn type,manufactured from semi-hard, silver alloyed copper of rectangular cross-section,with ventilation holes punched in it.Two individual conductors are placed one over the other are bent to obtain half turns.Further these half turns are brazed in series to form coil on the rotor model.

  The solid poles are provided with additionalslots used to accomdate finger of damper segment acting as damper winding.

 

3. INSULATION:

The individual turns are insulated from each other by large layer of glass prepag strips on turn of copper and backed under pressure and temperature to give a monolithic interturn insulation.Overall winding insulation with respect to rotor body is provided by epoxy glass laminated troughs located in the slots.

 At bottom of slot D-shaped liners are put to provide a plane-seating surface for conductors and to facilitate easy flow gas from one side to another.These liners are made from moulding material.The overhang portion are separated by glass laminated blocks called liners.The overhang winding atre insulated from retaining ring segment having L-shaped and made of glass cloth impregnated by epoxy resin.

 

4. ROTOR WEDGE:

The rotor slot wedges are used to protect the windings against the effect of centrifugal force.This slot wedges are made of an alloy featuring high strength and good electrical conductivity.They extend below the shrink seals of the retaining rings.They are short-circuited under retaining for short circuiting induced shaft current.The slot closing wedge laminate which also have drilled holes.Thus the cooling air entering sub-slots radially comes out through these holes, ensuring effective cooling of rotor.

 

5. RETAINING RING:

 The overhang portion of field winding is held by non-magnetic steel forging of retaining ring against centrifugal force.They are shrink fitted to the end of the rotor bady barrel at one end,while the other side of retaining ring does not make contact with the shaft.

 The centering rings are shrink fitted at the free end of retaining ring that serves to reinforce the retaining ring, securing end winding in axial direction at the same time.

 The retaining rings are made of non magnetic alloy steel forging with very high mechanical properties.This material is stress and corrision resistant.The material is worked to reduce stray losses.

 

6. FANS:

 Two axial fans located at the two ends of rotor shaft circulate the cooling in the generator.The blades are aluminium alloy die forgings.Threadings is made in the fan blades roots such that they can be fixed into the threaded pools provided on the rotor.The fans circulate required quantity of cooling air at required head to overcome the pressure drop inside the generator.

 

7. EXCITATION LEAD:

 The excitation lead provides electrical connections between rotor windings and output from brushless exciter.A flexible copper connector electrically joins the rotor windings to the excitation leads through radial current carrying studs.The radial stud is screwed to field lead in the shaft bore through a hole drilled radially in the shaft.The field leads run in the axial direction from the radial stud to the end of the rotor.They consist of two semi circular conductors insulated from each other by an intermediate plate and from the shaft by tube.The ends are suitably drilled to accomdate multi contact pins to receive output from the exciter.

 

8. BEARINGS:

 Bearings play an important role in the operation of the machines.The bearing should not develop any defect even continuous operation for years together and also should not show wear-outs.In the case of turbo-generators plain-bearings with rings for oil circulation are used.Large turbo generators have bearings lubricated by oil circulating under pressure.The oil flowing out of the bearingis cooled by means of water coolers and after cooling the same oil is pumped in.The bearing must be oil-tight as any leakage of oil from the bearing has the possibility of entering the machine,spoiling its insulation and resulting in shut-down.

To prevent damage due to shaft currents,bearings and oil pipings on either side of the non-drive end bearings are insulated from the foundation frame.For facilitating and monitoring the healtiness of bearing insulation,split insulation is provided.

 

9. SLIP-RINGS:

Excitation current to the rotor winding is supplied through 2 shrunk fit slip rings located beyond the main pedestal bearing on the exciter side.These are made of steel,adequately insulated from rotor body using mica strips insulation.Actual excitation leads from slip rings to the winding are carried with in the shaft bore and are provided with adequate insulation of class'B' type.For better heat dissipation the surface of slip rings has been helical machined.Self-ventilation arrangement has been for slip rings.

 

10. BRUSH GEAR:

 In order to effectively supply the excitation current to the rotor winding,the brush gears are robustly designed with radial type brush holders.Brushes of natural graphite composition with low co-efficient of friction and self lubricating properties are used,which ensure smooth and trouble free operation.The brush gear free operation.The brush gear is either fixed on to the end of the stator end covers or bearing or is mounted separately on a pedastal.

 Depending on the requirements,generators are provided with on-load brush changing gears.The advantages of this arrangement are:

 *Constant spring tension over the entire brush wear.

*Brush change-over without stopping the machine.

*Safety of the personnel due to provision of insulated handle.

