Cooling arrangements for rotating electrical machines

A rotating electrical machine has openings (105) in its stator teeth (101) occupying a substantial part of the surface area of each tooth so that the stator windings (103) have only a short thermal path to axial cooling ducts created by the openings. In another embodiment, each rotor tooth (109) is provided with a recess (110) at one corner of the tooth to form a recess axial cooling duct; the recess may be of such a depth as to expose part of an adjacent slot (118) to the recess duct. Cooling is by air, either in a single-ended or a double-ended axial ventilation system.

BACKGROUND OF THE INVENTION 
The invention concerns axial-flow cooling arrangements in rotating 
electrical machines, and in particular, though not exclusively, axial-flow 
cooling arrangements in a large cage induction machine employing 
exclusively air cooling of the stator and rotor. 
Many large rotating machines employ what is known as radial ventilation for 
cooling the stator and rotor. An example of this is shown in FIG. 1. In 
FIG. 1, a squirrel cage induction motor 10 comprises a stator core 11 and 
a rotor core 12, each having a number of sections 13 and 14, respectively. 
Both cores are made up of a large number of laminations. The stator core 
11 is firmly attached to a housing 15 and the rotor core 12 is secured to 
a shaft 16 which may drive a load (not shown). The stator core 11 is 
provided with a 3-phase winding 17, while the rotor core 12 is equipped 
with solid aluminium or copper rotor bars 18. The rotor bars 18 are 
shorted together in end rings, one at each end of the rotor. The stator 
core 11 and rotor core 12 are provided with radial ducts 19 through which 
air is forced in order to cool the stator and rotor. 
Air inside the machine 10 is made to circulate round the machine by a fan 
20 secured to the shaft 16. The air is drawn in through a number of ducts 
21 situated between the rotor core and the shaft 16, and at the same time 
through an airgap 22 between the stator and rotor, the air then passing 
through the ducts 19 and over the rear 23 of the stator core, before being 
returned to the fan 20. On the way from the fan 20 to the air inlet points 
of the stator and rotor, the air passes through a heat exchanger 24, 
cooling air being supplied from outside the machine by a further fan 25 
also mounted to the shaft 16. 
The laminations of both the stator core and rotor core appear as shown in a 
simplified representation in FIG. 2. The stator laminations comprise a 
body portion 31 and a number of teeth 32. (In practice, the number of 
teeth in a large rotating machine would be much greater than the number 
shown in the figure). Between the teeth 32 are slots 33 into which the 
3-phase winding 17 (see FIG. 1) is inserted. 
The laminations of the rotor are constructed in a manner similar to those 
of the stator, i.e. each lamination consists of a body portion 35 and a 
number of teeth 36 and slots 37. The slots 37 are suitable shaped to 
receive the solid bars 18. 
While radial ventilation has been used to good effect in large machines, it 
presents a number of drawbacks. Firstly, the use of a radial construction 
makes it difficult to achieve low vibration levels. This is because of the 
necessity to have, on the one hand, a special rotor arm construction for 
securing the rotor to the shaft while at the same time creating the duct 
spaces 21 for cooling the rotor, and, on the other hand, duct spacers 
(which may be in the form of "I" beams) separating the individual rotor 
core sections to create the radial ducts 19. Both these elements may 
create out-of-balance forces in the rotor assembly during operation, 
leading to vibration problems. Secondly, since there are radial ducts 19 
in both the stator and the rotor, a "siren" noise effect can be produced 
when the machine is running, especially if the two sets of ducts are in 
line with each other. This can be reduced by offsetting the ducts on the 
stator relative to those on the rotor, or by employing a different number 
of ducts on the stator and the rotor. However, this causes complexity of 
manufacture and adds to the cost of the machine. Thirdly, the necessity 
for ventilation ducts 21 between the rotor core 36 and shaft 16 means that 
the rotor diameter is increased, which in turn increases the windage loss 
of the machine. Fourthly, there is an increased risk of airgap sparking. 
