TITLEMethod Of Assembling A Permanent Magnet RotorBACKGROUND OF INVENTIONField of the InventionThe present invention relates to a synchronous permanent magnet machine (motor/generator) having a rotor which has segments of permanent magnet fixed around a shaft, and more particularly to a method of assembling a permanent magnet rotor in which segments of permanent magnet are held using high strength composite materials allowing for rotor peripheral speeds in excess of 200 m/sec.
Description of the prior art:In conventional synchronous permanent magnet machines, segments of permanent magnet are secured to a shaft by any of a variety of means. Care must be taken to assure that such securement prevents the segments of magnet from moving either axially or radially. If axial movement were permitted, one or more of the magnetic segments on the rotor may not properly align with the armature, with the consequence that machine efficiency would diminish. If any radial movement were to take place, then a dynamic imbalance would be generated, and could cause a bearing or shaft failure or a bad vibration. In the worst case, radial movement of the magnetic segments could prevent high speed rotation of the rotor. Furthermore any interfering contact between the rotor and the stator would cause frictional drag and/or damage to the machine parts.
The reliable securement of magnets increasingly becomes a problem with increased rotor speeds due to the increasing forces tending to cause magnet movement, particularly in the radial direction, with the increasing centrifugal force accompanying increasing rotational speeds.
Therefore, it has been proposed in, for example, US-A4674178 and US-A-3968390 that a retaining can made of high strength composite materials such as CFRP (carbon fibre reinforced plastic) is fitted onto the magnets in order to restrain the magnets in the radial direction.
Modern CFRP materials have an ultimate tensile strength (UTS) which can exceed 2000 MPa with currently commercially available fibres and resin systems. A high speed permanent magnet motor or generator operating at a peripheral speed in excess of 200 m/sec may require the retaining can to be so tight that 1000 MPa or more tensile stress (at) in the hoop direction (hoop stress) is necessary. The hoop stress can be obtained from equations such as those found in Roak & Young, "Formulas for Stress and Strain" fifth Edition published in 1985 byMcGraw-Hill. Anyone skilled in the art of stress analysis can determine the hoop stress in the carbon fibre composite retaining can which is caused by thermal effect, internal pressure caused by centrifugal loading of the magnets, centrifugal body stresses and internal pressure caused by an interference fit.It can easily be determined that rare earth permanent magnets such asNeodymium Iron Boron cause stress of 1000 MPa in a carbon fibre composite can when rotating at a peripheral speed in excess of 200 m/sec.
Given that typical CFRP composite material may have aYoung's Modulus E of around 140 GPa then a strain value of at/E = 0.007 is required in the can. That means that, for a rotor of magnet outside diameter 100mm, the can in its natural unstressed condition before fitting would require an inside diameter 0.7W less at 99.3 mm. In order to fit two components together in the so called shrink fit method as proposed in the above publications, the can must be expanded by heating or the rotor shrunk by cooling.
However, carbon fibre has a very small coefficient of expansion and the maximum temperature to which it can be heated is limited by the thermosetting cure termperature which is normally about a maximum of 1800C. Therefore one must consider shrinking the rotor by cooling. The calculation below, however, shows that cooling the rotor prior to fitting the retaining can is never going to attain the desired initial strain value in the can.
The equation defining the thermal expansion and contraction of a material is6L = a x Lo x AT where 6L is the change in a linear dimension (in thiscase the diameter of the rotor);a is the coefficient of thermal expansion; Lo is the initial linear dimension (in this casethe external diameter of the rotor); andAT is the change in temperature.
If the rotor were to have a coefficient of thermal expansion, a, of about 12 microns per metre per degreeCelcius, then the temperature reduction AT necessary to cause a 7% reduction in the diameter of the above rotor would be given by:AT = dL a x Lg = 0.7 12.10-3 = 584 OC Room temperature is only about 3000C above Absolute zero so that it is clearly impossible to establish the desired initial strain value in the can by the shrink fit method taught in the above US Patents. It has therefore until now been impossible to establish the above initial strain values for the proper running of high speed permanent magnet motors and generators at peripheral speeds of over 200 m/sec.
