High speed rotor system

A magnetically suspended rotor system is provided capable of operating at rotational speeds of 100,000 rpm or greater. The system comprises a rotor assembly and electric motor means for producing rotation of the assembly about an axis thereof. Electromagnetic means disposed near one end of the rotor is energized to exert an attractive force on the rotor to support it and preferably cooperates with permanent magnet means affixed to the rotor assembly near one (the upper) end thereof. An additional permanent magnet affixed to the rotor near the opposite (lower) end thereof cooperates with another permanent magnet disposed in juxtaposition thereto to exert an attractive force on the rotor tending to oppose the force exerted by electromagnetic means and produces a radial constraining force on the rotor tending to center the lower end of the rotor assembly. A conductive element is interposed between the lower end of the rotor assembly and the latter permanent magnet which cooperates with the magnetic field produced by the permanent magnet in the lower end of the rotor assembly to inhibit nutation of the lower end of the rotor assembly by virtue of eddy currents induced therein. Means are provided responsive to displacement of the rotor in the direction of its axis of rotation for controlling the currents supplied to the electromagnetic means so as to tend to restore the rotor to its position prior to such displacement. The rotor may be driven by a motor utilizing regenerative commutation of the sort disclosed in co-pending application Ser. No. 695,507, filed June 11, 1976. The system disclosed makes possible the achievement of unusually high rotational speeds by minimizing or eliminating undesired resonances in the rotor assembly.

This invention relates to improvements in magnetically suspended rotor 
systems, and particularly to such systems capable of operating at 
extremely high rotational speeds -- i.e., of the order of 100,000 rpm or 
greater. Such rotor systems are particularly useful in applications such 
as turbo-pumps, ultra-centrifuges, momentum storage systems, systems for 
testing and balancing rotating objects, laser scanners and the like. 
In the past considerable difficulties have been experienced in attempting 
to construct such systems capable of operating at extremely high speeds. 
One of the problems was that of providing a suitable drive motor capable 
of accelerating the rotor to high rotational speeds. Thus, if the rotor 
was made an element of an induction motor, large temperature rises were 
experienced in the rotor such that heat dissipation from the rotor became 
a limiting factor. This difficulty was overcome by using a special form of 
motor system employing regenerative commutation, as disclosed and claimed 
in co-pending application of Joseph Lyman, Ser. No. 695,507, filed June 
11, 1976. 
Another difficulty experienced was that of undesired resonances which 
tended to occur in the rotor and which tended to interfere with the 
acceleration of the rotor to the desired high rotational speed. The 
present invention is particularly concerned with overcoming this latter 
difficulty. 
Accordingly it is an object of the invention to provide a magnetically 
suspended rotor system in which the rotor can be caused to rotate at 
extremely high speeds -- i.e., of the order of 100,000 rpm or greater. 
Another object of the invention is to provide a system in which undesired 
resonances of the rotor system are substantially eliminated. 
A further object of the invention is to provide such a system in which 
undesired nutation of the rotor is substantially eliminated. 
In accordance with the invention, the foregoing objects and others which 
will appear are achieved, in a magnetically suspended rotor system 
comprising a magnetically permeable rotor and electric motor means for 
producing rotation of said rotor about an axis thereof, by providing: 
(a) electromagnetic means disposed near one end of said rotor, which is 
energized in response to electric current supplied thereto to exert an 
attractive force on said rotor to support it, 
(b) a first permanent magnet affixed to said rotor near the opposite end 
thereof and polarized in a direction substantially parallel to the axis of 
rotation of the rotor, 
(c) a second permanent magnet disposed in juxtaposition to said first 
magnet and polarized so as to cooperate with said first magnet and exert 
an attractive force on said rotor tending to oppose the force exerted by 
said electromagnetic means, 
(d) means responsive to displacement of said rotor in the direction of its 
axis of rotation for controlling the current supplied to said 
electromagnetic means so as to tend to restore said rotor to its position 
prior to such displacement, and 
(e) a conductive element positioned between said first and second 
electromagnets so as to have eddy currents induced therein upon the 
occurence of any nutation of said opposite end of said rotor relative to 
said second permanent magnet, whereby said nutation is effectively damped.