Fifthly, the forces exerted on the above-mentioned duct spacers (e.g. "I" 
beams) in a high-speed machine can cause these members to be dislodged 
from the end laminations to which they are supposed to be secured. 
Because of these drawbacks, a technique known as axial ventilation has also 
been used. One known axial ventilation arrangement is shown in FIG. 3. In 
this arrangement, a ventilation duct 40 is provided in each of the teeth 
of the stator 11 adjacent to the airgap 22. The duct 40 runs the entire 
axial length of the stator, and air is forced through this duct in order 
to cool the laminations of the stator core and the windings 17. Additional 
cooling may be provided by forcing air through small ducts 41, 42 made in 
the body of the stator core and rotor core, respectively. 
This technique enjoys the advantages of axial cooling, which include 
reduced windage due to the fact that the rotor 12 can be of smaller 
diameter, but suffers from the disadvantages manifest by the use of the 
duct 40, as will now be explained with the aid of FIG. 4. 
FIG. 4 is a partial view of the arrangement of FIG. 3 showing a stator 
tooth 32 and two associated stator slots 33. Each slot 33 comprises a 
winding section 51 and a ventilation duct section 52, which represents the 
duct 40 in FIG. 3. The ventilation duct section 52 is sometimes termed a 
"tunnel slot". The winding section 51 accommodates the stator winding 53, 
which in this example is composed of two sections 54, each made from a 
number of rectangular conductors held together by a suitable binding 
means. The two sections 54 are kept apart by a separator 55. The winding 
53 is prevented from moving down the slot 33 by a wedge 56 which runs the 
length of the stator core 11, or core section 12. 
The use of such "tunnel slots" in such an axial ventilation system makes 
for inefficient cooling. This is for several reasons: firstly, the 
cross-sectional area of the tunnel slot 52 is relatively small, which 
restricts the flow rate of the cooling air and produces an undesirably 
large pressure drop along the axis of the stator. The tunnel slot 52 may 
be increased in depth to allow a greater throughput of air, but with this 
must go a reduction in depth of the winding section 51 in order not to 
prejudice too much the mechanical properties of the stator core. This in 
turn means that the winding 53 must be made shorter and fatter, which 
necessitates a longer end-winding 17 in order to satisfy minimum clearance 
requirements at the end-winding itself. Secondly, the surface area of the 
tunnel slot 52 in contact with the air is restricted, which impairs the 
cooling efficiency of the arrangement. Thirdly, the top part 57 of the 
winding 53 and its adjacent lamination portions have long heat flow paths 
58 to the tunnel slot 52, which produces an undesirably high temperature 
gradient between these two parts of the slot. 
FIG. 4 also shows a pair of rotor bars 18 occupying the slots 37 of the 
rotor 12. 
Axial airflow has also been employed in a very large synchronous machine as 
an adjunct to the water cooling of the stator winding. This arrangement is 
shown in FIG. 5. In FIG. 5, which shows a stator tooth 32 and two adjacent 
slots 33, the stator tooth 32 is provided with two small ducts 61, 62 
running the whole length of the stator core. These ducts serve to provide 
nominal axial air cooling of the stator laminations only. Cooling of the 
stator winding (not shown) in the slots 33 is achieved by arranging for 
the conductors of the winding to be hollow and forcing water through them. 
In a further known axial cooling arrangement, a cage rotor 12 (see FIG. 6) 
is provided with small-diameter air ducts 71 in the rotor teeth 37. In 
order to supplement the inadequate cooling effect that this measure 
produces, the arrangement incorporates in addition larger ducts 72 
situated in the body of the rotor core below each rotor bar 18. Thus the 
ducts 71 perform essentially cooling of the laminations with some cooling 
of the rotor bars 18, while ducts 72 take away heat mainly from the rotor 
bars 18. 
It is an object of the invention to provide a rotating electrical machine 
with axial cooling of the stator and/or rotor which seeks to overcome or 
mitigate the drawbacks associated with the above known axial cooling 
arrangements. 
