Note: Descriptions are shown in the official language in which they were submitted.
106S94:1
This invention relates to reciprocating electric motors
of the type in which a magnetically permeable armature is disposed
to axially reciprocate within the air-gap of a fixed electromag- ~
netic circuit.
This application is a divisional application of copen-
ding application No. 235,045 filed Sept. 9, 1975.
In the art of moving-iron linear-motor compressors,
much effort has been expended without having achieved significant
commercial success (see P.W. Curwen, "Recent Developments of Oil-
Free Linear-Motor Resonant-Piston Compressors", AS~E publication
69-FE-36, June, 1969). The linear-motor compressor disclosed
herein has been subjected to extensive laboratory testing and the
design parameters have been verified through the use of iterative
computer programming techniques, and therefore, the requirements
of a commercially viable product are believed to have at last been
achieved.
The present invention provides a moving-iron linear-
motor compressor that is easy and economical to assemble.
The present invention also provides a magnetic core for
a moving-iron, linear-motor compressor which may be more easily
assembled and which has less waste material than a core o~ con-
~entionally stacked laminations. It is a related object to pro-
vide an economical method for manufacturing said magnetic core.
The present invention also provides a magnetic core
for a moving-iron, linear-motor compressor which is incompressible
in the direction of tightening of the compressor tie bolts in
order to insure that the tie bolts continue to exert the reten-
tion stresses to which they are initially set during assembly to
thereby maintain proper alignment of the core, armature and frame
of the compressor.
1- ~
1065941
The present invention also provides a moving-iron,
linear-motor compressor which is small in size and which may be
easily packaged to suit a wide variety of applications.
The present invention further provides a moving-iron,
linear-motor compressor in which the twisting forces imparted on
the reciprocating armature are reduced substantially to zero.
The present application is an improvement on the linear
compressor disclosed in U.S. Patent No. 3,947,155, ~ichael K.
~idol.
To reduce the twisting forces imparted on the recipr~-
cating armature of the compressor motor, the present invention
provides a moving-iron, linear-motor compressor wherein the means
for returning the armature at the end of its magnetically powered
stroke comprises a pair of linear internested coil springs each
having substantially zero pitch.
The present invention further provides a moving-iron,
; linear-motor compressor having a magnetic core which includes a
pair of abutting convoluted loops of magnetic strip material
spirally would on a plane perpendicular to the compressor axis of
2,0 reciprocation with the magnetic core air-gap formed in the area
of abutment of the two loops.
In copending application No. 235,045 there is provided
in an electromagnetic compressor of the type including a gas
pump having a pumping chamber, a magnetic circuit including means
fixedly attached to said pump and having an air-gap, and recip-
rocating means attached to a pumping member and disposed to
axially reciprocate within said air-gap, means for magnetically
activating said magnetic circuit to drive said reciprocating
means in a first direction, and return means operatively con-
nected to said reciprocating means to drive said reciproat-
1065941
ing means in a direction opposite to said first direction, said
return means including a pair of parallel coil springs, means
clamping one end of each said springs in fixed relation to said
megnetic circuit, and means clamping the other end of each said
springs to said reciprocating means, the improvement wherein said
coil springs comprise a pair of linear coil springs having sub-
stantially zero pitch.
According to one aspect of the present invention
there is provides in a reciprocating electric motor of the type
in which a magnetically permeable armature is disposed to axially
reciprocate within an air-gap of a fixed electromagnetic circuit,
the method of forming a magnetic core for said fixed electromag- r
netic circuit comprising the steps of spirally winding first and
second inner convoluted loops of magnetic strip material, placing
said first and second inner loops on a common plane perpendicular
to the axis of reciprocation of said armature such that adjacent ',
~ Cl- sides of said inner loops are in flatwise abutment on a plane
i perpendicular to said common plane, clamping said abutting inner
loops tightly together, and forming said air-gap in the area of
abutment of said inner loops, the central axis of said air-gap
being coaxial with said axis of reciprocation and perpendicular
to said common plane.
In another aspPct thereof the present invention pro-
vides in a moving-iron, linear-motor compressor of the type in- t
cluding a gas pump having a pumping chamber, a magnetic circuit
fixedly attached to said gas pump and a magnetically permeable
armature attached to a pumping member and disposed to axially
reciprocate within an air-gap in said magnetic circuit, the im-
proved magnetic core in said magnetic circuit comprising first
and second inner convoluted loops spirally would of magnetic v
strip material and placed on a common plane perpendicular to the
axis of reciprocation of said armature in flatwise abutment on a
1~
~ - 3 -
1065941
plane perpendicular to said common plane, means clamping said
abutting inner loops tightly together, and means forming said
air-gap in the area of flatwise abutment of said inner loops,
the central axis of said air-gap being coaxial with said axis
of reciprocation.
In a further aspect of the present invention there is r
provided in an electromagnetic circuit of the type in which a
magnetically permeable armature is disposed to axially recipro-
cate within an air-gap defined by a pair of spaced-apart, coaxial
poles on a fixed magnetic core, said air-gap comprising means
providing a first space between said poles, said space having a
circular cross section perpendicular to the axis of reciprocation
of said armature, and means providing opposing pairs of exposed
coplanar gap surfaces, the planes of said surfaces being dis-
placed on opposite sides of said axis of reciprocation such that
the maximum displacement between said pairs of surfaces is less
r~` than the maximum diameter of said first space, the space between
said pairs of surfaces being devoid of core material, the improve-
ment wherein said maximum displacement between said pairs of sur-
faces is greater than 16.7% of the cross-sectional area of said
poles as measured on a plane-parallel to said pairs of surfaces.