Referring now to FIG. 1, the high speed rotor system there shown comprises 
a rotor assembly 1 including a unitary main body portion 3 in the form of 
a solid of revolution having two circumferential ring portions 5 and 5' 
coaxially disposed and vertically spaced from each other, central web 
portions 7 and 7' joined to the inner circumferential surfaces of ring 
portions 5 and 5' respectively and each having a thickness vertically 
which may be of the order of one-quarter the vertical thickness of the 
ring portions 5 and 5'. The two web portions are interconnected by a 
tubular central portion 8. Extending from the outer circumferential 
surfaces of ring portions 5 and 5' are relatively thinner circumferential 
extensions 9 and 9', each of which may have a vertical thickness of the 
order of one-quarter the vertical thickness of ring portions 5 and 5', 
which function to increase the moment of inertia of the over-all rotor 
assembly. The outer portions 11 and 11' of ring portions 5 and 5' are 
tapered to distribute stresses created at the high speeds of rotation of 
the rotor assembly. 
Each of the web portions 7 and 7' is provided with four equally 
circumferentially spaced vertical holes extending vertically therethrough 
and positioned adjacent the inner circumferential surfaces of ring 
portions 5 and 5', as shown more clearly in the exploded view of FIG. 2. 
In these holes are inserted axially polarized permanent magnets 13 which, 
as will be explained hereinafter, cooperate with suitable windings to 
provide motor means for causing the rotor assembly to rotate. The entire 
main body 3 of the rotor may be machined in a single piece from a suitable 
non-magnetic material, preferably of high tensile strength, such as 
non-magnetic stainless steel, such steel commercially designated as being 
particularly suited for this purpose. As explained in my co-pending 
application Ser. No. 695,507, filed June 11, 1976 for Electric Motor 
System, permanent magnets 13 preferably may be made of a suitable rare 
earth alloy such as samarium cobalt. 
Positioned respectively above and below web portions 7 and 7' and 
immediately adjacent thereto are discs 15 and 15' of magnetically 
permeable material, such as ordinary cold-rolled steel, the vertical 
thickness of each disc being of the order of one-quarter the vertical 
thickness of ring portions 5 and 5' and their diameters being 
substantially equal to the internal diameter of the inner circumferential 
surfaces of said ring portions. Each disc is provided with a central hole 
aligned with a similar hole in the main body 3 through which is inserted a 
pin 17, also of magnetically permeable material such as cold-rolled steel 
for magnetically interconnecting discs 15 and 15'. 
Directly above disc 15 is positioned an annular permanent magnet 19 which 
also may be formed of a rare earth magnetic alloy such as samarium cobalt, 
the external diameter of magnet 19 being somewhat less than the internal 
diameter of ring portion 5. Into the space between the outer periphery of 
magnet 19 and the inner surface of ring portion 5 is inserted a retaining 
ring 23, the purpose of which is to centrally position annular magnet 19 
and counteract any tendency for it to rupture by reason of the high speed 
of rotation of the rotor assembly. Another similar annular permanent 
magnet 21 is positioned immediately below and adjacent the lower disc 15 
and is provided with a similar retaining ring 25 which performs the same 
function as ring 23. Retaining rings 23 and 25 may be made of any suitable 
high tensile strength non-magnetic material such as aluminum or an aramide 
resin such as Kevlar-49 sold by E. I. DuPont de Nemours. Discs 15 and 15' 
provide low-reluctance magnetic paths between magnets 13 and magnets 19 
and 21 respectively. If desired either or both of permanent magnets 19 and 
21 may be modified in the manner shown in FIG. 3 by cutting them into a 
plurality, e.g. eight, of segments 61, 63, 65, 67 by making radial cuts 
69, 71, 73 preferably at equally spaced intervals, through the body 
thereof before they are assembled into the rotor assembly. This 
modification has the advantage of reducing or eliminating "hoop" or 
tangential stresses in the magnets which may occur at high speeds of 
rotation and reduces the likelihood of rupture thereof at such speeds. 