SUMMARY OF THE INVENTIONIt is an objet of the invention to provide a new and improved method of assembling a permanent magnet rotor which overcomes the above drawback.
The invention provides a method of assembling a permanent magnet rotor, comprising the steps of: (a) forming a rotor hub; (b) holding segments of permanent magnet on the rotor hub; (c) machining the segments of permanent magnet so that a continuous shallow angle taper is formed over the outer diameter of the segments of permanent magnet from one axial end of the rotor hub to the other axial end; and (d) axially fitting around the outer circumference of the permanent magnet segments, from the smaller diameter end thereof, a plurality of retaining rings made of high strength composite materials and having a decremental change in inside diameter equivalent to the change in magnet outside diameter due to the effect of the taper, the retaining rings being axially moved to engage the outer circumferential surface of the segments of permanent magnet with a predetermined interference.
BRIEF DESCRIPTION OF THE DRAWINGSFig. 1 is a sectional view of a permanent magnet rotor made according to the present invention;Fig. 2 is a sectional view taken substantially along the line A-A of Fig. 1;Fig. 3 is a view similar to Fig 1, but showing the permanent magnet rotor at an intermediate point in its assembly;Fig. 4 and Fig. 5 are schematic views for explanation of the present invention; andFig. 6 is graph of hoop stress and interference pressure versus speed in the carbon fibre ring in the high speed generator.
DESCRIPTION OF THE PREFERRED EMBODIMENTA method of assembling a permanent magnet rotor in accordance with a preferred embodiment of the invention will be described with reference to attached drawings.
In Fig.l and Fig. 2 a permanent magnet rotor 10 is provided with a rotor hub 11 which is made of a magnetic material. The rotor hub 11 includes a pair of shaft portions 12, 13 and a central portion 14 which is formed between the shaft portions 12, 13 and which has a larger diameter than that of the shaft 12, 13. Plural segments of permanent magnet 15 made of a rare earth material such as Nd, Sm-Co are secured around the central portion 14 of the rotor hub 11 with an adhesive. The rotor hub 11 may have a central portion 14 of uniform diameter from end to end as shown, or it could have a recess or annular groove between its ends in which the plural segments of permanent magnet are adhered.
As previously explained the forces and stresses required in a modern carbon fibre can, to restrain the segments of permanent magnet in the radial direction, are very large.
For very high speed rotors it is no longer possible to use techniques which involve temporarily heating or cooling one component to make the fit possible by thermal differential expansion. Fig. 1 and Fig. 3 show how the segments of permanent magnet 15 are secured in place by a plurality of retaining rings 16a-16h in accordance with the present invention.
The outer circumferential surfaces of the segments of permanent magnet 15 are machined by an accurate method such as grinding so that a progressive shallow taper is formed over the outer diameter of the segments of permanent magnet from one axial end of the central portion 14 to the other axial end. In the embodiment as illustrated, there are no side plates made of non-magnetic material fixed to the shaft portions 12, 13 in order to restrain the magnets 15 in the axial direction. However, the provision of such side plates is possible according to the invention, in which case the side plates are ground to the same taper as the segments of permanent magnet 15.
Next, on the outer circumferential surface of the segments of permanent magnet 15, a plurality of retaining rings 16a-16h made of high strength composite materials such as carbon fibre and curable epoxy resin are fitted thereon with a predetermined interference. The retaining rings 16a-16h have a decremental change in inside diameter equivalent to the progressive change in magnet outside diameter due to the effect of the taper. This inside diameter can be simply machined or determined by the mandrel upon which each ring is orignally filament wound.
The inside diameter of each retaining ring 16a-16h is smaller than the approximate outside diameter of the portion of each corresponding segment of permanent magnet 15 by a predetermined amount. However, each ring could have an inside diameter which has a taper to match that of the magnet outside diameter so long as it is sized to have the predetermined interference.
In this embodiment, the axial length of each of the retaining rings 16a-16h is half of that of each of the segments of permanent magnets 15. Thereby, even though one of the rings 16a-16h might be destroyed in use, since the segment of magnet 15 cannot move in the radial direction, the adjacent ring will be able to prevent axial movement of the magnet segment 15. Therefore, it is desirable that each of the segments of permanent magnet 15 is secured by at least two retaining rings.