 

 

SPECIAL FEATURES  OF ROTOR

1. Rotor slots are made tapped to increase mechanical strength of teeth part.

2. Windings are connected in particular fashion to create opposite poles which is shown here.

3. In order to minimize windage losses,which tend to be high at turbine,the surface of the finished rotor are made as smooth as possible.

 

EXCITER

 The basic use of given exciter system is to produce necessary DC for turbo generator system.Principal behind this is that PMG is mounted on the common shaft which generates electricity and that is fed to yoke of main exciter.This exciter generates electricity and this is of AC in nature.This AC is converted into DC and than fed to turbo generator via c.c.bolts.For rectification purpose we have got RC block and diode circuit.

 The most beatiful feature of this type of exciter is that it automatically decides the magnitude of current to be circulated in rotor circuit.This happens with the help of A.V.R. system ,which means automatic voltage regulator.A feedback path is given to this system which compares theoritical value to predetermined and then it sends the current to rotor as per requirement.

 

                                                         TESTING

1. TEST PROCEDURE FOR HIGH VOLTAGE TEST ON STATOR   WINDINGS OF TURBO GENERATOR:

 

SCOPE:

    This procedure covers the high voltage test on stator winding.

 

PURPOSE:

    This test is conducted to check whether the insulation is properly placed or not. The insulation placed over the windings should be in such a way that they make half overlaps with the next wrapping of the tape.

 

THIS TEST IS EXCLUSIVELY CONDUCTED TO TEST THE HEALTHINESS OF INSULATION OF WINDINGS.

 

2.TEST PROCEDURE FOR HIGH VOLTAGE ON ROTOR WINDING OF TURBO GENERATORS:

 

SCOPE:

    This procedure covers the high voltage test on rotor winding.

 

PURPOSE:

     This test is conducted to check whether the insulation is properly placed or not.

 

3.TEST PROCEDURE FOR MEASUREMENT OF DC RESISTANCE OF STATOR AND ROTOR WINDINGS OF TURBO GENERATORS:

 

SCOPE:

     This procedure covers the measurement of dc resistance of stator windings.

 

PURPOSE:

     This test is conducted to measure the resistance content present in the conducting material of stator and rotor.

 

TEST PROCEDURE FOR MEASURMENT OF IMPEDANCE OF ROTOR WINDING WITH 5O AND 500 Hz SUPPLY:

 

SCOPE:

      This procedure covers the measurement of impedance of Rotor winding 50 and 500 Hz source.

 

TEST EQUIPMENT:

1.50Hz (Power frequency)/500Hz AC source.

2.AC voltmeter (0-30,75,150V and 0-150,300,450V)

3.AC Ammeter (0-5 A)

4.Multimeter

5.Current transformer (50/5 A or 100/5 A)

6.Connecting leads

 

CALCULATIONS:

 

                         Z = V/I   OHM

 

Where          Z = Impedance in ohms

                    V = Voltage in Volts

                     I = Current in Amperes

 

With 50 Hz,this test is done in following conditions:

 1. Rotor outside the stator at standstill.

 2.Rotor inside the stator at standstill.

 3.Rotor inside the stator at speeds 1/3, 2/3, 3/3 of rated speed.

 500 Hz source is used only during process testing.

 

ACCEPTANCE LIMITS:

 IMPEDANCE WITH 50 Hz:

  As the impedance of the rotor winding depends on physical geometry of the rotor,it is not possible to fix at particular value for acceptance.But a peculiar characteristic that impedance increase with increase of voltage can be assured.Value of increase is less in cylindrical rotors than in salient pole rotor.

 IMPEDANCE WITH 500 Hz:

 This measurement is only to observe the voltage drop across each pole.Difference in voltage drops upto 2V is acceptable across two identical poles.

 

5.TEST PROCEDURE FOR MEASUREMENT OF LEAKAGE REACTANCE OF TURBO GENERATOR STATOR WINDING:

 

SCOPE:

      This procedure covers the leakage reactance measurementb of stator winding after completion of winding.

 

PURPOSE:

1.To determine total leakage reactance(xb)

2.To determine bore leakage reactance(xa)

3.To compute potier reactance(xd)

 

6.TEST PROCEDURE FOR CAPACITANCE AND TAN$ MEASUREMENT OF STATOR WINDINGS:

 

SCOPE:

This procedure covers the measurement of capacitance per phase with respect to ground and tan$ of the stator winding.

 

PURPOSE:

To generate refernce value for comparison in future at site.

 

PRINCIPLE:

The stator windings have two values of capacitances

1. Cg :Capacitance with respect to ground called ground capacitance

2. Cm:Capacitance with respect to other windings called mutual capacitance.

The measurement of capacitance is done using Schering Bridge and a Standard capacitor.

 

 

 

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