In yet another aspect thereof the present invention
provides in an electromagnetic compressor of the type including
a gas pump having a pumping chamber with a central axis, a mag-
netic circuit having an air-gap coaxial with said pumping chamber,
and means fixedly attaching said magnetic circuit to said pump,
said attaching means being oriented to exert retention stresses
on said magnetic circuit in a direction parallel to said central
axis, the improvement wherein said magnetic circuit comprises
laminationslayered in a direction perpendicular to said axis
whereby said magnetic circuit is substantially incompressible in
the direction of said axis.
- 3a -
1065941
The invention itself, however, together with additional
objects, features and advantages thereof, will be best understood
from the following description when read in connection with the
accompanying drawings in which:
FIG. 1 is a perspective view of one embodiment of the
linear-motor compressor hermetically encased within a protective
housing in accordance with the present invention;
FIG. 2 is an exploded perspective view of the linear
compressor motor and gas pump shown in FIG. l;
FIG. 3 is an elevational view of the compressor shown
in FIG. 1 taken partly in axial section along the line 3-3 of
FIG. l;
FIG. 4 is an axial sectional view of the compressor
shown in FIG. 3 in a stage of partial assembly;
FIG. 5 is a fragmentary axial sectional view of the
compressor shown in FIG. 3 in a second stage of partial assembly;
FIG. 6 is a graph used to explain the operation of the
compressor shown in FIGS. 1-5;
FIG. 7 is a graph of the reluctance curve of one embodi-
ment of the compressor of FIG. l;
FIG. 8 is a fragmentary axial sectional view of analternative embodiment of the compressor of FIG. 1 which includes
a pair of zero-pitch internesting springs;
FIG. 9 is an end view of the compressor of FIG. 8 which ..
!
- 3b -
1065941
shows the internested relation~hip of the springs in greater
detail;
FIG. 10 is a plan view of a modified magnetic core which
may be used in the compres~or of FIG. 1:
FIG. 11 is a perspective view of the air-gap in the core
of FIG. lO at an intermediate stage of fabrication;
FIG. 12 is a perspective view of the completed air-gap
in the core of FIG. lO; and
FIG. 13 i8 a graph used to explain the operation of the
modified core shown in FIG. 10.
In the various figures, identical reference numerals
indicate identical part~. Referring to FIGS. 1-7, there i8
shown an exemplary embodiment of a linear-motor compressor
20 constructed pursuant to the diæclosure of the aforementioned
U.S. Patent No. 3,947,155 which is disclosed herein to provide
a better understanding of how to make and use the features of
the present invention, which are disclosed in detail subse-
quently herein in conjunction with FIGS. 8-13, and repre~ent
modifications to compres~or 20. Compressor 20 is suspended
within a protective enclosure 21 by the suspension springs 22
which ideally provide a zero retarding force to the axial oscil-
latory movement of compressor 20 and an infinite retarding force
to lateral or radial oscillatory movement thereof. Enclo~ure
21 is hermetically sealed and may be formed of sheet steel or
aluminum or molded plastic, and may assume a ~hape most con-
venient for the particular application. Lubricating oil is
preferably provided in a sump 23 at the bottom of the enclosure
at a depth ~ufficient to contact the lower portion of compressor
20. The oil will be splashed onto the moving parts by the axial
reciprocating action of the motor. Alternatively, the oil may
be channeled to lubricated surfaces by other means known in the
art.
1(~6S94~
The detailed description of compressor 20 may be best
understood with reference to FIGS. 2-3. Magnetic circuit 30,
which includes magnetic core 31 and windings 32 and 33, has
a pair of spaced-apart poles 32a and 32b defining an air-gap
34 with the opposed ~urfaces or pole faces of poles 32a and
32b defining a poxtion of a frustoconical surface of revolu-
tion. Attached to opposite sides of the magnetic circuit by
means of bolts 35 and 36 is an outboard bearing and spring
retainer plate 37 and a cylinder block 38 having a pump chamber
or cylinder 39 formed therein. As shown in FIG. 3, the taper
of air-gap 34 converges in the direction of chamber 39 with the
center axi-~ of gap 34 being coaxial with chamber 39.
Mbvable in air-gap 34 i8 a frustoconical armature 40 car-
ried by an armature rod 41. Armature 40 may be made of either
solid magnetically permeable material or stacked lamination~
as shown. It has been found that the use of stacked laminations
increases the efficiency of the compressor by 15 per cent when
compared to an identical compressor with a solid magnetic core.
Armature rod 41 may be made of nonmagnetic material, such as
stainless steel, or, preferably, magnetically permeable material.
Mbunted on one end of rod 41 and slidable in chamber 39 i~
a piston 42. Fbr m ximum compre~ion efficiency, the sliding
clearance between piQton 42 and the side wall of chamber 39
must be small: a nominal clearance of .0003 inche~ i9 preferred.
Mbunted in plate 37 is Zl sleeve bearing 43 disposed about
rod 41 at the end thereof remote from piston 42. Because of the
clo~e sliding clearance between piston 42 and the wall of
chamber 39, the piston will cooperate with bearing 43 to main-
tain rod 41 and armature 40 centered in air-gap 34 during axial
3~ di~placement of the armature, rod and piston.
