Dynamoelectric machine with flux screen

Laminated flux screens disposed between finger plates and end plates of a generator to reduce the axial flux striking the end region of the generator core.

BACKGROUND OF THE INVENTION 
This invention relates to a flux screen for a dynamoelectric machine and 
more particularly to a flux screen for the end region of a large 
generator. 
The increasing size of large turbine generators has led to higher losses in 
the ends of the laminated stator core. Unless appropriate design 
innovations are incorporated into this region, these higher losses will 
lead to increased temperatures. Normally, these increased temperatures 
will not adversely affect laminations that have been coated with inorganic 
insulation. However, these same temperatures in the vicinity of the stator 
coil can cause a reduction in the life of the stator coil insulation. If 
not properly controlled by design, this could limit the range of leading 
power factor operation of large generators. 
Historically, end iron heating was resolved in the stator tooth area by 
splitting the last few inches of core iron and by stepping the tooth end 
packs. Providing radial cooling ducts in the ends of axially ventilated 
stator cores has proven to be satisfactory for existing ratings and 
machine sizes. However, the combination of several slits and radial ducts 
in the end pack region of the stator has the adverse effect of 
mechanically weakening the core end structure. 
Other schemes had utilized flux shields in the tooth portion of the stator 
core. These eddy current shields are typically copper plates or loops 
situated directly in front of the stator teeth. They are conductive 
members of low magnetic permeability arranged to provide circulating 
currents which divert stray flux away from the tooth region. For highly 
rated machines, the currents induced in this type of shield produces 
extremely high losses, many times higher than that normally seen in the 
stator end iron. In most cases, liquid cooling of the shields may be 
required. 
Additional losses in the stator core end packs result from axial magnetic 
flux caused by the rotor and stator end winding currents. Under normal 
machine operation, the terminal voltage is produced by magnetic flux which 
travels radially across the air gap and is distributed uniformly over the 
length of the core. In the end regions, this flux fringes from the ends of 
the rotor body to the stator end packs. This fringing flux enters the end 
packs at right angles to the plane of the lamination of the core. The loss 
generated by the ensuing eddy currents is considerably higher than the 
losses due to the same flux density entering parallel to the laminations. 
The net fringing flux is a vector sum of both the rotor and the armature 
components, varying in magnitude with both load and power factor. This 
flux continues to penetrate axially into the end pack until it can turn 
radially and join the peripherally traveling main synchronous flux system. 
The flux screen hereinafter described assists the fringing flux traveling 
to the periphery and joining the main synchronous flux system. 
SUMMARY OF THE INVENTION 
In general, a dynamoelectric machine having a stator and a rotor disposed 
within this stator with a gap therebetween, when made in accordance with 
this invention comprises a plurality of laminations forming a core. The 
core laminations have slots disposed to extend radially outwardly from the 
radial inner margin. The slots are aligned to form axially extending 
grooves and teeth. Conductors are disposed in the grooves and extend 
axially therein and beyond the grooves. The dynamoelectric machine further 
comprises finger plates having fingers which extend over the teeth and fit 
between the conductors for transmitting compressive forces to the core 
laminations including the teeth, end plates disposed outboard of the 
finger plates for transmitting compressive forces to the finger plates, 
and flux screens disposed between the end plates and the finger plates. 
The flux screens are formed of laminations having digital portions which 
extend radially inwardly over the fingers of the finger plates. The 
radially inner margins of the flux screen laminations are stepped so as to 
ascend radially and axially outwardly from the inner margin thereof, 
whereby the teeth of the core are shielded from fringing flux which would 
enter the ends of the core region generally perpendicular to the plane of 
the core laminations.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawings in detail and in particular to FIG. 1 there 
is shown the end region of a dynamoelectric machine such as a large 
generator which comprises a stator 1 having a core 3 and a rotor 5 
encircled by the core 3 with a gap 7 disposed therebetween. 
The core 3 comprises a plurality of sheets or laminations 9 electrically 
insulated with a surface coating and stacked on building bolts 11 and 
firmly clamped together by insulated through bolts 13 which apply pressure 
through non-magnetic finger plates 15. The core laminations 9 have a 
plurality of slots disposed to extend radially outwardly from the radially 
inner margin. The slots are aligned to form an axial groove or slot 17 for 
conductors 19 which extend axially through the core and therebeyond and 
are connected at the ends by end turns 21 to form the stator windings. The 
conductors 19 are held in the slots 17 by slot wedges (not shown) between 
the slots 17 are portions of the laminations called teeth 22. The finger 
plates 15 have fingers 15f, which extend over the teeth 22 and fit between 
the conductors 19 in order to transmit compressive forces to the teeth 
portions 22 of the lamination 9. 
Outboard of the finger plates 15 are end plates 23, end shields 25 and core 
support ring 27. The end shields 25 are made from a plurality of 
laminations which are stacked to form steps 29 which ascend radially and 
axially outwardly as they extend from the core 3. 
The end of the core laminations 9 are also disposed to form steps 31, which 
ascend radially and axially outwardly. 
The rotor 5 comprises a shaft 33 having a plurality of axial grooves 35 
extending radially inwardly from the outer periphery. Conductors 37 are 
disposed in the axial grooves 35 and are retained therein by rotor wedges 
39. The conductors 37 extend beyond the grooves 35 and are connected by 
rotor end turns 41 to form the field windings. A retainer ring 43 
encircles the rotor end turns 41 to support them against centrifugal 
forces induced by rotation of the rotor 5. 
Flux screens 51, as shown in FIGS. 2, 3, and 4 are disposed between the end 
shields 25 and finger plates 15 and comprise a plurality of laminations 53 
disposed in stacks 55 and 57 and having digital portions 59 which extend 
radially inwardly over the fingers 15f of the finger plates 15 and between 
the conductors 19. Two stacks 53 and 55 are shown, however, multiple 
stacks could be utilized with a cooling duct 61 disposed between adjacent 
stacks. The radially inner end of the digital portions 59 are stacked to 
form steps 63, which ascend radially and axially outwardly from the 
radially inner end of the core 3. 
The laminations 53 are bent so that at least a portion thereof is bent to 
form an angle with the plane of the core laminations 9. The bend is such 
that the laminations 53 generally conform to the outer surface of the 
finger plates 15. The laminations 53 have at least one and in the 
embodiment shown have 3 slots 65 which extend radially outwardly from the 
radially inner margins thereof. 
A clamping plate 67 is disposed on the axially outer side of the digital 
laminations and cooperates with insulated bolts 69 or other fastening 
means to compress the laminations 53 against the finger plates 15 the 
latter being drilled and tapped to receive the bolts 69. 
Auxiliary vent holes 71 are disposed to align with axial vent holes in the 
core 9 to improve cooling. 
The screen laminations 59 are made of a high permeable magnetic material 
and are coated with an insulating material so that they are electrically 
insulated from each other. 
FIGS. 5 and 6 schematically show the fringing flux in the end region of a 
dynamoelectric machine which does not incorporate flux screen 51 and FIG. 
7 schematically shows the fringing flux in the end region in a 
dynamoelectric machine which incorporates the flux screen 51. The flux 
screen 51 hereinbefore described minimizes the amount of axial flux which 
strikes the end of the stator core in the tooth region generally 
perpendicular to the plan of the stator laminations 9 and simplifies and 
mechanically improves the stator core construction by reducing the number 
of slits in the end teeth to a minimum and the axial distance that the 
teeth are slit is also drastically reduced. The need for radial vent ducts 
in the end portion of the core can be either reduced or eliminated, thus 
resulting in a mechanically sound end region.