Closed channel axial vent for radially ventilated generator rotor

An axial vent channel (30) for installation in a generator rotor winding slot, the channel (30) being in the form of an elongate, closed tube of polygonal cross section enclosing a longitudinal passage (32), the tube being formed to have a closed bottom wall and an opposed, substantially closed, top wall provided with a plurality of radial vent slits (36) spaced apart along the longitudinal passage (32), the channel (30) being configured to be installed in the rotor winding slot with the passage (32) extending parallel to the axis of the rotor, the bottom wall facing toward the axis of the rotor and the top wall facing away from the axis of the rotor.

BACKGROUND OF THE INVENTION 
The present invention relates to ventilated rotors for generators, and is 
particularly concerned with structures for creating ventilation paths in 
such rotors. 
Generator rotors are provided with windings to which a current is applied 
in order to create the magnetic flux required in order to generate output 
power. Typically, the windings are constituted by copper straps which are 
installed in slots formed in the rotor circumference, or winding face, the 
straps being connected together at their ends to form the windings. The 
flow of current through the windings results in the generation of heat 
which must be dissipated. Efficient heat dissipation enables higher 
current levels to be carried by windings in a generator having a given 
size. 
According to one known technique for effecting such heat dissipation, 
cooling gas is caused to flow axially beneath each stack of conductive 
straps, and is then directed radially outwardly via spaced radial vents 
formed in the straps, as well as in insulators disposed between the 
straps. This is known as radial path rotor ventilation. 
For a given current level, the rate at which heat evolves decreases as the 
effective current flow cross section of the copper conductors increases. 
Conversely, the larger the effective cross sections of the various vent 
passages, the better the cooling. Thus, for a given rotor slot 
cross-sectional area, the desirability of installing copper conductors 
having a large cross section in the direction of current flow conflicts 
with the desirability of providing as large a cross section as possible 
for the flow of cooling gas. 
Various arrangements known in the art seek to resolve these conflicting 
considerations with varying degrees of success. One known arrangement is 
illustrated in FIG. 1 and includes a slot cell, or liner, 2 of insulating 
material which is configured to fit snugly in an axially extending rotor 
slot with a bottom portion 3 which rests against the slot bottom, and 
which extends axially along the entire length of that slot. Within cell 2 
there is disposed a channel 4 of copper which is formed by bending a flat 
strip into approximately a C-shape, so that the upper surface of channel 4 
is open along its entire length. Upon channel 4 there is disposed a stack 
composed of copper straps 6 and 8, with an insulating strip 10 being 
interposed between successive copper straps. In practice, a larger number 
of copper straps 8 and insulating strips 10 than illustrated will be 
provided in each rotor slot. 
Copper strap 6 rests directly on channel 4 so that channel 4 and strap 6 
act together as a single conductor. Straps 6 and 8 and strip 10 are 
provided with mutually aligned radial vent passages 12 for radially 
conducting cooling gas which initially flows axially along the passage 
defined by channel 4. 
The structure shown in FIG. 1 requires that strap 6 be relatively thick to 
assure that it does not buckle during rotor assembly. Since each 
conductive strap 6, 8 will carry the same current level, it is desirable 
for all of the conductive straps to have the same current flow cross 
section and heat generation will be minimized by making those cross 
sections as large as possible. However, for a rotor slot having a given 
size, the larger the conductive straps, the smaller the gas flow passage 
provided by channel 4. Therefore, in order for the passage provided by 
channel 4 to have the necessary cross-sectional area, strap 8, as well as 
the straps thereabove, must be made thinner than strap 6. As a result, the 
rotor will operate at higher temperatures and will require greater input 
power. 
A second prior art approach is illustrated in FIG. 2. Here, each rotor slot 
has a channel 14 milled in its bottom to provide an axial flow path for 
cooling gas. Upon channel 14 there is placed a relatively thick insulating 
creep spacer 16 which supports slot cell 18, of insulating material, and a 
stack composed of copper straps 20 and interposed insulating strips 22. 
Here again, the number of straps 20 and strips 22 provided in practice 
will be greater than that illustrated in FIG. 2. All straps 20 and strips 
22 are provided with mutually aligned vent passages 24 for radially 
conducting cooling gas which initially flows axially through channel 14. 
The structure illustrated in FIG. 2 includes a considerable amount of 
insulation, thereby reducing the space available for conductive straps. 
Moreover, there is a considerable danger, with this structure, that the 
high velocity cooling gas traveling axially through channel 14 will cause 
flaking of material from spacer 16, resulting in the danger of blockage of 
the radial vents, which will cause thermal imbalances. Moreover, in order 
to create the required radial vent passages, holes must be punched in the 
bottom of slot cell 18, and this compromises the structural integrity of 
that component. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to eliminate, or at least 
minimize, the drawbacks of known generator rotor ventilating systems. 
Another object of the invention is to achieve an improved compromise 
between winding conductor cross-sectional area and cooling gas flow area. 
A more specific object of the invention is to provide a novel axial vent 
channel component which makes available, in a rotor slot of given size, a 
relatively large area for installation of conductive straps, while 
providing an axial vent passage having a substantial cross-sectional area. 
The above and other objects are achieved, according to the present 
invention, by an axial vent channel for installation in a generator rotor 
winding slot, the channel being in the form of an elongate, closed tube of 
polygonal cross section enclosing a longitudinal passage, the tube being 
formed to have a closed bottom wall and an opposed, substantially closed, 
top wall provided with a plurality of radial vent slots spaced apart along 
the longitudinal passage, the channel being configured to be installed in 
the rotor winding slot with the passage extending parallel to the axis of 
the rotor, the bottom wall facing toward the axis of the rotor and the top 
wall facing away from the axis of the rotor.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 3 shows a preferred embodiment of the invention, which is constituted 
by an axial vent channel 30 having a generally trapezoidal cross-section 
which encloses an axial vent passage 32 and is essentially completely 
closed around its periphery, except for a succession of radial vent slits 
36 spaced apart along the length of channel 30, the length being 
perpendicular to the plane of FIG. 3. One such slit 36 is shown in FIG. 4. 
Channel 30 is inserted in a slot cell 38 of insulating material and is 
surmounted by a stack composed of insulating strips 40 alternating with 
copper rotor winding straps 42. Here again, the number of insulating 
strips 40 and winding straps 42 employed in practice will be greater than 
that illustrated. 
Insulating strips 40 and copper straps 42 are provided with slits aligned 
with slit 36 to form radial vent passages 44. 
The channel illustrated in FIG. 3, because of its essentially closed 
structure, allows the amount of electrically inactive insulating material 
provided in the rotor slot to be minimized while providing secure 
mechanical support for the stack composed of insulating strips 40 and 
copper straps 42. The support provided by channel 30 allows all of the 
copper straps 42 to be given the same current flow cross section. 
Moreover, substantially closed channel 30 eliminates the possibility that 
high velocity cooling gas flowing through passage 32 can cause flaking of 
insulating material. 
Further, this channel structure eliminates the need to punch holes in the 
bottom 45 of slot cell 38. 
Channel 30 can be readily constructed by extrusion of a single copper 
billet, followed by cold working to retain desired mechanical properties. 
Radial slits 36 can be subsequently be punched in the channel by means of 
mating male/female dies. 
According to a further feature of the invention, slits 36 are given the 
elongated shape shown in FIG. 4, whereby their dimension along the length 
of channel 30 is substantially greater than in the direction transverse 
thereto. Since the slits formed in copper straps 42 will have the same 
size and configuration as slits 36, the result is that the presence of the 
slits in straps 42 will produce only a minimal reduction in their current 
flow cross section, while the length of slit 36 in the axial direction of 
channel 30 enables the resulting radial cooling gas vent paths to be given 
a sufficiently large cross section. By way of example, a channel 30 for 
installation in a rotor having an axial length of the order of 91 cm 
(36"), channel 30 having the same length, can be provided with slits 36 at 
a spacing, along the rotor axis, of the order of 3.53 cm (1.39"), and each 
slit 36 can have a length, in the axial direction of the rotor, of the 
order of 2.16 cm (0.85") and a width of the order of 0.25 cm (0.1"). 
An exemplary embodiment of channel 30 having a generally trapezoidal 
cross-section is shown in FIG. 5, and could have the following dimensions, 
which are given by way of non-limiting example, for the case where channel 
30 is to be installed in a rotor having an axial length of 91 cm, channel 
30 having the same length: 
H=2.92 cm .+-..0127 (1.150".+-.0.005) 
W1=2.637 cm .+-..0127 (1.038".+-.0.005) 
W2=0.635 cm (0.25") 
W3=1.81 cm .+-.0.0127 (0.712".+-..005) 
A=60.degree. 
R1=0.163 cm 
R2=0.15 cm (0.06".+-.0.016) 
R3=0.27 cm .+-.0.041 (0.106".+-.0.016) 
T=0.229 cm .+-.0.0127 (0.090".+-.0.005) 
Cross-sectional area of passage 32=4.12 cm.sup.2 (0.639 in.sup.2) 
Cross-sectional area of metal forming channel 30=2.34 cm.sup.2 (0.362 
in.sup.2) 
As shown in FIG. 5, the upper extremities of the side walls of channel 30 
are provided with thickened portions 46, delineated by width dimension W2 
and angle A, which assure that the channel top wall 48 will provide the 
required support for the stack composed of insulating strips 40 and 
winding straps 42. 
In further accordance with the invention, the axial vent channel can serve 
as a rotor winding conductor, thereby maximizing the utilization of each 
rotor slot. For this purpose, each channel end can be dimensioned to 
project a short distance beyond the axial end of the associated rotor body 
and, as shown in FIG. 6 for one channel end, the top wall 48 of channel 30 
can be cut away, for example, down to the level of thickened portions 46, 
to provide a seat 49 for the end of a copper strap 50 which forms a part 
of a winding end turn. Strap 50 is conductively secured to top wall 48 and 
thickened portions 46, for example by brazed joints 52. According to an 
exemplary embodiment of the invention, the length of strap 50 which 
contacts thickened portions 46 may be of the order of 5 cm. 
The novel peripherally closed channel according to the present invention 
provides an axial vent passage for a radial path rotor which, for a given 
rotor slot size, provides maximum space for the copper winding straps, 
thereby allowing the current flow cross-sectional area of the straps to be 
maximized and reducing the required exciter power. Moreover, the novel 
channel according to the present invention reduces the amount of 
insulation required by known arrangements, while eliminating the need for 
punching holes in the slot cell. 
While the description above shows particular embodiments of the present 
invention, it will be understood that many modifications may be made 
without departing from the spirit thereof. The pending claims are intended 
to cover such modifications as would fall within the true scope and spirit 
of the present invention. 
The presently disclosed embodiments are therefore to be considered in all 
respects as illustrative and not restrictive, the scope of the invention 
being indicated by the appended claims, rather than the foregoing 
description, and all changes which come within the meaning and range of 
equivalency of the claims are therefore intended to be embraced therein.