Slidably mounted on plate 37 and clamping one end of a pair
of return springs 44 ana 45 is an adjustable clamp bracket 46.
Bracket 46 may be tightly clamped to plate 37 by means of screw
-5-
106594~
47 which is threadably received in a split or slotted offsetportion of the clamp. The respective straight end-tang termina-
tion~ 44a and 45a at the outboard end of springs 44 and 45 are
clamped into associated holes 46a and 46b of bxacket 46 by means
of ~crews 48 and 49 which traverse associated bra~ket splits
leading to each of the clamp holes. One end of each of return
springs 44 and 45 is thus fixedly clamped in relation to magnetic
circuit 30 and air-gap 34. The other straight end-tangs 44b
and 45b of each return spring 44 and 45 is operatively clamped
~o armature 40 by means of a spring clamp plate 50 which is
mounted on rod 41 against the large diameter face of conical
axmature 40.
End-tangs 44b and 45b are inserted into associated holes
50a and 50b re~pectively and clamped therein by means of screws
50c and 50d threadably received into respective split portions
of clamp 50. It should be noted that springs 44 and 45 are
coiled in the same direction but that each spring enters bracket
46 and clamp 50 from a direction 180 from the direction of
entry of the other. In this configuration the bending forces
imparted upon armature 40 by the spring~ during axial recipro-
cation of the armature tend to cancel each other, thereby assist-
ing bearing 43 and the bearing action of pi~ton 42 to center
armature 40 in air-gap 34 during reciprocating axial movement.
It should be further noted that end-tang~ 44a, 44b and 45a,
45b extend in a direction parallel to the central axi~ of the
~prings ~rom the periphery of the respective spring~. This
feature allows bracket 46 and clamp 50 to as ume a reduced
diameter, thereby reducing the required dimensions of enclosure
21. While several methods of terminating and affixing springs
44 and 45 will be evident to those skilled in the art, it has
been found that the use of straight end-tang terminations 44a,
44b and 45a, 45b on the return springs and the associated split
~065941
clamp mounting facilitate~ adjustment and assembly.
Compressor 20 is suspended in sump 23 as detailed above
with reference to FIG. 1. Because the lateral dimension of
block 38, that i8, the dimension perpendicular to a line between
bolts 35 and 36 and perpendicular to the axis of reciprocation,
is less than the corresponding lateral dimension of core 31,
which relationship is best seen in FIGS. 1 and 2, and becauYe
of the access via side openings provided by the axial spacing ,
of block 38 from core 30 due ~o mounting pads 38c and 38d (FIG.
2), oil splashed upwardly by the reciprocating action of com-
pressor 20 will enter the chamber 31a (FIG. 3) between core
30 and block 38. Splashed oil which contacts the minor diameter
face of armature 40 will be thru~t into chamber 39 again~t the
back of piston 42 by the reciprocating action of the armature.
mi8 oil will lubricate the side~ of chamber 39 in the area
of sliding contact with piston 42.
A valve plate and cylinder head assembly 51 i8 mounted on
cylinder block 38 by means of bolt~ 52. The suction and dis-
charge valves, the valve plate assembly, and the cylinder head
may each be any one of the several ~tandard designs known to
the art and do not ~orm a part of this invention. In a 450
BTU/Hr working embodiment of compressor 20 to be discussed in
detail hereinafter, valve plate assembly Sl is an adaptation
of the valve ~ystem from a commercially available Mbdel AE
Compressor manufactured by Tecumseh Products Company of
recumseh, Michigan. Valve assembly 51 will not be discu~ed
further except by reference during the discussion of the assembly
and operation of the compressor.
m e economical method of as~embling compressor 20 may be
best understood by reference to FIG. 4 in which compressor 20
is shown being assembled on an assembly surface 80. Cylinder
block 38 is first placed head-end down on the assembly surface.
~06594~
m en magnetic circuit 30 is loosely placed on the accurately
machined seating surfaces 38a and 38b of block 38 with the
respective bolt holes of the core and block roughly aligned.
The armature rod assembly, consisting of clamp 50, armature 40
and piston 42 all mounted on armature rod 41, is then seated
in the magnetic circuit by being piloted piston-end fir-~t into
chamber 39 until the piston extends sufficiently into chamber
39 such that the conical armature is seated against the pole
faces 32c and 32d which define conical air-gap 34. Nbte in
FIG. 4 that in this fully inserted co~ition piston 42 extends
beyond the head-end face 38c of cylinder block 38 by an amount
of distance indicated "b" when armature 40 abuts the pole faces.
The purpo e of thi~ extension will be explained in the discussion
of the operation of the compressor motor hereinafter. As the
armature is being thu~ seated, the geometry of the armature
and air-gap and the tight tolerance between the piston and
chamber wall cause~ the armature rod assembly to act as a set-
up jig which cam~ core 31 sideways so as to 3hift it laterally
on faces 38a and 38b to thereby automatically center the mag-
netic circuit and cylinder block with one another and with thearmature, rod and pi~ton. me outboard bearing plate 37 and
bearing 43 i8 next mounted on the magnetic circuit, and then
bolts 35 and 36 are inserted through plate 37 and core 31 and
threaded into block 38, thereby automatically aligning bearing
43 with the common axis of the air-gap and compression chamber
and bringing the parts into accurated angulax registry. Bolts
35 and 36 may be then tightened down to secure the sub-assembly.
In the next stage of assembly shown in FIG. 5, end-tangs
44a, 44b and 45a, 45b of return springs 44 and 45 are inserted
and tightly clamped in adjustable bracket 46 and clamp 50.
Bracket 46 at this stage is loosely received on a mounting post
37a of plate 37 ~o that it can move thereon as piston 42 i9
1065941
raised to rest upon a jig block 81 which i8 in~erted below the
piston in the pocket of the assembly surface 80. The piston
and armature will then be in the de~ired rest position, and
clamp 46 is then tightl~ clamped to post 37a after the valve
plate and cylinder head assemblie~ 51 are mounted to the cylinder
block, the motor will be ready for operation.
T~ operate the linear compressor motor, windings 32 and
33 mu~t be connected to a source of alternating current. In
the embodiment of the invention illustrated herein, the source
of alternating current is half-wave rectified utility power at
a frequency of 60 Hz. The motor thus operates at 3600 recipro-
cations per minute. It i8 well known in the art that maximum
compre~sor efficiency will be achieved when the reQonant fre-
quency of the compressor during normal operation approaches the
line fre~uency of the exciting voltage. Thu~, the natural
03cillating frequency of the piston, armature, rod and return
springs taken together with the normal ~uction and discharge
pressures in the compression chamber should approach 60 Hz.
me natural frequency of the return springs together with the
rod, pi#ton and armature must, therefore, be less than the
frequency of exciting current. In the disclosed embodiment,
the natural frequency of the return springs and the rod, piston
and armature is preferably substantially equal to 38 Hz.
Operation of compressor 20 may be best understood with
reference to FIG. 6 which i8 a timing diagram depicting the
relationships of ~elected parameters of compressor 20 during
one cycle of line voltage. me line voltage 60 describes a
sub~tantially sinusoidal pattern over the duration of a 360
cycle time. Because compressor 20 present~ an inductive load
to line voltage 60, it is to be expected that the current 61
will lag voltage 60 and describe a rectified half wave which is
periodic but not ~inusoidal. ~he flux 62 through magnetic circuit
106594~
30 follows, but slightly lags, current 61. The ordinates of
voltage 60, current 61 and flux 62 are measured in units of
volts, amps and kilomaxwells re~pectively and are not to scale.
However, voltage 60, current 61 and flux 62 have a common zero
ordinate reference for clarity of understanding. The armature
displacement 63 is measured in units of inches with the zero
displacement reference being the abutment po~ition of armature
40 against pole faces 32c and 32d which reference position is
depicted in FIG. 4. The magnetic force 64 is measured in units
of pounds with reference to positive displacement of armature
40. m us, magnetic force 64 which tends to move armature 40 in
a negative direction, that is, a direction toward the zero dis-
placement referenc~, is shown executing a negative excursion
from the zero magnetic reference point. Similarly, spring
force 65, which is the force exerted upon armature 40 by springs
44 and 45, and pressure force 66, which is the force exerted
on the compression face of piston 42, are measured in unit~ of
pounds with reference to a positive axial displacement armature
40, that i8, a spring or pressure force which tends to move
armature 40 in the direction of positive axial displacement i~
considered to be a positive force. Magnetic force 64, spring
force 65 and pre~sure force 66 have a common zero ordinate
reference for clarity of understanding. The abscissa of FIG. 6
i8 measured in units of electrical time in degrees of a single
cycle of line voltage 60. It should be noted with respect to
FIG. 6 that, while the signals shown therein are not to ~cale,
the geometry of each ~ignal is duplicated from test results
based upon the 450 BTU/Hr working embodiment to be set forth in
detail hereinafter~
In the operation of compressor 20 voltage 60 begins a
positive excursion at electrical time zero degrees and induce~
current 61 in the windings of magnetic circuit 30. Current 61
--10--
1065941
induces, in turn, flux 62 in core 31 and armature 40. Thus,
starting at zero degree3 electrical time, magnetic force 64
gradually increases (in the negative direction) and urges armature
40, and therefore pi~ton 42, in the negative displacement direc-
tion. It will be noted from FI~. 6 that, at time zero degrees,
armature 40 is moving in the positive displacement direction
which mean~ that, at the beginning of an electrical cycle, the
armature is executing its retuxn stroke, as opposed to it~
compre~sion stroke, as a re~ult of the momentum imparted to
the moving a~sembly comprising armature 40, rod 41, piston 42
and clamp 50 by return spring~ 44 and 45 during the preceding
electrical cycle. Spring force 65 is negative at time zero
degrees indicating that spring~ 44 and 45 are in compression
and exert a force on armature 40 in the negative displacement
direction. Thus, shortly after time zero degree~, magnetic
~orce 64 co~perates with spring force 65 to work against the
momentum of the a~sembly to arrest positive displacement thereof
and begin movement in the negative direction.
At an electrical time of 90 degrees, displacement 63 has
reached it~ maximum value and the moving a~sembly ha~ reached
its "top dead point" of operation. The a~sembly will begin
to~move in the negative direction. As i8 to be expected, at
time 90 degree~ spring force 65 has reached its maximum negative
or compression value and will begin to move in the positive
direction. Magnetic force 64 will continue to increase in a
negative direction as current 61 and resulting flux 62 increases.
