"METHOD AND APPARATUS FOR AIR GAP SETTING"

TECHNICAL FIELD The present invention relates to the setting of the air gap in electric machinery with particular relation to electric motors used in hermetic and semi-hermetic compres¬ sors employed in vapour compression refrigeration systems.

BACKGROUND ART Ever since the electric motor (in which a rotor, attached to a shaft, and surrounded by a number of pairs of magnetic poles) has come into use it has been widely recognized that the air gap between the surface of the rotor and the adjacent surfaces of the poles should be uniform and of the smallest dimension which will not allow contact of the rotor surface with any pole surface since this minimum air gap results in the lowest reluctance of the magnetic circuit and so provides the maximum torque on the rotor with all other things being equal.

The usual construction of an electric motor in a hermetic or semi-hermetic compress'or consists of a frame containing a bearing supporting a cantilever shaft to which is attached a rotor of cylindrical shape. Also attached to the frame is a stator having a bore which is larger in diameter than that of the rotor by twice the desired air gap dimensions. The axial length of the stator is equal to or slightly less than that of the rotor. The stator is posit¬ ioned on the frame so that the distance between the rotor surface and the surface of bore in the stator is the same for all positions. The space between the rotor and the stator is the air gap and the operation performed to establish a uniform air gap is referred to as setting the air gap.

The rotor and stator are constructed by stacking pre- shaped laminations on top of each other until the required stack height is obtained. Each lamination is about 0.5 mm thick and is pierced or slotted to accept the electrical conductors. In the case of the rotor the laminations are a circular disc in which a central hole is pierced and a plurality of equally spaced holes pierced on a circle concen¬ tric with the central hole. The rotor electrical conductors are placed in the outer holes and these together with end rings serve to hold the rotor stack together. The central

2 hole and the outside diameter of the rotor stack are machined to obtain the finished sizes and to establish the best concentricity between the axis of the central hole and the cylindrical surface so machined. In the case of the stator, the laminations are stamped to obtain the desired shape which can be of any convenient form and to pierce a large bore which is approx- ^' imately central with the outer form. A plurality of radial slots which communicate with the central bore as well as plurality of holes near or slots at the outer edge are pierced The stator stack is formed by selecting and stacking the number of laminations required to give the necessary height after which one half of the stack is rotated 180 relative to the outer half with the object of cancelling out most of the wedge effect brought about by the taper in the lamination sheets. The central bore in the stator laminations and the radial slots leading from it are aligned after which preformed electrical insulating film is inserted into each slot. Magnetic winding wire is then wound into the slots to produce the desired number of magnetic poles.

The insulating film and the winding wire inserted into each slot will hold the laminations in approximate alignment.

It is common practice to bring the laminations into better alignment by positioning the stator stack in a fixture so that a close fitting mandrel locates in the large central bore. In some cases the mandrel is equipped with an expand¬ ing device which takes up all the clearance between the mandrel surface and the surface of the large central bore. Furthermore the mandrel is positioned perpendicular to the surfaces in the fixture on which the stator stack end surface contacts. An externally activated clamp holds the stator stack in firm contact with the end contacting surfaces. While the stator stack is held firmly with the bore aligned and perpendicular to one end surface, two or more welding runs are made over the full length of the outer surface of the stator stack in such a way as to weld the adjoining laminations together thereby restraining the whole stator stack in the approximate shape it had taken in the fixture.

Other methods have been developed to restrain the

CYΓI

individual laminations in alignment after clamping in the fixture. One method is to notch the outer edge of each lamination so that a plurality of dovetail openings are formed. When the stack is aligned and clamped in the fixture the notches are also aligned to form straight dove¬ tail grooves into which cleats are inserted and expanded so as to be in firm contact with the sides of the groove. A still further method of restraining the individual lamin¬ ations in alignment after clamping in the fixture is to dip the stator stack into a varnish which will penetrate the crevices between the lamination and be held there by capillary attraction. After dipping and removal of excess surface varnish the stator stack and fixture are heated in an oven to bake the varnish thereby bonding all laminations into a solid stack.