Inserted in the vertical central hole in magnet 19 is an insert 27 of 
aluminum or other non-magnetic material having a slightly rounded surface 
at its upper end, the function of which will be explained hereinafter. 
Positioned concentrically above and spaced from the upper surface of rotor 
assembly 1 is a support electromagnet assembly comprising a core 31 of 
suitable magnetically permeable material such as cold-rolled steel and an 
encircling winding 33, which in a typical embodiment of the invention 
comprised 330 turns of No. 20 wire. As shown, this support electromagnet 
assembly is of "pancake" configuration having a diameter considerably 
greater than its vertical thickness, which has been found to be 
particularly desirable in certain applications of the high speed rotor 
system for conservation of space, and also because it is essentially 
coextensive with the upper portion of the rotor assembly and therefore 
tends to provide substantial lateral restraint or centering force for the 
rotor assembly in addition to providing the desired upward force on the 
rotor assembly for supporting it. 
Current for energizing winding 33 of support electromagnet 29 is supplied 
by a suitable amplifier 33 which is so designed as to supply a constant 
direct current sufficient to cause support electromagnet 29 to produce a 
magnetic flux sufficient to cooperate with the flux produced by permanent 
magnet 19 to provide the necessary over-all force needed to support rotor 
assembly 1. At the same time, the provision of the permanent magnet 19 in 
the rotor assembly itself, reduces the amount of energy required to be 
supplied to winding 33 to provide such support. In addition, means are 
provided for controlling amplifier 33 so as to compensate any tendency for 
rotor assembly 1 to become displaced either upward or downward along its 
vertical axis from its normal position. 
While various arrangements may be used to achieve this result, in the 
embodiment illustrated such control is provided by means of an 
electrooptical system comprising a light source 35, a collimating lens 
system 37, prism 39, a photoelectric cell 41 and an integrating circuit 43 
comprising the parallel combination of a capacitor 45 and a resistor 46. 
In this arrangement any vertical displacement of rotor assembly 1 will 
produce a variation in the amount of light reaching photoelectric cell 41 
from light source 35 as the rounded upper portion of insert 27 moves into 
or out of the path of the light beam from prism 39 to photocell 41. As a 
result, the output from photocell 41 will vary in response to the vertical 
displacement of the rotor assembly and this output is partially integrated 
in integrator circuit 43 to yield a signal which varies in response to 
both the displacement and the rate of displacement of rotor assembly 1, 
which in turn is supplied to the input of amplifier 33 to control the 
current supplied by it to winding 33 so as to compensate for any tendency 
of the rotor assembly to become displaced either upward or downward from 
its normal operating position. It will be apparent that other systems may 
be employed for controlling the current in winding 33. For example, a 
system employing a laser beam rather than a light beam may by advantageous 
in some instances. 
Concentrically positioned below rotor assembly 1 is a further permanent 
magnet which is axially polarized and of such strength that in cooperation 
with magnet 21 in the rotor assembly it exerts a small downward force on 
the rotor assembly, by virtue of which and by virtue of the fringing 
fields produce between these two magnets a radial constraining force is 
produced tending to center the lower end of the rotor assembly. Like 
permanent magnets 19 and 21 in the rotor assembly, magnet 47 may be made 
of a rare earth alloy such as samarium cobalt. 
Further there is interposed between the lower end of the rotor assembly and 
permanent magnet 47 a disc 49 of a highly conductive metal such as copper. 
This cooperates with the magnetic field produced by permanent magnet 21 in 
the rotor assembly so that whenever any nutation of the lower part of the 
rotor assembly tends to occur, eddy currents will be induced in conductive 
element 49 so as to cause any such nutation to be damped out. 
