BACKGROUND OF THE INVENTION
The present invention relates to dynamoelectric machines and, more
particularly, to a stator lamination and stator cooling arrangement for such machines.
Dynamoelectric machines such as rotary electric motors are generally
provided with some form of cooling in order to extend the operating capability
of the machines. In general, it is desirable to maintain the temperature of such
machines below a predetermined limit in order to prevent deterioration of the
machine through thermal breakdown of insulation or thermal distortion due to thermal
expansion of elements of the machine. In air cooled dynamoelectric machines, air
is forced through air passages in the stator core as well as along surfaces of
the rotor and adjacent windings in the machine. The air may be forced by external
means but is generally drawn in through apertures or vents in the end bell or
caps by a fan coupled in driving relationship with a rotating assembly and rotor
of the machine. Air passages are formed in the stator core for passage of this
cooling air so as to carry heat from the machine. In general, the stator core
air passages are formed as round or oval passages adjacent the outer periphery
of the stator core.
As increased horsepower has been demanded from such dynamoelectric
machines while the physical size has been maintained or, in some instances, reduced,
there has been a concurrent requirement to provide more efficient cooling of the
machines. Such demand and requirement have necessitated development of improved
cooling without impacting electromagnetic design of such machines.
SUMMARY OF THE INVENTION
Among the several objects of the present invention may be noted the
provision of an improved cooling arrangement for a dynamoelectric machine which
is achieved without detrimental effect on the electromagnetic design of the machine.
In one form, the improvement is illustrated in an electric motor of the 1-5 horsepower
class in which the stator assembly thereof is formed from a plurality of stack
laminations, the available horsepower being determined by the stack height. The
stator lamination comprises a generally flat disk having a centrally located bore
for passage of a rotor therethrough. A plurality of uniformly spaced winding slots
extend radially outward from the bore with adjacent ones of the winding slots defining
a pole piece extending from the bore to a radially outer portion of the lamination.
A plurality of cooling air passages are formed in the lamination adjacent a radially
outer termination of at least some of the winding slots. Preferably, the passages
having a radially inner boundary conforming generally to the radially outer termination
of the winding slots and
a radially outer boundary arcuately shaped for minimizing effects on the electromagnetic
flux path extending radially through the pole pieces. More particularly, the passages
have a generally crescent shaped configuration.
In an illustrative embodiment, the invention is disclosed as an improved
cooling arrangement for a dynamoelectric machine of the type having a plurality
of stacked laminations forming a stator core. The stacked laminations have a central
bore for passage of a rotor therethrough and a plurality of uniformly spaced winding
slots extending radially outward from the bore and terminating at an end at least
partially through the laminations. Adjacent ones of the slots define pole pieces
extending from the bore into a radially outer portion of the laminations. A plurality
of cooling air passages extend axially through the stacked laminations generally
parallel to the central bore. Each of the air passages are positioned adjacent
the terminating end of a corresponding one of the winding slots and each air passage
has a radially outer shape corresponding generally to a radially outer shape of
the winding slots. Preferably, each cooling air passage extends circumferentially
between a pair of radius lines of the machine co-extensive with opposite circumferential
edges of a respective one of the winding slots and each air passage has a crescent
shape when viewed axially. Each said air passage is also positioned to minimize
effects on electromagnetic flux lines in the stacked laminations.
The invention is further disclosed in a dynamoelectric machine having
an outer housing and a pair of end shields attached to opposite ends of the housing
with each of the end shields having a plurality of air vents passing therethrough.
A stator assembly is positioned in the housing generally axially spaced from the
end shields and has a plurality of windings adapted for excitation upon the energization
of the dynamoelectric machine. The stator assembly comprises a plurality of stacked
laminations having a plurality of axially aligned, circumferentially spaced, radially
inner slots through which the winding means extend. A plurality of crescent shaped
air passages are positioned adjacent a radially outer end of at least some of
the slots and a blower is provided for forcing cooling air through the air passages
for cooling the windings. The stator assembly is assembled in heat exchange relationship
with the outer housing and the air passages are positioned such that a major portion
of the mass of the stacked laminations lies outside a cylinder defined by an outer
periphery of the air passages. The windings include a pair of opposite end turns,
each of which are formed with radially outwarding extending portions. The air
passages positioned adjacent the winding slots causes the air to blow into and
over the end turns to improve heat transfer from the end turns to the air. In
one form, the dynamoelectric machine stator assembly has an outer diameter of about
6.4 inches and the cross-sectional area of each of the air passages is about 0.0475
BRIEF DESCRIPTION OF THE DRAWINGS
For a better understanding of the present invention, reference may
be had to the following detailed description taken in conjunction with the accompanying
drawings in which:
DETAILED DESCRIPTION OF THE INVENTION
- FIG. 1 is a simplified cross-sectional view of a dynamoelectric machine incorporating
the improved cooling arrangement of the present invention;
- FIG. 2 is a plan view of a prior art stator lamination for use in a dynamoelectric
- FIG. 3 is a plan view of a stator lamination in accordance with one form of
the present invention;
- FIG. 4 is a plan view of a stator lamination of another embodiment; and
- FIG. 5 is a graph comparing the heat transfer characteristics of the present
invention to the prior art.
Referring now to the drawings in general, and in particular to FIG.
1, there is shown a cross-sectional view of a dynamoelectric machine 10 incorporating
the teachings of the present invention. The dynamoelectric machine 10 includes
an outer housing 12 and a pair of opposite end shields 14 and 16. The machine
10, which may be operated as a generator but is preferably operated as an electric
motor, includes a stator assembly 18 preferably formed of a plurality of stacked
ferromagnetic laminations 20. The stacked laminations 20 are generally circumferentially
encompassed by outer housing or shell 12 of dynamoelectric machine 10 and predeterminately
spaced between the opposite end shields 14 and 16. The laminations 20 are also
desirably fitted within housing 12 so as to be in heat exchange relationship with
the housing. The stator assembly further includes a plurality of winding means,
indicated generally at 22, which are adapted for excitation upon the energization
of the dynamoelectric machine across a power source (not shown). A rotatable assembly,
indicated generally at 24, comprises a shaft 26 having a squirrel cage rotor 28
or the like for instance mounted thereon so as to be conjointly rotated with the
shaft. However, it is contemplated that rotors other than the squirrel cage type
may be utilized with the shaft 26 of the rotatable assembly. The rotor 28 of the
rotatable assembly 24 is arranged or otherwise associated in magnetic coupling
relation with stator assembly 18 upon the energization of dynamoelectric machine
10, and means, such as bearing device 30 or the like for instance, is associated
with each end shield 14 and 16 and adjustably alignable for journaling a part
such as shaft 26 of the rotatable assembly.
The winding means include a pair of opposite end turns 32, 34 which
are formed so as to be disposed in a radially outwardly directed orientation with
a close spacing between radially outer boundaries 36,38, respectively, of the end
turns and the outer housing 12. A plurality of air passages 40 are formed through
the stacked laminations 20 of the stator assembly 18 for passage of cooling air
therethrough. The stator assembly 18 further includes a plurality of radially inner
circumferentially spaced and axially extending slots 42, shown in FIGS. 2-4, within
which the stator winding means 22 is disposed. The air passages 40 are positioned
so as to be adjacent at least some of the slots 42 such that air passing through
the slots affects a cooling of the adjacent stator structure and the associated
winding means 22. The arrows 44 indicate the flow of air through the air passages
40 and around the outwardly extending end turns 32, 34 of the winding means 22.
The cooling air is forced through the stator assembly 18 by means of a blower
means 46 or similar type fan arrangement connected in rotating relationship with
the motor shaft 26 whereby rotation of the rotor assembly 24 affects conjoint
rotation of the fan or blower means 46. The air is drawn into the dynamoelectric
machine or motor through air vents 48 circumferentially spaced about the end shields
14 and blown through the stator assembly 18 exiting through air vents 50 in the
opposite end shield 16.
It will be noted that the air passages 40 through the stator assembly
are positioned adjacent and very near to the winding means 22. A large extent
of the mass of the laminations of the stator assembly 18 is located between the
air passages 40 and the outer housing 12. The position of the air passages 40
is selected so as to minimize their effect on magnetic flux within the laminations
20 of the stator assembly 18 and further to provide a maximum heat transfer between
the major mass portion of the stator laminations 20 and the outer housing 12.
Concurrently, the end turns 32, 34 are formed to arch radially outward toward the
outer shell 12 so as to force the flow of cooling air through a path over and
around the end turns thereby to affect a better transfer of heat from the winding
end turns to the cooling air.
To better explain the present invention, reference is now made to
FIG. 2 which illustrates an exemplary prior art stator lamination 50 for a dynamoelectric
machine such as that shown in FIG. 1. The lamination 50 includes a plurality of
circumferentially spaced, radially inner winding slots 42 through which selected
ones of the windings of the winding means 22 pass. The slots 42 circumscribe a
central aperture 54 which receives the rotor 28 of the rotatable assembly 24. Adjacent
an outer perimeter 56 of lamination 50 there are positioned a plurality of elongated
air passages 58 for passage of cooling air through the stacked laminations 20
of an assembled stator as described generally with respect to FIG. 1. Lamination
50 also includes apertures 60 for passage therethrough of fasteners, such as an
elongated bolt (not shown), which may be used in the assembly of the dynamoelectric
machine. Slots 62 along the perimeter of lamination 50 are used to align the stacked
laminations 20 of the stator assembly.
During operation of the dynamoelectric machine 10, electric current
energizing the winding means 22 results in heating of the winding means due to
the inherent resistance of the winding means. The amount of electric power dissipated
in the winding means 22 is a function of the value of current I and the winding
resistance R, i.e., power dissipated is proportional to I²R, where I is the magnitude
of current supplied to the winding means. As the horsepower requirements for the
machine 10 are increased, the power dissipated in the form of heat increases as
a square of the current. Such increased heat can result in insulation breakdown
and thermal distortion of the machine and result in failure.
Cooling air forced through passage 58 picks up heat energy from the
lamination 50 and carries the heat to outside the machine 10 thus extending the
power capability of the machine. It will be seen in FIG. 2 that the heat energy
transfers from the winding means 22 to the stator lamination 50, passing through
the primary mass of the lamination before reaching the passages 58. Although the
electromagnetic material of the lamination 50 is a relatively weak heat conductor,
cooling air adjacent the perimeter of the lamination is effective in cooling of
the winding means 22 by transference of heat energy from the winding means to the
lamination and then to the cooling air in passages 58. However, as electric power
is increased to the machine 10, a limit is reached at which the cooling air cannot
keep the winding temperature below a critical value.
One method of improving heat transfer is to increase the cross-sectional
area of the air passages 58 so as to expose more surface to the flow of cooling
air. FIG. 4 illustrates one such method in which the air passages 58 are redesigned
into stretched or further elongated passages 63 in a lamination 64 thereby increasing
the surface area exposed to cooling air. The stretched passages 63 have proven
to be more effective, as will be shown hereinafter, than the passages 58. However,
one disadvantage of the passages 63 is that they tend to block heat flow from
the laminations 64 to the outer housing or shell 12 of the machine 10. Accordingly,
the stretched passages 63 reduce the amount of heat which has heretofore been
transferred to the outside environment via the housing 12.
One possible modification of the lamination 64 is to move the air
passages 63 from adjacent the outer periphery to a position closer to the winding
slots 52. However, moving the passages 63 radially inward will place them in a
location which will detrimentally interfere with electromagnetic flux in the lamination.
More particularly, the winding means 22 establishes electromagnetic flux in the
stator lamination with the flux path extending radially outward into the major
mass portion of the lamination, i.e., that portion between the slots 52 and the
outer shell 12, and further extending through the teeth 66 between the slots 52.
The flux projected radially inward from the teeth 66 define the magnetic poles
of the stator assembly which affect rotation of the rotatable assembly 24. Blockage
of the flux path detrimentally affects the magnetic pole strength and the power
capability of the machine 10.
Applicants have discovered that improved cooling capability can be
achieved by redesigning the lamination air passages such that they can be moved
to a position adjacent the lamination slots without detrimentally interfering with
the stator electromagnetic flux paths. Referring to FIG. 3, there is shown a stator
lamination 70 in accordance with the teaching of the present invention in which
a plurality of cooling air passages 72 are formed adjacent at least some of a
plurality of radially inner stator winding slots 74. The slots 74 are formed with
a conventional configuration, i.e., being slightly pie shaped with a somewhat wider
radially outer portion. The air passages 72 are formed adjacent at least some
of the slots 74, being separated from the slots by a relatively thin bridge 76.
The passages 72 are formed with a crescent shape having a radially inner perimeter
which conforms generally to the radially outer perimeter of an adjacent slot 74.
A radially outer perimeter of passage 72 is formed as though it were an outer
perimeter of the adjacent slot. By forming the passages 72 in this manner, the
passages do not extend into the magnetic flux path defined between the slots 74
by intermediate teeth 78.
Tables 1, 2 and 3 compare the effectiveness of the air passages 58,
62 and 72, respectively, in a dynamoelectric machine or motor having stator laminations
of about six inches in diameter and a rotor diameter of about three inches. Such
motors are typically about from one to five horsepower where the horsepower can
be increased by extending the lamination stack height and the length of the rotor.
In the Tables, DH is hydraulic diameter (equal to four times cross-sectional area
divided by perimeter), V is air velocity in feet per minute, H is a film coefficient,
HA is the coefficient of heat transfer (i.e., H multiplied by A, the heat transfer
surface area of the air passage) and CFM is the cubic feet per minute of cooling
Table 1 illustrates the characteristics of the cooling air passages
of FIG. 2 for an exemplary motor having air passages with a cross-sectional area
of 0.0714 in² and a perimeter of 1.028 inches. Note that the values of HA are lowest
for this configuration of air passage. Table 2 shows the effect of stretching
the air passages such that the cross-sectional area decreases to 0.0678 in² but
the perimeter increases to 1.514 inches as shown in the lamination 62 of FIG.
4. The configuration of FIG. 4 actually has the highest values of HA and is therefore
more effective in heat transfer via the cooling air. However, the effect of blocking
heat transfer to the shell 12 overcomes the other advantages.
Table 3 shows the improvement attained with the crescent shaped passages
72 of FIG. 3. The value of HA is improved by about fifty percent over the conventional
passages of FIG. 2. Further, the reduction in HA as compared to FIG. 4 (Table 2)
is relatively small and offset by improved heat transfer to the machine shell
12 since the outer air passages are eliminated. More importantly, magnetic saturation
tests of a motor using the lamination of FIGS. 3 and 4 have shown virtually no
difference when compared to the saturation curve for a similar motor using the
prior art lamination of FIG. 2. Referring to FIG. 5, a graph of thermal dissipation
capacity for identical motors, one using conventional air passages as in FIG.
2 (Line A) and another using crescent shaped air passages of FIG. 3 (Line B),
shows significant improvement with the crescent shaped passages 72. The vertical
axis of FIG. 5 measures heat dissipation in watts per degree Celsius while the
horizontal axis measures dissipated losses in watts.
It will be appreciated that the crescent shaped air passages 72 are
placed or formed as near to the slots 74 as practical. The bridge 80 separating
the slots 74 from passages 72 must have sufficient strength to support the winding
means 22 during the assembly thereof since the windings are forced downward to
substantially fill the slots 74. In the exemplary motor lamination 70, the bridge
80 is about 0.0625 inches thick, i.e., the distance from slot 74 to passage 72.
The cross-sectional area of the passages 72 is selected to maximize air flow without
detrimentally affecting the electromagnetic structure or flux path in the lamination.
More specifically, the total flux area is the distance from the lamination outer
diameter to the outer perimeter of the air passage 72 times the stack length.
While horsepower can be increased by increasing stack length, industry standards
generally define motor diameter and thus limit the ability to increase flux area
radially. Accordingly, the cooling requirements become a balance against desired
flux and control the size of cooling air passage 72. The crescent shaped passage
adjacent the winding slot improves cooling without adversely affecting power output.
While the invention has been described in what is presently considered
to be a preferred embodiment, many variations and modifications will become apparent
to those skilled in the art. Accordingly, it is intended that the invention not
be limited to the specific illustrative embodiment butt be interpreted within
the full spirit and scope of the appended claims.