The slight taper of the magnetic segments causes a variable spacing between the rotor and the stator from one end of the rotor to the other, but the variation is very small and has no significantly adverse influence on performance of the motor/generator.
As shown in Fig. 3, the taper of the outside diameter of the segments of permanent magnet 15 provides a slope up which the retaining rings 16a-16h can be pushed. In order to be able to fit all of the retaining rings 16a-16h, a starting tool 17 is provided which is designed to locate accurately with the rotor hub 11 at the axial end where the magnet outside diameter is least. The starting tool 17 has a taper which matches that of the magnet outside diameter.
The interference between each of the retaining rings 16a-16h and each of the segments of permanent magnet 15 is so tight that the segments of permanent magnet 15 do not need to be retained by any other means. The taper angle a can be chosen to suit the application, but it must always be shallow. The taper angle a is limited as follows:Referring to Fig. 4 the reaction R (in Newtons) is equal to the product of the inside surface area of the ring and the interference pressure, ieR = 7rx d x lx Pwhere d is the mean internal diameter of the ring;1 is the axial length of the ring; andP is the interference pressureThe force F (in Newtons) required to overcome friction is given by: F=xR=xirxdxlxP where 4 is the coefficient of frictionIf a is the included taper angle then the maximum value of a to ensure adhesion of the ring is given by: 4 Rcos(a/2) > Rsin(a/2) p > tan(a/2)Furthermore, the frictional resistance caused by the interference pressure requires a force to install the rings. Since the rings are not strong in directions perpendicular to the lay of the fibre, the axial length 1 of each ring is therefore dictated by the maximum axial force allowed during assembly.Referring to Fig. 5, the allowable maximum assembly force Fmax is limited as follows:The assembly push load F (in Newtons) for a taper mandrel of included angle a is given by: F = R/cos(a/2) = x x K x d x 1 x P/cos(a/2) The compressive assembly stress a of the retaining ring is given by:a = F/(X/4(D2-d2)) where D is the mean external diameter of the ring; andd is, as before, the mean internal diameter of the ring.
Since the allowable maximum compressive assembly stress is amax: amax = Fmax/(X/4(D2-d2)) Fmax = amax x w/4(D2-d2) As shown in the above equations, in order to achieve high interference pressure P, the length 1 of the retaining ring needs to be reasonably small. If the length 1 is large and the assembly push load F becomes larger than the allowable maximum assembly force Flax, the compressive assembly stress a of the retaining ring exceeds the allowable maixmum compressive assembly stress amax and the retaining ring is destroyed. Therefore, it is impossible to apply a single long-length retaining can over the whole axial length of the rotor.
Fig. 6 shows a graph of hoop stress and interference pressure in the carbon fibre ring versus speed in a high speed generator having the above permanent magnet rotor.
As shown in Fig. 6, the segments of permanent magnet are As shown in Fig. 6, the segments of permanent magnet are securely held in place in the normal temperature range of use of the high speed generator. Thus although the interference pressure decreases with increasing rotational speeds, nevertheless it is maintained within acceptable limits up to an operating speed of 50,000 r.p.m.
Because according to the invention it is possible to obtain the required interference fit without using techniques which involve temporarily heating or cooling one component to make the fit possible by thermal differential expansion, the invention enables the segments of permanent magnet to be more surely and easily held in place.
Furthermore, since according to the invention each of the segments of permanent magnet is preferably secured by at least two retaining rings, even if in use one of the rings might be destroyed, the segment of magnet is held by the other ring or rings and cannot move in the radial direction. Therefore, the invention prevents damage caused by contact between the magnets and the stator, and safety is therefore improved.
Furthermore, according to the invention one may select and vary the material of the retaining rings to suit the surrounding temperature of the rotor. For example, if there is in normal use a temperature gradient in the axial direction of the rotor, one may select rings with appropriate heat-resisting properties to tolerate that temperature gradient. Furthermore, one may vary the interference pressures between different retaining rings.
Therefore, if the retaining ring which is fitted to the rotor hub at the axial end where the magnet outside diameter is least has the largest interference pressure, this retaining ring may function as a stopper.