The object of these operations is to obtain a stator stack in which the bore surface lies on a cylindrical plane and one end surface is perpendicular to the axis of the cylinder, this being the surface which will locate on the motor frame. Some manufacturers resort to a final machining of the mounting face and the large bore to achieve satis¬ factory flatness and perpendicularity. The motor frame is machined so that the surface on which the stator stack is located is perpendicular to the axis of the bearing support- ing the rotor shaft.

When the stator is assembled to the motor frame, the axis of the rotor shaft will be parallel to the axis of the stator bore. When the shaft and the rotor are assembled, the stator can be moved radially until the air gap between the surface of the rotor and the surface of the stator bore is uniform in all positions.

With the stator so positioned, threaded bolts are fitted to the holes or slots in the outer portion of the stator and engaged with threaded holes in the frame. The bolts are tightened to exert sufficient force to hold the ^' stator in the set position without allowing movement when the frame and the stator are subjected to the forces exper¬ ienced during normal operation and transit.

The preceding methods have a number of shortcomings which Drevent the attainment and holding of a uniform air

OMPI Sλ . ^' 1JWDΛ

gap, or they substantially add to the production cost.

Firstly, the taper and lack of flatness of the lamina¬ tion sheets in the stator may create a lean to one side of he stator notwithstanding the rotation of half of the stack with the object of cancelling any taper.

Secondly, the welding of the outer edges of the stator laminations introduce stress which may cause the stator to lean to one side.

Thirdly, the bonding of the laminations by varnish is an expensive operation.

Fourthly, the machining of the stator bore and face is an expensive operation.

Fifthly, the friction force between the frame face and the stack face is not high enough to resist the disturb- ing forces imposed by rough handling of the assembled motor.

DISCLOSURE OF THE INVENTION One of the objects of this invention is to prepare the stator stack so that improved perpendicularity between the bore and the mounting face (and thus a more uniform air gap) is obtained without resorting to the high cost of machining.

A desirable result achieved by the preferred embodi¬ ment of this invention is to perform a low cost operation which will bind the first lamination in the stator stack to the prepared face on the frame and then bind each lamination to each other so that relative movement of the whole or part of the stack to the frame will not take place even under the most arduous conditions of transport.

According to one aspect of the present invention there is disclosed a method of setting the air gap in an electrical machine having a frame, a shaft rotatably mounted by said frame, and a cylindrical rotor carried by said shaft and intended to be rotatable therewith within a stator supported by said frame, having a cylindrical bore and being formed from a stack comprising a plurality of laminations, said method comprising the steps of:

1. aligning the bore of each lamination in said stack and then pressing said stack to provide an end surface thereof perpendicular to the longitudinal axis of said bore, 2. locating said end surface on a prepared surface

of a frame for said machine, and placing said shaft within said bore,

3. externally gripping said shaft and simultaneously internally gripping said bore to position said stack with the longitudinal axis of said bore coincident with the longitudi¬ nal axis of said shaft,

4. securing said stack to said frame, releasing said internal and external gripping, and placing said rotor on said shaft. Preferably the stack is secured to said frame by mechanical means such as bolts which provide a frictional force preventing movement of said stack relative to said frame. In order to provide additional rigidity and thereby prevent relative movement between the laminations, and/or the stack, and/or the frame, the laminations are preferably adhered to each other and the stack is also preferably adhered to the frame.

According to another aspect of the present invention there is disclosed an aligning press apparatus for the stator stack, said apparatus comprising expandable cylinder means located interior of the bore of said stack, a press within which said stack is located, and means to operate said expandable cylinder means prior to, and during, operation of said press whereby the bore of each lamination forming said stack is aligned to form a cylindrical stack bore and said stack is pressed to provide a desired stack shape relative to said cylindrical stack bore.