Armature 40 and piston 42 now move in the negative di~placement
or working direction toward the head-end of pump chamber 39,
compressing the gas in chamber 39 to a de~ired discharge pre~sure
at which the discharge valve will open.
When moving in the negative displacement direction, armature
40 will eventually pa~s its neutral position 30 that springs 44
10659~1
and 45 go into tension and begin to retard further negative
displacement of the moving assembly. In FIG. 6 this neutral
or zero spring force po~ition is achieved at an electrical
time of approximately 208 degrees. It should be noted that
at time 208 degree~ flux 62 has already pa~sed it~ maximum point
and has begun to decline toward zero.
When magnetic force 64 and the rate of change of momentum
of the moving mass 40, 41, ~2 and 50 is equal to the sum of
spring force 65 exerted on armature 40 by return springs 44 and
45 in tension and pressure force 66 exerted on the face of
piston 42 by the compressed gas in chamber 39, positive dis-
placement is arre~ted and the armature and piston reach their
"bottom dead point" of operation. In FIG. 6 this occurs at an
electrical time of approximately 265 degrees. It should be
noted that at this "bottom dead point" time flux 62 in magnetic
circuit 30 i5 less than haif of its maximum value.
Magnetic force 64 will continue to decline after bottom
dead point time 265 degrees 30 that spring force 65 and pressure
force 66 govern movement of the armature and piston and return
the moving assembly in the positive displacement or return
direction. Winding current 61 reaches a zero value at time
320 degrees. ~ecause the current i8 rectified, voltage 60
returns to zero at this tlme. The moving assembly comprising
armature 40, rod 41, piston 42 and clamp 50 continue motion
in the positive displacement or return direction under the
influence of pressure force 66 and spring force 65. Positive
displacement 63 will continue to increase until the moving
assembly reaches its top dead point of operation under the
influence of the spring and magnetic forces as outline~ above.
In-prior linear-motor compressor~ of the ~ype which include
a cylindrical armature and air-gap, the armature is attracted
into the air-gap and made to do work until it reache6 a point
-12-
~06594~
at which it~ top and bottom end faces are flush with the faces
defining the axially opposite ends of the air-gap. At this
point, the armature completely fills the air-gap and, since
the air space between the armature and pole faces i~ constant,
the reluctance of the total magnetic circuit is at a minimum.
The armature can thu~ be made to do no further work in that
cycle. It ha~ apparently been assumed by others in the art
that this constraint will also apply to a linear motor having I
a conical armature and air-gap; this, however, is not the case.
Indeed, maximum compressor efficiency is obtained when the
conical armature "fills the air-gap" at the point of maximum
flux and, since this maximum flux point will not necessarily
occur at the "bottom dead point" of operation, it is advan-
tageous to have the armature continue through the air-gap beyond
this flush point. Since the air space between the conical
armature and opposing pole faces is no longer constant and is,
in fact, a function of axial diqplacement, the reluctance of the
total magnetic circuit will continue to decrease even though
part of the armature is moviny out of the air-gap.
Returning to FIG. 4, it can be seen that armature 40 extends
out of the air-gap a distance "a" when pi~ton 42 extends a
distance "b" beyond the end face of cylinder block 38. m e flush
condition will exi~t when the ~inimum diameters of the armature
and air-gap are coplanar--i.e., when a . 0. It i8 undesirable
to allow armature 40 to ~trike the pole faces; for this reason,
distance "a" is made much larger than distance "b". The piston
will thus strike the valve plate before the armature can reach
the pole faces, which prevents the armature from striking the
pole faces.
Referring to FIG. 6, it will be ~een that the "bottom
dead point" of operation is achieved at an electrical time of
-13-
106S94i
about 265 degree~. At this time flux 62 in magnetic circuit 30
is le~s than half of its maximum value. Armature 40 is to be
positioned on rod 41 so that the ar~ture is flush with pole
pieces 32a and 32b at an electrical time of approximately 180,
at which time flux 62 achieves itB maximum value. This may be
accomplished by modifying the diameter of air-gap 34 vis-a-vis
the diameter of armature 40, while maintaining identical included
angles of taper, so that, when piston 42 is in the ~et-up posi~ion
shown in FIG. 4, armature 40 extends through the air-gap a
distance calculated to achieve the desired flush position at the
desired time based upon the test re~ults ~hown in FIG. 6. Re-
ferring again to FIG. 4, in the 450 BTU/Hr working embodiment
of the compressor, armature 40 is positioned to extend approxi-
mately .350 inches beyond pole pieces when piston 42 extends
.030 inches beyond the head-end of cylinder block 38.
The minimum air space between the pole faces and the armature
will exist when the piston abuts the valve plate assembly. In
the disclosed embodiment this minimum space, that is, the minimum
distance from a pole face to the armature as mea~ured in a direction
perpendicular to the pole face, is sub~tantially .0035 inches.
It would, of course, be undesirable to allow the p$ston to
continually strike the valve plate during normal operation.
However, as is well known in the art, compression efficiency
is optimized when the distance between the piston face and the
valve plate approaches zero at the "bottom dead point" o,f opera-
tion. Magnetic force, spring force and compression force must
be thus optimized to achieve maximum compression efficiency with-
out allowing the piston to 3trike the valve plate.
While it has been stated for purposes of explaining the
operation of the compres~or of FIGS. 1-5 that the armature
moves "into" and "out of" the air-gap, it will be appreciated
from the discussion immediately above that the armature need
-14-
~06594~L
not move "entirely out of the air-gap" nor for that matter
need "a major portion thereof" be located outside of the air-gap
at the "top dead point" of operation, contrary to the dis-
closures in the United States Earthalon patent~ 3,542,495
and 3,461,806 respectively. Indeed, in the embodiment dis-
closed herein, which operates at 450 BTU/Hr at standard rating
point conditions, the total compression stroke is only .8
inches, and the armature exposure at the "top dead point" of
operation is less than 50 per cent.
When the magnetic circuit reluctance characteristics de-
tailed above have been defined--i.e., a substantially linear
reluctance curve over the entire stroke length and an armature
flush condition at the time of maximum flux--then the included
angle of taper of armature 40 and air-gap 34 may be specified.
As stated above, it has been found that, under the above re-
cited conditions, a piston extension dimension "b" of .030
inches y~elds good results. To achieve this dimension, the
included angle of taper of the armature and air-gap should be
at lea~t 10, and a range of taper included angles between 10
and 14 is preferred.
me aforementioned ~arthalon patents teach that the
efficiency of a linear motor will be optimized if the reluc-
tance of the magnetic circuit varies linearly with armature
movement. As the aforementioned U. S. Patent ~o. 3,947,155
teaches, the stability of a pump which may occasionally
operate below atmospheric pre3sure, such as a refrigeration
compressor, will be enhanced if the linear reluctance curve
also has a low slope. The various design parameters have been
optimized in the present compressor motor to achieve this
desired re~ult. While it is not necessary to have the angle
1065941
of taper of the armature identical to that of the air-gap, it
has been found that this condition gives the best overall
results. It has al80 been ~ound that the best results are
achieved if the net cross ~ection of the armature, that is, the
cross sectional area of the armature taken on a plane through
the center of the armature parallel to the axis of movement
and excluding the armature rod, is equal to about 80 per cent
of the effective cross sectional area of the pole piece. The ,
effective cross sectional area of the pole piece i~ that area
taken on a plane parallel to the axis of movement of the arma-
ture and perpendicular to the flux through the pole piece.
The shape of this cross section should be ~ubstantially ~quare
rather than rectangular to achieve the minimum winding length
per unit of de~ired flux. me gross cross sectional area of the
armature, that i8, the cross sectional area of the armature
taken as above but including the armature rod, should be
greater than the effective cross sectional area of the pole
piece. Thi~ arrangement yields good results, particularly when
an armature rod of magnetically permeable material is used to
increase the "magnetic cross section" of the armature.
The reluctance curve of the above-mentioned ~S0 BTU/Hr
embodiment i~ ~hown in FIG. 7. In the curve 70 of FIG. 7 the
abgci8~a i8 in inches of displacement a~ measured from the
condition of FIG. 4 when the armature is seated in the magnet-
ic core. The ordinate measurement of reluctance indicates
that minimum reluctance at the position of FIG. 4 is approxi-
mately .001 ampere-turn~ per maxwell. It ~as been found that
an excessive ~lope angle 71 is accompanied by freguent impact
of pi~ton 42 upon valve plate 51, while an insufficient slope
results in loss of mechanical efficiency and a reduced range
of conditions for successful operation. It will be noted
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that reluctance curve 70 is substantially linear over the
entire stroke of .8 inches and has a slope of approximately
.022 ampere-turns per maxwell-inch. The parameters of this
450 BTU/Hr working embodiment which contribute to this low-
sloped, linear reluctance curve, and the con~equent high com-
pressor efficiency, are ~et forth in the discu~ion of the
working embodiment detailed hereinafter.
An alternative to the three-turn paired spxing arrangement
in the compressor of FIG. 1 is shown in FIGS. 8 and 9. An
outboard bearing and spring retainer plate 100 i9 clamped to
magnetic circuit 30 and cy~inder block 38 by the tie bolts
102 and 104. Plate 100 has a pair of spring retainer~ 106
and 108 each of which fixedly clamps one end of the zero pitch
linear springs 110 and 112. Respective straight end-tang ter-
minations llOa and 112a at the outboard end of springs 110 and
112 are clamped into associated holes 106a and 108a of clamps
106 and 108 by means of screws 114 and 116 which traverse
associated bracket splits leading to each of the clamp holes.
End-tangs llOb and 112b are similarly clamped to armature 40
by means of spring clamp plate 50.
It will be appreciated by those skilled in the art that,
depending upon the manufacturing technique used to fabricate
the springs, a "zaro pitch" spring will have a pitch between
zero and the diameter of the spring material. Where straight
end-tangs are required, the spring is usually first coiled
on a circular mandrel or jig with the end-tangs extending tan-
gentially from the coil. The end-tangs are then bent to
positi~ns perpendicular to the plane of the coil. The pitch
of the ~pring thus formed will be sub~tantially equal to zero
within ~ome tolerance range which depends upon the resilience
106594~
of the material u~ed to wind the spring.
There are approximately .92 turns of spring material in
spring~ 110 and 112. End-tangs llOa and llOb of spring 110 are
thus laterally spaced from each other allowing room for spring
112 to pass tharethrough before terminating in clamp 50. Sim-
ilarly, end-tang~ 112a and 112b are spaced to allow passage
of spring 110 therebetween, thereby internesting the ~pring~.
In this geometry the coils of springs 110 and 112 are aligned
with a line connecting tie bolt~ 102 and 104 rather than being
perpendicular therewith and are contained within the lateral
perimeter of compressor 20 defined by magnetic circuit 30,
thereby reducing the lateral and axial dimensions of the com-
pressor. Furthermsre, with the coils of springs 110 and 112
disposed in axial proximity to magnetic circuit 30, housing 21
which encompasses compressor 20 may assume an eliptical shape
which is believed to reduce the level of acoustical noise
e ~nating from an operating unit.
me zero pitch internesting spring~ ~hown in FIGS. 8 and
g have the additional advantage of reducing the twisting forces
imparted upon armature 40 almost to zero. This reduction in
the torsion or twisting forces on the armature and spring~
results in long spring life and helps maintain armature 40
within air-gap 34 during axial reciprocation thereof.
As shown in FIG. 2, magnetic core 31 compri~es stacked
laminations attached in a manner well known in the art. Al-
ternatively and preferably, the magnetic core may be comprised
of first and second inner loops spirally wound of magnetic
strip material with the loop~ placed in abutment and banded
together by an outer loop of the same magnetic ~trip material.
Such a core 120 is shown in FIG. 10 and i~ constructed by
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~0ti5941
first separately winding a pair of identical inner loops 122
and 124 of magnetic strip material to orm spiral wrap
pattern 126. When loop 122 has reached the desired thickness,
the strip material may be terminated and tacked as shown at
128. When loop 124 has reached the desired thickness, the
strip material is to be tacked a~ at 130, but need not be ter-
minated. Loops 122 and 124 are then placed in flat end abut-
ment on plane 129 and the magnetic strip material extending
from tack 130, or a separate strip material tacked onto either
loop at a convenient attachment point, is wound around the ex-
posed periphery of the dual loop subassembly to form an outer
convoluted loop 132 which holds inner loops 122 and 124 tightly
together as disclosed in U.S. Patent 2,431,128, which issued
on November 18, 1947 in the name of E. A. Link. Conical air-
gap 134 i8 then machined in the area of abutment of inner loops
122 and 124. Windings 32 and 33 will be wound about the opposing
pole pieces and will have magnetic communication carried entirely
by the inner loops. For this reason, outer loop 132 may be of
any convenient m~terial. me magnetic core ~hown in FIG. 10
is more easily as~embled and has less waste material than ~tacked
lamination core 31.
FIG. 11 is a perspective view of conical air-gap 134 after
the air-gap i8 first machined into the area of abutment of
first and second loops 122 and 124. When the minor diameter
of gap 134 is less than the width of the core (i.e., the dimen-
sion perpendicular to plane 129), then the pole pieces 136 and
138, rather than being isolated from each other, are connected
by the magnetic bridges or connections 140 and 142 on either
side of the machined gap. In order to prevent a short in the
magnetic circuit and to facilitate the mounting of windings
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32 and 33 upon pole pieces 136 and 138, connections 140 and
142 must be removed at a second machining stage in the fabri-
cation of magnetic core 120.
FIG~ 12 iæ a perspective view of air-gap 134 in magnetic
core 120 after bridges 140 and 142 have been removed. Bridges
140 and 142 have been removed by machining across the faces of
pole piece~ 136 and 138 in a pair of planes X and Y respec-
tively perpendicular to the central axi3 of poles 136 and 138
and parallel to but displaced on opposite ~ides of the axis
of reciprocation. ~hen the distance between planes X and Y
i8 less than the maximum diameter of gap 134, this machining
will produce in these planes the triangular coplanar exposed
gap surfaces 140x, 142x, and 140y, 142y upon opposing face of
pole pieces 136 and 138 respectively. When the gap between
planes X and Y is to be only sufficient to allow insertion of
windings 32 and 33, a distance between the plane~ of 16.7 per
cent of the cro~s-sectional area of the pole~ is sufficient,
or, in other words, the ratio of the distance between the
planes and the cross-sectional area of the poles is 0.167 ins. 1.
Hbwever, it has been discovered pur~uant to the present
invention that compre~sor operation is enhanced when the dis-
tance between planes X and Y is increased beyond this 16.7 per
cent figure. In a specific 450 BTU/Hr working embodiment of
the present invention having 1.5 inch-square poles, the di~-
tance between planes X and Y was increased to .8 inche~ or
approximately 35.5 per cent of the cro~s-sectional area of
the poles. This arrangement yielded the results ~hown in FIG.
13 when compared to a similar 450 BTU/Hr unit with a planar
gap of .375 inches or 16.7 per cent. In FIG. 13 BTU/Hr output
is plotted versus evaporation temperature. Dashe~ curve 150
depicts the output of the .375 inch unit over a wide range of
evaporation temperatures while curve 152 represents the output
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of the .8 inch unit over the same xange. It can be ~een that
the two units perform equally at rating point conditions--point
154--and perform similarly at evaporation temperature~ lower
than rating point. However, at higher evaporation temperatures
the performance of the .375 inch unit falls off much more rapidly
than th~ performance of the .8 inch unit. It should be noted
that the curves of ~IG. 13 were plotted from actual test re~ult~
and are to scale.
Strip wound core 120 may replace laminated core 31 in
compressor 20 of FIG. 2. In thi~ preferred compressor assembly,
tie bolts 35 and 36 pass through a pair of substantially tri-
angular apertures 131 and 133 which are formed in the area of
abutment of inner loop~ 122 and 124 and are bounded by the
inner loops and outer loop 132 a~ best seen in FIG. 10. Aper-
tures 131 and 133 afford core 120 a greater degree of lateral
"slop" in the asse~ly stage, thus facilitating the automatic
alignment process discussed above with respect to FIG. 4. In
addition, since the individual wraps or laminations of core 120
are layered in a direction perpendicular to the axis of recip-
rocation and to the axes of tie bolts 35 and 36, the retention
stresses exerted by the tie bolts are taken edgewise by each
wrap in~tead of parallel to the thickness dimen~ion of each
wrap. Hence, the core laminations or wraps are not subjected
to bolt forces tending to squeeze them together. Due to this
orientation of the bolts parallel to the lateral or width
dimention of the wraps, the strip wound core is not compressible
in the direction of tightening of the tie boltq. For this
reason, it i~ easier to hold alignment tolerances when core 120
is used.
q~he material disclosed above with reference to FIG~. 1 to
~0659~
7, as well as the general concept of fabricating the air-gap
as shown in FIGS. ll and 12, are the subject of the afore-
mentioned U. S. Patent Nb. 3,947,155. ~his material is dis-
clo~ed herein to facilitate understanding of the pre3ent
invention and because it is the best method presently known
for practicing the invention.
Several working embodiments of compres~or 20 have been
built and tested; one such embodiment i~ the 450 BTU/Hr (nominal)
; unit mentioned above and drawn to scale in FIGS. 1-5. By way
of example and not by way of limitation, the parameters which
contribute to the low-slope linear reluctance curve and the
resulting high compressor efficiency at rating point conditions
are as follows:
mass of piston 42. . . . . . . 0.17 lbm
mass of armature 40. . . . . . 0.8 lbm
mass of rod 41 . . . . . . . . 0.13 lbm
mass of clamp 50 . . . . . . . 0.12 lbm
effective mass of springs 44
and 45 (1/3 actual mass) . . 0.08 lbm
rate of spring~ 44 and 45. . . 200 lb/in
material of rod 41 . . . . . . 1060 ~teel
net cross-eectional area of
armature 40. . . . . . . . . 1.76 sq. in.
gros~ cross-sectional area
of armature 40 (and 41). . . 2.32 sq. in.
effective cro~s-sectional
area of pole pieces 32a and
32b. . . . . . . . . . . . . 2.25 sq. in.
resist~nce of windings 32
and 33 . . . . . . . . . . . 2.10 ohms
number of turns in windings
32 and 33. . . . . . . . . . 400
refrigerant suction pressure . 4.4 p9ig
refrigerant discharge pressure 180 psig
~o6594~
refrigerant temperature enter-
ing compressor housing . . . 90~F
bore . . . . . . . . . . . . . 1.156 inches dia.
flux path area . . . . . . . . 2.25 sq. in.
In the working embodiment with the above exemplary parameters,
the following results were measured at refrigeration industry
standard rating point condition~ after 10,000 hour~ of oper-
ation:
capacity . . . . . . 485 BTU/Hr
power input. . . . . 134 watts
efficiency . . . . . 3.62 BTU/watt-hour (Weston3
In addition, the following results, which are difficult to
accurately measure in a working linear compressor, were cal-
culated from a computer analysis of the 450 BTU/Hr model, the
analysi3 being similar to that set forth above with reference
to FIG. 6:
lengt.h of stroke . . . . . . . 0.54 in.
position of A/C power
cycle at "top dead
point" of operation. . . . . 91 degrees
position of A/C power
cycle at flush position. . . 207 degreeQ
current at flush position. . . 4.9 amps
flux at flush position . . . . 213 kilomaxwells
pesition of A/C power
at maximum flux. . . . . . . 180 degrees
current at maximum flux. . . . 7 amps
maximum flux . . . . . . . . . 231 kilomaxwells
spring force at "top
dead point" of
operation. . . . . . . . . . -70 lbf
spring force at "bottom
dead point" of operation . . +38 lbf
106594~
position of A/C power at
opening of discharge
valve. . . . . . . . . . . . 252 degrees
A3 discu~sed above, reluctance curve 70 at FIG.7 indicates
that this embodiment achieved the objective of having a low-
~loped, linear reluctance curve. Furthermore, the above data
indicate~ that the objective of achieving maximum flux at
the flu3h po~ition ha~ been achieved within 8 per cent.
From the foregoing de~cription, it will now be apparent '
that there has been provided, in accordance with the invention,
an improved moving-iron linear compressor motor that fully
satisfies the objects and advantages ~et forth above. While
the invention has been de~cribed in conjunction with 3pecific
embodiments thereof, it is evident that many alternatives,
modifications, and variations will be apparent to those skilled
in the art in light of the foregoing description. It will be
further apparent that, while the invention ha~ been disclosed
and exemplified in connection with a refrigeration system,
the invention i8 equally applicable to other type~ of refrig-
erant systems and that, indeed, many principle~ of the inven-
tion may be applied generally to gas pumps, such a~ air com~
pressors or the like. Accordingly, the invention is intended
to embrace all such alternatives, modifications, and varia-
tions as fall within the spirit and broad scope of the
appended claims.
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