SUMMARY OF THE INVENTION 
According to a first aspect of the invention, there is provided a rotor 
core for a cage induction machine, comprising a plurality of teeth and a 
plurality of rotor-bar slots, each tooth being provided with a recess in 
an outer corner of the tooth, the recess forming a ventilation duct which 
extends along the axial length of the rotor core. 
This has the advantage of increasing local airgap turbulence and improving 
the transfer of heat from both the stator windings and rotor bars to the 
circulating air. In addition, the mass of the rotor, and therefore also 
its inertia, is reduced, and also saturation of the leakage reactances of 
the stator and rotor can be eliminated. Further, the use of such a recess 
helps to reduce high-frequency losses in the rotor bars. 
Preferably, the recess is of such a radial depth as to expose part of an 
adjacent slot to the duct. This has the effect of directly exposing part 
of a rotor bar occupying that slot to the cooling air, thereby greatly 
increasing efficiency of cooling of the bars. 
Preferably, the rotor teeth are configured so that an unrecessed corner of 
any one tooth completely bounds the top (short) edge of its associated 
rotor bar. This means that, even with the presence of a recess in an 
adjacent tooth, as described above, that recess does not result in the 
absence of any lamination from the top edge of the bar. This has the 
benefit that it provides full mechanical support of the bar and therefore 
reduces crushing stresses, especially at high speed. A further benefit is 
that this unrecessed part of the tooth acts as a heat sink when the rotor 
is stalled. This is significant because when the rotor is locked, rotor 
slip frequency is at a maximum and most of the current flowing in a rotor 
bar is crowded toward the top end of the bar. Having laminations all the 
way along that short top edge helps to dissipate a substantial part of the 
heat which is thereby produced. 
According to a second aspect of the invention, there is provided a stator 
core for a rotating electrical machine, comprising a plurality of teeth, 
each tooth having one or more openings extending along an axial length of 
the core to form one or more corresponding axial ventilation ducts, the 
one or more openings of each tooth being dimensioned such that they 
provide a total radial opening depth which extends over a major part of 
the radial depth of the tooth. 
The inventors have recognised that, on 2-pole machines especially, the flux 
densities in the stator teeth are relatively low, which means that the 
teeth are under-utilised. It has been found that, by providing the teeth 
with fairly large openings, ducts may be thereby formed for the influx of 
air for cooling the windings, while at the same time leaving the magnetic 
performance of the core relatively unaffected. 
The rise in flux density in the teeth occasioned by this measure can be 
somewhat offset for a given opening area by narrowing the slots and 
thereby increasing the flux-carrying width of the teeth. Narrowing the 
slots also makes for narrower windings in the slots. This means that the 
stator end windings can be made shorter, and therefore the machine can be 
made smaller. This in turn has the advantage of reducing the length of the 
shaft carrying the rotor, and thus the bearing centres can be brought 
closer and the stiffness of the shaft increased. 
The chief benefit that arises from having an opening area in the tooth that 
occupies a major part of the tooth depth (which is the same as the slot 
depth) is that virtually the whole radial depth of the winding (or rotor 
bar, in the case of a cage rotor) in a slot has only a short path to the 
cooling duct formed in an adjacent tooth, which makes for efficient 
cooling. This is in contrast to the "tunnel slot" arrangement of FIG. 4, 
for instance, where the upper part of the winding has a long path to the 
ventilating tunnel slot. 
A number of configurations are possible for the one or more openings. A 
preferred configuration is to have two rectangular or trapezoidal holes 
disposed end-to-end along the radial axis of the tooth and separated by a 
bridge section, the holes being situated approximately midway along the 
width of the tooth. Alternatively, two such pairs of holes may be employed 
side by side along the width of the tooth and suitably spaced from each 
other and from the adjacent slots. 
Instead of having holes punched in the stator laminations, each lamination 
making up the stator core may be provided with one or more recesses along 
one edge of the tooth, i.e. where the tooth borders onto an adjacent slot. 
Such recesses may be configured in a manner similar to the above holes. 
Thus, a preferred arrangement is to have two rectangular or trapezoidal 
recesses disposed end-to-end along one radial edge of the tooth and 
separated by a bridge section. Alternatively, two such pairs of recesses 
may be employed along opposite edges of the tooth. 
A further possible opening configuration is to have one opening or recess 
only running along most of the radial depth of the tooth. As before, the 
single opening may be a hole situated approximately midway along the width 
of the tooth, or a recess disposed along one radial edge of the tooth. 
This latter arrangement is not a preferred configuration, however, since 
it makes for difficulties in preventing migration of the adjacent winding 
into the recess. The use of a bridge section in the double (end-to-end) 
recess configuration has the advantage of controlling such migration. 
A further possible configuration is to use two such single openings side by 
side. Again, the preferred arrangement in this case is to have two long 
holes suitably spaced along the width of the tooth. The use of two long 
recesses along opposite edges of the tooth would suffer from the same 
winding migration problem as the use of a single recess along only one 
edge. 
An opening area created by the one or more openings may extend beyond the 
radial depth of the tooth. This has the effect of increasing the available 
duct volume per tooth and is a possible measure, especially in the stator, 
where the flux density in the core will not thereby be made excessive or 
where mechanical stiffness considerations of the stator core will not be a 
prohibiting factor. This latter proviso is mitigated somewhat by the 
presence of part of the tooth around the opening, which tends to increase 
the stiffness of the stator core. 
According to a third aspect of the invention, there is provided a rotor 
assembly comprising a rotor core as described above, and according to a 
fourth aspect of the invention, there is provided a stator assembly 
comprising a stator core as described above. The stator or rotor core may 
comprise one or more radial ventilation ducts for use in a double-ended 
axial ventilation arrangement. This is useful where the invention is 
required to be used in a very large machine with a correspondingly large 
cooling requirement. 
According to a fifth aspect of the invention, there is provided a rotating 
electrical machine comprising a rotor assembly as defined above, and 
according to a sixth aspect of the invention there is provided a rotating 
electrical machine comprising a stator assembly as defined above. A 
seventh aspect of the invention is a rotating electrical machine 
comprising both stator and rotor assemblies as described earlier, the 
inventors having found that particularly good results are obtained using 
both the axial ventilation ducts as described for the stator and the axial 
tooth recesses as described for the rotor.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 7, FIG. 7 shows a partial view of a stator and rotor core 
assembly in a preferred embodiment of the invention. The two cores are 
part of a 2-pole cage induction motor and, since cooling is by axial, not 
radial, ventilation, the cores consist of only one core section. Each core 
comprises a large number of laminations placed together in a stack. The 
diagram shows a stator tooth 101 bounded by two stator slots 102, and a 
winding 103, which is divided into two layers 117, is shown inserted into 
the slots 102 in the stator core. The windings are held in place by wedges 
104. In the tooth portion 101 of each lamination of the stator core are 
stamped two openings in the form of holes 105 with a bridge piece 116 left 
between them, such that there is formed along the axial length of the 
stator core two ventilation ducts per tooth allowing the flow of air 
through the tooth portion of the core. The bridge piece 116 helps to 
maintain adequate rigidity of the tooth 101. The upper opening 105 is 
arranged to extend beyond the radial depth of the slots 102 into the body 
of the stator, and the total end-to-end radial depth of the two openings 
is greater than one-half of the depth of the slots 102. 
In operation of the machine, a fan (not shown) blows air through the ducts 
made by the holes 105 and carries heat away from the winding 103 via very 
short thermal paths 106 in each lamination. These paths run in parallel 
all the way along the axis of the stator, there being a total cooling 
surface area per tooth equal to the sum of the measurements of the 
perimeters of the two openings 105 in the tooth multiplied by the axial 
length of the stator. This has the effect of cooling very efficiently not 
only the tooth laminations, but also the windings 103. The cooling air is 
circulated around the machine and dissipates its heat to a heat exchanger 
(see FIG. 1). 
At the same time, the laminations which make up the rotor core are arranged 
to have their own openings 110 in the rotor teeth 109. The openings 110 
are recesses which create, in a manner similar to the holes 105 in the 
stator laminations, ventilation ducts along the axial length of the rotor 
core. As in a conventional cage rotor, bars 111 are mounted in the rotor 
slots 118 and shorted to each other at either end of the rotor core. By 
arranging for the recesses 110 to start at some point 112 along the radial 
length of the respective rotor bars 111, a significant proportion of one 
face 113 of each bar is exposed to air and is thereby directly cooled by 
the contacting air. Also, the other face 114 of each rotor bar 111 is 
separated from the next recess in the series by only a comparatively short 
thermal path 115, which also assists in cooling the bar. This provides 
efficient removal of heat from the bars and rotor laminations. 
In addition to this, when the machine is running, the recess 110 in each of 
the rotor laminations increases the local turbulence in the airgap 112, 
which improves the transfer of heat away from the rotor laminations and 
bars 111. 
A further effect of the recesses 110 is that they reduce the mass and 
therefore inertia of the rotor. 
The presence of laminations against the top side 119 of the rotor bars 
helps greatly in the event that the rotor stalls. A locked rotor has its 
rotor-bar current concentrated in the upper part of each bar, which abuts 
onto the corner of an adjacent tooth in which there is no recess. Under 
these conditions, the unrecessed corner of the tooth acts as a heat sink 
and helps to dissipate the heat away from the bar. 
While it may be doubted that the total cooling surface area in the stator 
assembly, for instance, of FIG. 7 would be of the same order as that in 
the radially ventilated stator assembly of FIG. 1, in fact calculations 
made by the inventors on the sum of the surface area of the stator ducts 
formed by the openings 105 of a prototype design of the invention proved 
that the available cooling area was almost equal to the apparently much 
greater surface area afforded by the massive end lamination faces of each 
stator section 13 exposed to the radial ducts 19 in the known 
radial-ventilation arrangement (see FIG. 1). 
An examination of thermal coefficients in the two systems also highlights 
the advantages of the invention. From FIG. 1 it can be seen that the 
conduction of heat from laminations in the middle of a stator core section 
13 (which may be taken to be the hottest part of the stator) must take 
place across a large number of insulated laminations, which in practice 
are approximately 25 mm deep in total (half the depth of a core section). 
The thermal conductivity of the laminations is approximately 2.2 W/mK. In 
the arrangement according to the invention, however, the hottest part of 
the stator is the winding 103, and the distance from the winding to the 
nearest airflow is, in the embodiment of FIG. 7, about 6 or 7 mm, which 
applies to all the laminations in the core. In addition, since the heat in 
the arrangement according to the invention flows through the plane of the 
laminations (i.e. solely through metal) and not through the thickness of 
the laminations (i.e. through metal plus insulation) the thermal 
conductivity of the metal used for the laminations must be used in 
calculations, not that of the metal and insulation in series. Thermal 
conductivity for the pure metal is approximately 30 W/mK. This means that 
there is, in the invention, a total gain in cooling efficiency of 
25/7.times.30/2.2.congruent.55 when comparing the hottest parts of the two 
arrangements. 
A second embodiment of a stator assembly according to the invention is 
shown in FIG. 8. In this embodiment, the stator laminations have stamped 
in them two openings 120 in the tooth 101. These openings, which are 
recesses, differ from the holes 105 in the first embodiment of the stator 
assembly in that they are arranged to be a continuation of the slot 102, 
thereby leaving a substantially narrowed radially extending tooth portion 
121. This configuration functions in a manner similar to the rotor 
recesses in FIG. 7: one face 122 of the windings 103 is directly cooled by 
air flowing through the ducts formed by the recesses 120, while the other 
face 123 has a short thermal path 124 to the recesses of the next tooth. 
The openings in the stator may be configured in other ways. Examples of 
other configurations are shown in FIG. 9. FIGS. 9a, b and c show 
alternative forms of openings in the form of holes, while FIGS. 9d, e and 
f show the equivalent in the form of recesses. The holes/recesses are, in 
each case, shown hatched. Briefly, a single long opening only may be used, 
though in the case of the single recess shown in FIG. 9d, e and f this may 
entail difficulties with regard to the anchoring of the winding in the 
adjacent slot. Alternatively, two long but narrower openings may be used 
side by side; this has the same disadvantage as the single-opening 
arrangement. Finally, four openings may be employed in the form of two 
pairs of the openings shown in the first embodiment of the stator 
assembly, FIG. 7. These alternative configurations function in exactly the 
same way as the preferred configurations illustrated in FIGS. 7 and 8 and 
will therefore not be discussed further. 
It is possible to employ the openings, which have hitherto been described 
in connection with the stator, for the rotor also. This is illustrated in 
FIG. 10, in which the rotor teeth 130 are provided with holes 131 situated 
between the rotor bars 111, in a manner similar to the stator holes 105 in 
the first embodiment (FIG. 7). This second embodiment still provides 
effective air cooling, but lacks the specific advantages outlined earlier 
associated with the open recess shown in FIG. 7 (i.e. increase in local 
airgap turbulence, elimination of leakage reactance saturation, etc.). 
FIG. 11 shows a simplified sectional view of a rotating machine according 
to the invention highlighting the airflow which takes place inside the 
machine. Air enters the machine compartment from the right, having been 
forced through a heat exchanger 24 by a fan 20. The air then follows three 
parallel paths, A, B and C: A is a path behind the body of the stator 
core; B is a path through the openings in the stator teeth, and C is a 
path through the recesses in the rotor teeth and through the airgap. 
The fact that the air paths are all in parallel is one of the great 
advantages of axial ventilation, since if it is found that too much heat 
is being generated in the stator teeth, for instance, but little heat is 
being generated to the rear of the stator core, airflow along path A can 
be restricted and that along path B can be increased, and so on. 
While it has so far been assumed that a machine according to the invention 
will employ both the stator as described with its openings and the rotor 
as described with its recesses, in practice it may employ only one of 
these. In particular, because the rotor-cooling arrangement of FIG. 7, for 
instance, entails so many benefits for the performance of a machine into 
which it is incorporated, it may be used by itself in conjunction with a 
conventionally cooled stator in an existing machine, the stator having, 
for example, water-cooled windings. 
Also, in order to increase the cooling surface area of the ducts created by 
the openings, it is possible to provide the openings with corrugated 
edges, as shown in FIG. 12. The corrugations may be on the longer edges or 
on all the edges of an opening, depending on ease of manufacture. 
Where the invention is to be used on a very large machine, it is possible 
to employ a combination of axial and radial ventilation to obtain 
sufficient cooling. In this arrangement, which is known as double-ended 
axial ventilation, one or more radial ducts are placed in the centre of 
the stator and/or rotor which allows air to flow into both ends of the 
machine at the same time. This has the advantage of reducing the pressure 
drops and equalising the temperature variations which occur along the axis 
of the machine. 
An example of double-ended axial ventilation is shown in FIG. 13. In this 
figure, the stator 11 is equipped with three radial ducts 19, the rotor 12 
with none. A fan 20 is placed at either end of the machine on the shaft 
16, and these fans draw air into both ends of the machine simultaneously 
along the same three paths illustrated in FIG. 11. Air flowing along the 
stator tooth ducts and the rotor tooth ducts (where this is a recess) and 
airgap is then forced through the radial ducts 19 when it reaches the 
central part of the machine and is taken up through the heat exchanger 24 
before recirculating through the machine. 
It should be mentioned that the inventors have built a 3.7 MW, 60 Hz, 
2-pole cage induction motor using the principles embodied in FIG. 7, and 
performance in all aspects met or exceeded expectations. In particular, 
cooling was found to be very satisfactory, vibration was minimal and noise 
was at a very low level, due partly, for example, to the reduced windage 
loss. Actual figures obtained for the various parameters were: efficiency, 
97.5%; vibration, &lt;1 mm/sec.; noise, 78 dBA.