According to another aspect of the present invention there is disclosed stator bore and shaft aligning apparatus comprising an expandable cylinder means to grip the interior of said stator bore and make same cylindrical, and an inter¬ nally contractable cylinder means to grip the exterior of said shaft, the longitudinal axis of both said cylinder means being coincident. Preferably the abovementioned cylinder means each comprises a collet.

BRIEF DESCRIPTION OF THE DRAWINGS The preferred embodiment of the present invention will be further described with reference to the accompanying drawings in which: Fig. 1 is a fragmentary vertical sectional viev

6 an electric motor incorporating a stator, a rotor, a shaft and a mounting frame with integral bearing.

Fig. 2 illustrates a stator positioned in a press fixture. Fig. 3 is a sectional view of a stator positioned on a mounting frame with a locating tool fitted to the motor shaft.

Fig. 4 is a plan view of typical mounting bosses on a mounting frame. Fig. 5 is a plan view of a stator lamination.

Fig. 6 is a part sectional elevation of a locating tool,

BEST MODE OF CARRYING OUT THE INVENTION

Referring in more detail to the accompanying drawings, Fig. 1 illustrates one exemplary embodiment of an electric motor constructed in accordance ith the present invention and particularly adapted for use in hermetic or semi-hermetic compressors used in vapour compression refrigeration systems. The motor consists of the usual rotor 1 and stator 2 each built up by stacking pre-punched steel laminations on each other until the required stack height have been achieved, each lamination being about 0.5 mm thick.

The rotor 1 has diecast conductor bars 4 and end rings 5 which also serve to hold the assembly firmly together. It is usual to machine the external surface of the rotor 1 and the central hole 70 in it to generate a cylindrical outer surface 71 concentric with the axis of the centre hole 70 through which the shaft 60 is fitted. An interference fit of the rotor 1 on the shaft 60 with sufficient friction to transmit the maximum motor torque is obtained by machining the hole 70 in the rotor 1 smaller than the shaft diameter and then expanding the rotor 1 by heating it until the hole 70 is large enough to slip over the shaft 60.

The shaft 60 is supported in a bearing 7 which is machined in the cast mounting frame 8 which in this embodi¬ ment is integral with the compressor crankcase casting. The mounting frame 8 also incorporates a plurality of raised bosses 9 on the side where the rotor shaft 60 extends,

AU81/00013

7 bosses 9 are machined to generate flat co-planar surfaces on which the stator will locate. These surfaces are perpendicular to the axis of the shaft bearing.

Some of the bosses 9 are drilled and tapped to receive threaded screws 10 which are fitted through holes

11 {Fig. 5) pierced in the stator laminations 12. The position of the holes 11 in the laminations 12 and the mounting frame 8 relative to the axis of the bore 72 in the laminations 12 and to axis of the bearing 7 in the mounting frame 8 are the same so that when the screws 10 are fitted to the holes 11 in the laminations 12 and engaged with the threaded holes in the bosses 9, the axis of the bore 72 in the laminations 12 will be approximately coincident with the axis of the bearing 7 in the mounting frame 8. Clearance between the screws 10 and the holes 11 in the laminations 12 is provided so that some small movement of the stator stack 73 can be made to compensate for any errors in positioning the holes in both the laminations 12 and the bosses 9. The stator stack 73 is constructed from the requi¬ site number of laminations 12 needed to produce the desired stack height. Initially, all the laminations 12 are oriented in the stack 73 in the same direction they occupied in the sheet from which they were stamped. After building up the stack 73 approximately one half of the number of laminations is rotated 180° relative to the other half, the plane of rotation being parallel to the plane of the laminations. This operation cancels some of the lean in the axis of the bore 72 in the stack 73 caused by taper in the sheets from which the laminations 12 were stamped. With the slots 13 in the stator stack 73 approximately aligned, preformed insulation 14 is inserted into each slot 13 after which the magnet winding wire 15 is placed in position on the slot insulation 14. The cumulative stiffness of all the pieces of slot insulation 14 and the wire 15 in the slots 13 holds the laminations 12 in position and permits the handling of the stator 2 without allowing undue disturbance of the lamina¬ tions 12 relative to each other.

The stator stack 73 is now located in a press fixture as shown in Fig. 2. A preferred method o ^~ construction a

OMPI

8 operation of the press fixture is also shown in Fig. 2. The fixture base 16 has hardened steel bosses 17 similarly disposed as the bosses 9 on the mounting frame 8 and having a shape which is larger in all directions than bosses 9 on the frame 8.

The base 16 is machined to give clearance to the stator windings (not shown) and has a boss 74 which is approximately central for mounting of an expanding collet 18, a device well known to those skilled in the art, which in the loading position, has assumed its smallest diameter and will permit the loading of the stator 2 as shown in Fig. 2. The axis of the collet 18 is perpendicular to the bosses 17.

A top tool 19 is attached to the moving ram of a press (not illustrated) and is of generally tubular form with an inner flange 20. A sliding punch 21 with a flange 22 located in the top recess 23 in the top tool 19. Radial clearance is provided between the flange 22 on the sliding punch 21 and the recess 23 in the top tool 19 and also between the body of the sliding punch 21 and the bore 24 in the inner flange 20.

The contact surfaces of the flange 22 in the top tool 19 and the sliding punch 21 are machined flat and perpendicular to the axis of the press fixture.

The lower end of the sliding punch 21 is machined to a conical surface 25 substantially identical to an interior conical surface 25 in the collet 18. The lower extremity of the sliding punch 21 is machined to form a spigot 26 which will closely fit a recess 27 machined in the base 16. Chamfers 28 are machined on the spigot 26 and the recess 27 to lead one into the other. An elastic pad 29 made from a suitable grade of polyurethane or similar elastomer is placed above the flange 22 on the sliding punch 21. A back plate 30 is attached to the top tool 19 by screws 31. The upper surface of the top tool 19 is machined perpendicular to the axis of the fixture.

A removable tubular section 32 is attached to the top tool 19 by screws, (not illustrated). The axial length of the removable tubular section 32 is varied as required to suit various stator heights. in operation the press ram descends and

9 top tool 19 onto the base 16, guidance being done by a conventional die set (not illustrated) . The first engage¬ ment of the sliding punch 21 occurs when the spigot 26 enters the recess 27. Should there be any misalignment between the parts, side movement of the sliding punch 21 will be caused by the lead of the spigot 26 into the recess 27 until correct engagement and alignment occurs. Further downward movement of the top tool 19 results in engagement of the two conical surfaces 25. Still further movement results in the expansion of the collet 18 until contact between its external surface and the bore of the stator 2 takes place, the first contact being with those of the laminations 12 which have the greatest displacement from the mean position. Continued movement of the top tool 19 and the collet

18 will cause the displaced laminations 12 to be moved until they all have their bores on a common axis. The rigidity of the laminations 12 will resist any significant enlargement of the bores but will cause an increase in loading between the flange 22 on the top end of the sliding punch 21 and the elastic pad 29. The length of the removable section 32 is chosen so that contact by its lower surface is about to be made with the top of the stator stack at this point of travel of the top tool 19. The continued movement of the press ram results in further reduction in thickness of the elastic pad 29 with little expansion of the collect 18 but establishes firm contact of the lower surface of the removable section 32 with the top of the stator stack 73 and the application of a force sufficient to cause permanent deformation of the bottom lamination 61 where it is in contact with bosses 17. Since the faces of the bosses 17 are perpendicular to the collet 18, the corresponding surfaces on the stator 2 will now bear the same relationship. After this operation has been performed the stator stack 73 will have been brought into a condition where the face mounting areas are flat and co-planar, the bore 72 is cylindrical and perpendicular to the mounting areas and there is a metal to metal path around each mounting hole 11. No attempt is made to restrain the

10 this condition after removal of the forces applied to the fixture and after removal of the stack 73 from the fixture other than the mild restraint of the slot insulators 14 and the winding wire 15 in the slots 13. During the assembly of the motor, the partly assembled compressor including the frame 8 and the shaft 60, is positioned with the mounting face of the frame 8 horizontal and the shaft 60 vertically upwards. A conventional guide (not illustrated) is placed over the shaft 60, the guide having a bore slightly larger than the shaft diameter and an outside diameter slightly smaller than the diameter of the stator bore 72.

A liquid adhesive is applied by any one of a number of known suitable means to each of the flat surfaces of bosses 9 taking care to avoid the adhesive running into the bolt holes 11. The adhesive chosen is preferably one which will not degrade when in contact with oil or refrigerant or at the temperature expected at the motor and frame during normal operation. It also preferably has a suitable viscosity and does not act as a catalyst which would promote chemical decomposition of the oil or the refrigerant. Two part epoxy resins are suitable for this purpose.

The stator 2 is then fitted over the guide on the shaft 60 and allowed to rest on the adhesive smeared mounting bosses. Approximate orientation of the stator 2 on the frame 8 is achieved by manually turning the screws 10 so as to achieve initial threaded engagement with the holes in the bosses 9.

The guide is removed from the shaft 60 and a loca- ting tool 33 (Fig. 3) fitted to the shaft 60. The locating tool 33 has a plurality of pieces forming a collet 37 which can be moved radially inwards until they contact the shaft surface and also a plurality of pieces forming a collet 39 which can be simultaneously moved outwards until they contact the surface of the bore 72 in the stator 2. The contacting surfaces of the moveable pieces are machined to form part of cylindrical surfaces corresponding to the shaft 60 and the stator bore 72 respectively. The movement of the pieces of the locating tool 33 is accomplished by the application of an external force which can be supplied in known fashion b^_

"BUREAU

11 ^{' •} - mechanical, pneumatic or hydraulic means or a combination of these means.

A preferred construction of the locating tool 33 is shown in detail in Fig. 6. The tool 33 consists of a piston 34 capable of sliding in a cylinder 35. The lower bore of the piston 34 is machined to provide a conical surface 36. A collet 37 containing a plurality of bars which are capable of radial movement has a conical outer surface to match that on the piston 34. The bore of the collet 37 is machined to suit the motor shaft 60. The cylinder 35 is machined to form a conical surface 38 on the lower outer end. A collet 39 containing a plurality of bars capable of radial movement has a conical inner surface to match that on the cylinder 35 and an outer diameter to suit the stator bore 72.

When a fluid such as air, or preferably a hydraulic liquid, is forced under pressure through a hole 40 and passage 41 into the space between the end of the piston 34 and the end of the closed cylinder 35, equal and opposite forces are applied to the piston 34 and the cylinder 35 which overcome friction and cause relative movement.

Longitudinal movement of the collet 37 relative to cylinder 35 is prevented by two or more clips 42 engaging in grooves in the collet bars and held in position by screw 43. In a similar fashion longitudinal movement of collet 39 relative to piston 34 is prevented by sleeve 44 attached to piston 34 by screw 45 and to collet 39 by a garter spring 46.

Due to the preferential longitudinal restraint on both collets 37 and 39, they are forced to move radially until contact is made with the motor shaft 60 and the stator bore 72 respectively. Application of further pressure to the fluid will increase the force applied to collet bars and will push displaced laminations 12 into the mean position. Leakage of fluid between the piston 34 and cylinder 35 is prevented by seal 63.

Initial movement of the piston 34 relative to the cylinder 35 is achieved by manual rotation of the handle 47.

A helical surface 62 on boss 48, formed on handle 47, engages with helical surface 64 on boss 49, formed on the cylinder 35. Rod 50 is attached to handle 47 and piston 34

^"

12 and serves to move the piston 34 axially in the cylinder

35 when the handle 47 is rotated. Seal 51 prevents leakage of the liquid past the rod 50 and the clearance hole in cylinder 35. A torsion spring 52 applies a torque to handle 47 to return the handle 47 to the position which gives the maximum collet clearance on the motor shaft 60 and the stator bore 72.

By applying sufficient force to the moveable pieces, the stator bore 72 is restored to the cylindrical shape arrived at in the previous sizing operation and the stator 2 is moved until the axis of the bore coincides with the axis of the shaft 60. With the stator 2 held in this position all the bolts 10 are tightened to full torque.

The clamping force is then removed from the locating tool 33 and this is then withdrawn from the stator bore. Liquid adhesive, preferably an epoxy resin, is applied to the lamination edges in a continuous strip about 10 mm wide running the full height of the stack. The line of application can be either at the outer surface or in the bore 73 of the stack 72. One or more stripes, preferably four, can be applied. The application of adhesive can be by brush, spray, nozzle or contact. The liquid adhesive will penetrate the crevices between adjacent individual laminations 12 by capillary attraction which will result in a comparatively large surface of the laminations 12 being wetted by the adhesive.

The rotor 1 is heated to a temperature which will expand the central hole 70 sufficiently to enable it to slip over the shaft 60. The rotor 1 is held in the correct position on the shaft 60 until it has cooled down sufficiently to contact and grip the shaft.

Dehydration of the assembled motor or compressor is usually accomplished by placing the compressor in an oven which is heated to a temperature above the boiling point of water. This operation can be utilized to accelerate the curing of epoxy type adhesives. Alternatively a special oven or a continuous heating tunnel can be used for the purpose of raising the temperature of the adhesive with the object of bringing about an accelerated curing time. Slow curin or dr in adhesives should be iven

13 sufficient time to attain full or near full strength before the compressor is likely to be subjected to external forces of a magnitude which could cause the stator 2 to move. The motor stator 2 of a compressor assembled in the manner described will have a bore 72 which is cylindrical and whose axis is coincident with the axis of the rotor shaft 60. The air gap will be uniform throughout the motor stack allowing the diameters of the rotor 1 and the stator 2 to be selected to provide the minimum air gap consistent with the tolerances of machining, the stiffness of the cantilever shaft and the clearances given between the shaft and the bearing, thereby resulting in greater motor starting torque. Furthermore. the stator 2 will have the individual laminations 12 used in its construction bonded together and have the bottom lamination 61 of the stack 73 bonded to the frame 8 with sufficient strength to withstand the disturbing forces which would be imposed on it by the roughest handling likely to be met. - •

INDUSTRIAL APPLICABILITY It will be apparent to those skilled in the art that the above described preferred embodiment achieves two ends. Firstly, the stator bore and rotor shaft are located co-axial so that a uniform air gap is created. This over¬ comes the problem of the rotor sometimes touching the stator bore thereby preventing starting of a low torque motor. In- addition, smaller air gaps are possible giving rise to larger motor torques. Secondly, the manner of securing the stator lamina- tors, stator stack and frame together prevents the uniform air gap becoming non-uniform because of mechanical shocks applied to the motor during transport and handling. This overcomes the prior art problem of an aligned motor with uniform air gap becoming misaligned during transport and thereby possibly failing to start at its destination.

The foregoing describes only one embodiment of the present invention and modifications obvious to those skilled in the art, can be made thereto without departin scope of the present invention.

For example, the frame 8 can be provided with only two bosses 9 even though four screws 10 are provided in the stator 2 instead of the one-to-one correspondence between bosses 9 and screws 10 as described. Those screws which do not threadably engage a boss 9, engage a nut instead. A smaller number of bosses permits a smaller frame 8 and hence a saving in material cost.

In addition, it will also be apparent that the electric machine construction described can be used for both motors and generators as well as hermetic and semi-hermetic compressors. Also the rotor shaft can be supported by rigid bearings at both ends, by only one bearing located in the machine frame, or by one bearing located in the motor frame and another bearing at the other end of the motor stack, such bearing being self aligning and attached to the stator stack.