The means for producing rotation of the rotor assembly in the system 
according to the present invention preferably may be an electric motor 
system employing regenerative commutation as fully described in my 
aforementioned co-pending application Ser. No. 695,507. Accordingly it is 
not deemed necessary to describe such a motor system in detail in the 
present application. Briefly, as explained in said prior application, such 
a motor system comprises a rotor including magnetic means providing a 
plurality of correspondingly directed magnetic poles angularly displaced 
around said rotor, a stator comprising first and second windings adapted 
to cooperate with the poles of said rotor, each of said windings having 
transversely disposed conductive portions for intercepting the magnetic 
fields of said poles during rotation of said rotor, and an amplifier 
having its input coupled to said first winding so as to be energized by 
alternating voltage induced in said first winding during rotation of said 
rotor, and having its output coupled to said second winding to supply 
alternating current therethrough in synchronism with the interception of 
the magnetic fields of said poles by the transverse portions of said 
second winding. Also means may be provided for initiating rotation of the 
rotor, which may comprise Hall effect sensors disposed adjacent the 
transverse portions of the first winding, the outputs of which are 
combined and supplied to the input of the amplifier to cause alternating 
current to be supplied to the second winding to initiate rotation of the 
rotor. Such means are disclosed only schematically in FIG. 1, the stator 
windings being indicated at 51 and cooperating with magnetic poles 
provided by permanent magnets 13 in the rotor assembly. The Hall effect 
sensors for initiating rotation of the rotor are shown at 53 and the 
amplifier for controlling energization of the stator driving coil is shown 
at 55, being supplied with input through connection 57 and supplying its 
output to the stator driving coil through connection 59. 
In operation of the system hereinbefore descrbed, the electromagnet 
assembly 29 cooperates with permanent magnet 19 in the upper end of rotor 
assembly 1 to support the rotor assembly in a position intermediate 
electromagnet assembly 29 and the disc of conductive material 49 so that 
the rotor assembly is free to rotate substantially free of any friction. 
If desired, the entire system may be enclosed in an evacuated enclosure 
(not shown) so that friction caused by the surrounding atmosphere also may 
be eliminated or substantially reduced. Permanent magnet 47 disposed 
concentrically below rotor assembly 1 and conductive disc 49 cooperates 
with permanent magnet 21 in the lower portion of the rotor assembly to 
exert a smaller amount of force on the rotor assembly tending to pull it 
downward. The relative forces exerted by electromagnet assembly 29 and 
permanent magnet 47 are so proportioned as to yield a net upward force 
just sufficient to support the rotor assembly in the desired position. Any 
tendency for the rotor assembly to be displaced axially either upward or 
downward is counteracted, as hereinbefore explained, by the operation of 
the photoelectric control circuit comprising light source 35, the optical 
system comprising lenses 37 and prism 39, photocell 41, integrating 
circuit 47 and amplifier 33, which controls the current supplied through 
winding 33 of electromagnet assembly 29. By reason of the provision of the 
lower permanent magnet 47, the "stiffness" of the system is increased and, 
as hereinbefore pointed out, any tendency toward nutation of the rotor 
assembly is reduced. Such action is further enhanced by the damping effect 
obtained by inclusion of the conductive element 49 interposed between 
permanent magnets 21 and 47. As a result, undesired resonances of the 
rotor assembly are reduced as its speed is increased, and it is possible, 
using a motor system of the sort described in my aforementioned co-pending 
application Ser. No. 695,507, to achieve exceedingly high speeds of 
rotation of the rotor assembly up to 100,000 rpm and even in excess 
thereof. 
While the embodiment of the invention described does not shown any specific 
application therefor, as hereinbefore mentioned systems in accordance with 
the invention are useful in numerous applications such as for turbo-pumps, 
ultra-centrifuges, momentum storage systems, systems for testing and 
balancing rotating objects, laser scanners and the like. It will be 
apparent that in any such specific application, suitable structure may be 
provided for coupling the rotor assembly of the system in accordance with 
the invention to the apparatus to be driven thereby, and various suitable 
arrangements for accomplishing this purpose, depending upon the 
circumstances, will be apparent to those skilled in the art. 
While the invention has been described with reference to a single preferred 
embodiment thereof, it will be apparent that numerous modifications and 
alternative structures may be devised by those skilled in the art within 
the scope of the invention as defined by the following claims: