HOMOPOLAR GENERATOR

This invention relates to electrical generators. More particularly, the present invention relates to an improved energy density homopolar generator in which inertial energy is stored over a long period of time for delivery to a load in a short period of time.

Homopolar generators (HPGs) are well known and described in the literature. For example, the article entitled "The Design, Fabrication, and Testing of a Five Megajoule Homopolar Motor-Generator", authored by W. F. Weldon, et al. , and presented at the International Confer- ence on Energy Storage, Compression and Switching in

Torino, Italy in November of 1974, presents an overview of HPGs in general. The article "Compact Homopolar Gener¬ ator", authored by J. H. Gully, which appeared in the IEEE Transactions on Magnetics, Volume MAG-18, No. 1, January, 1982, discloses an improvement in the design of a HPG.

These and other recent design changes in homopolar generators have resulted in a substantial improvement in the HPG energy density which could be achieved over typical prior art existing designs. HPG's are now being considered as power supplies for electromagnetic launchers and other applications in the field rather than in the laboratory. Consequently, energy density has now become

important. One such improved design is known as the All Iron Rotating (A-I-R) homopolar generator, and represents a design approach which eliminates, as far as possible, the non-rotating parts of the generator, concentrating the mass of the generator in the energy storing parts

(rotor(s)). The rotor of the A-I-R HPG weighs typically about 1,600 pounds out of a total generator weight of 3,400 pounds, or about 47% of the total mass. Such a machine stores about 4 kilojoules (KJ) of energy per kilogram (kg) of machine mass. This may be compared to the rotor of a typical prior art homopolar generator which occupies approximately 10% of the generator mass. Such a typical prior art HPG has a total energy capacity of approximately 5 MJ (megajoules) for a specific energy density of less than one Kj/kg (kilojoules per kilogram).

Since the A-I-R homopolar generator approach now uses almost half of its weight to store energy it will never be able to improve its specific energy density by more than a factor of approximately two by following the design approach of minimizing the stator mass. Once the generator mass is reduced near the rotor mass, the only apparent method for increasing energy density is to increase the rotational speed of the rotor. As in any generator, the rotational speed of the rotor is controlled by an external input drive force, such as that provided by a connected hydraulic or electric motor.

While it may be possible to theoretically increase the specific energy density of the generator by increasing the rotational speed of the motor, such a design approach presents design limitations caused by the wear rate of the electrical brushes used as current collectors on the rotor surface. These electrical brushes are typically con- structed of a sintered copper-graphite composite, and are poor thermal conductors making it difficult to remove the

frictional heat created at the brush-to-slip ring interface.

This surface heating of the electrical brushes is a result of two phenomena, coulomb frictional heating and heating due to the brush-to-slip ring interface voltage drop. As the surface temperature of the brush reaches the melting temperature of the binders in the copper-graphite sinter, substantial wear occurs. Because of the physical pheonomena occurring at the interface between the brushes and the slip ring surface, it has been found for a typical HPG application that rotor surface velocities cannot exceed approximately 220 meters per second. Although the technology is attempting to discover exotic brush materials which are more amenable to the removal of this surface heating, it is anticipated that brush wear will continue to limit HPG rotor speeds.

Since it is desirable to continue to increase the specific energy density of homopolar generators, and since brush wear prevents increasing rotor speed (the most promising route to increased energy density), it would be attractive to decouple the brush/slip ring speed from the allowable rotor tip speed.

Another way to improve the effective overall energy density of an HPG powered system is to reduce the size of auxiliary components or combine them with other components. The concept of a self excited HPG achieves this by using the generator output current to provide field excitation for the HPG itself. Energy stored inertially in the rotor is converted to energy stored magnetically in the field coil. In some cases the ferromagnetic rotor and stator material can be eliminated allowing the magnetic field to rise faster and to higher levels than can be achieved in ferromagnetic machines.

This in turn allows the machine to develop higher output voltage and therefore higher power density as well. Of course, the elimination of ferromagnetic material can also reduce the weight of the machine, also improving the energy density.

The arrangement described above is particularly attractive if the HPG is to be used to charge an intermediate inductive energy store such as is used to power an electromagnetic railgun. In this case the HPG field coil can serve as the inductive store so that the total energy stored inertially in the rotor is transferred to energy stored inductively in the field coil and then transferred to the load. Not only does this eliminate the need for a separate excitation supply, but it eliminates a separate intermediate storage inductor as well. Such a machine is described in U.S. Patent No. 4,503,349 by Miller.

A problem with self excited, air core HPG's such as that described in the Miller patent is the balance of energy required in the field coil for excitation versus the energy originally stored in the rotor. In order to take advantage of the higher magnetic flux density (and therefore higher HPG power density) that can be achieved in the air core (nonferromagnetic) excitation coil, the energy required typically exceeds the energy that can be stored in rotor(s) fitting inside the coil and operating at conventional (about 200 m/s) surface speeds dictated by present day brush technology. This usually forces the rotors to be operated at higher than standard surface speeds in order to increase rotor energy density to the point needed to excite the field coil(s). This in turn compromises ^' the durability of the HPG by causing the brushes to operate well above their capabilities.

The present invention addresses the limitations on HPG energy density described above by separating the inertial energy storage function from the voltage generating function so that the flywheel tip speed is decoupled from the brush speed limitation. Furthermore, by removing the inertial energy storage elements from the bore of the air core self-excited field coil, the energy storage of the rotor can be increased without increasing the size and therefore energy requirements of the excitation coil.

In accordance with the present invention, an improved energy density homopolar generator is disclosed. The homopolar generator responds to a rotational drive input for generating a voltage and a current at a pair of output terminals. The generator includes a stator assembly for generating a magnetic field within an interi¬ or region. A rotor responds to the drive input and to the magnetic field generated by the stator assembly to gener- ate the voltage and current.

The rotor includes a current generator means which responds to the rotation of the rotor in the magnetic field for collecting the voltage and current to the output terminals. A separate energy storage means responds to the rotational speed of the rotor for storing inertial energy where the energy storage capacity of the storage means is determined independently of collection of the voltage and current by the current generator means.

The current generator means comprises a conductive cylindrical or squirrel-cage armature which is positioned within the stator assembly for rotation therein. The armature has disposed on its outer surface a pair of slip rings for collecting the voltage and current in response to rotation of the armature in the magnetic field. Two

sets of conductive brushes coupled to the output conductors, with each brush set associated with and contacting one of the slip rings, is provided. The brushes conduct the collected current to the exitation coil. The radius of the armature brush-to-slip ring determines the surface velocity at the interface of each brush.

The energy storage means of the rotor comprises a pair of energy storage wheels, one disposed at each end of the rotor armature for rotation therewith. The radius of the wheels is independently selectable from the radius of the armature thereby permitting optimization of the energy storage capacity of the generator independently of the surface velocity of the armature at the brush-to-slip ring interface. Furthermore, since the energy storage means of the rotor must no longer be ferromagnetic and must no longer conduct electrical current as in conventional HPG's, it can now be constructed of suitable materials to optimize its energy storage function such as graphite filament reinforce epoxy composites.

The stator assembly of the homopolar generator of the present invention includes a pair of coils encircling the current generator means portion of the rotor, with each coil associated with one associated brush and slip ring pair. Responsive to an excitation current within the stator coils, the magnetic field is generated within the interior region. A bearing assembly is mounted to the stator assembly for supportably receiving the armature therein.

In accordance with one aspect of the invention, the stator coils are operated at cryogenic superconducting temperatures in order to reduce their resistance so that the coils may generate the required magnetic field

strength to provide the necessary power conversion from the generator. In a different aspect of the invention, the current collected by each brush is conducted through its associated stator coil to its associated output terminal thereby to obtain a self-excited homopolar generator. In yet another aspect of the invention, the stator coils are excited in opposite directions.

For a fuller understanding of the present invention, reference should be had to the following detailed description taken in connection with the accompa¬ nying drawings in which:

Figure 1 is a cross-sectional view of one embo- diment of the improved energy density homopolar generator constructed in accordance with the present invention in which self-excited air gap stator coils are used to gener¬ ate the magnetic field in the rotor assembly;

Figure 2 is a cross-sectional view of an alternate embodiment of a homopolar generator constructed in ac¬ cordance with the present invention in which the stator field coils are operated at cryogenic temperatures to obtain superconducting coils;

Figure 3 is a more detailed cross-sectional view of a homopolar generator constructed in accordance with the principles as illustrated in Figure 1 in which the rotor assembly includes a squirrel cage armature for the generator's current generator means; and

Figure 4 is a graphical presentation of the expected performance of the homopolar generator illustrated in Figure 3.

Similar reference numerals refer to similar parts through the drawings.

Turning now to the figures and first to Figure 1, there is shown in cross-section a homopolar generator constructed in accordance with the principles of the pre¬ ferred embodiment of the present invention. The generator is basically constructed of two elements, a stator assembly comprised principally of the stator coils 16,17, and a rotor assembly comprised principally of a rotating shaft 18 and a pair of energy storage wheels 12 and 13 supported on bearings 24 and 25.

In a homopolar generator, there are two principal functions performed. First there is the energy storage function. Inertial energy is stored over a relatively long time period by steadily increasing the rotational speed of a large rotating mass. In the typical prior art homopolar generator, this mass is the mass of the rotor assembly which will rotate in the magnetic field developed when excitation current is applied to the stator field coils. Eventually, the rotor reaches the desired oper¬ ating speed where its rotational inertia represents a quantity of stored energy which can be delivered to a load as a voltage and current by a current generator means--the second principle function performed by the generator.

The current collection function of a homopolar generator is provided by a set of electrical brushes which, at the time of unloading of the stored energy, are lowered into contact with slip ring surfaces which are a part of the conductive elements of the rotating rotor mass. At the point of contact 26 between the brushes and slip ring surfaces, there is substantial heating and wear. As pointed out above, these pheonomena are directly related to the surface velocity of the rotor at the

brush-to-slip ring interface. This velocity is, in turn, directly related to the radius r _R of the rotor at the interface 26. Because brush wear limits surface speeds of the rotor, prior art homopolar generators have been unable to substantially increase their specific energy density and maintain any acceptable size.

In accordance with the present invention, the energy storage function of the rotating within the stator coils has been partitioned from the voltage and current generation function. Disposed at opposite ends of the rotating shaft 18 of the rotor are two energy storage wheels 12,13 which are attached to the shaft 18 for rotation therewith. The radius of the energy storage wheels 12,13, r W, is controlled independently of the radius, r„, of the shaft 18. The energy wheels 12,13 rotate at the same speed as the rotor shaft 18, and since r is controlled independently of r„, the amount of energy stored within the generator is independent of the surface speed of slip ring surfaces at the brush-to-slip ring interface 26.

This partitioning of the energy storage function in accordance with the invention has removed essentially all of the mass of the rotor from rotation within the magnetic field of the stator coils 16,17. (While there will always be some rotating mass of the rotor in the magnetic field of the stator coils, the inertial storage provided by such mass is negligable in comparison to the energy storage provided by the wheels 12,13, less than 1% of the total energy storage. ) This has reduced the flux cutting area of the rotor, and accordingly, has reduced the ability of the rotor to produce an acceptable voltage and current at the output terminals for a typical exci- tation current to the stator field coils. Because present designs are operating at the limits of ferro-magnetic

circuit performance, simply increasing the field exci¬ tation current would not achieve the desired power output from the generator which the potential energy density would suggest was possible.

In the case of the present invention, this problem is even more acute because of the reduced conductive material of the rotor 18 cutting the flux lines of the magnetic field created by the stator coils. Accordingly, to enable the generator constructed in accordance with the invention to convert the stored inertial energy into a .deliverable power at the output terminals, the magnetic field created by the stator coils 16,17 must be increased over those in prior designs.

To achieve the necessary magnetic field strength increase to permit the required energy conversion oper¬ ation at the discharge time, the present invention offers two alternative designs, a self-excited air core homopolar generator (see Figures 1 and 3) and a super cooled super¬ conducting coil arrangement (Figure 2). Yet a further arrangement could be a combination of both superconducting coils and self-excited coils (not shown). In such an arrangement, the superconducting coils would provide the initial flux in the rotor 18, but would be isolated from the transient magnetic field of the self-excited air core coil by the use of a superconducting magnetic shield.

Still referring to Figure 1 which illustrates the preferred embodiment of the invention, a self-excited air core homopolar design, the brushes 28,29 are each shown contacting associated slip ring surfaces 19,21 on the rotor shaft 18. In the self-excited arrangement, each brush 28,29 conducts collected current to an associated output terminal 30,31 through an associated stator field

coil 16,17, respectively. In this manner, there is "self-excitation" of the stator coils.

Self-excited, air core HPG's have always, in principle, offered an attractive alternative to more con¬ ventional iron core HPG since they essentially achieve their excitation "for free", and avoid the two tesla (2.0 T) saturation limit of ferro-magnetic materials while at the same time eliminating the weight of iron based materials. However, these features are not the primary reason for selecting a self-excited air core coil arrange¬ ment. The primary advantage of the self-excited air core HPG is its ability to operate at magnetic flux densities substantially above the saturation limit of ferro-magnetic materials. A related advantage is the fact that the linear magnetic behavior of non-ferro-magnetic materials eliminates the phenomenon of armature reaction which affects iron core HPG's at high output currents ( 1 MA).

For air core HPG's, the field excitation require¬ ments increase by a factor of approximately 1000 due to the difference in relative permeabilities of iron and air. This puts a severe constraint on the performance of certain self-excited air core HPG components. This is most easily seen by comparing the volumetric energy density of the air core field coil with that of the HPG rotor(s) .'

The energy density in the bore of the air core storage inductor/field coil can be expressed as

E _ B ² hprf- = vb.ore

where, E-. _ore ⁼ Magnetic Energy in Bore of Coil bore Volume Enclosed in Bore of Coil

2 bore bore

B Root Mean Square Magnetic Flux Density in

Coil Bore μ © = Permeability of Free Space = 4-TT x 10 ^"7

For solenodial coils with length-to-diameter ratios around 00..55,, tthhee ttoottaall iinndduuccttiivvee energy E _IN _. is about twice that in the coil bore. Thus,

'IND _ B'

bore

If the rotor(s) are 0.9 of the coil bore in diame¬ ter and 0.9 of the coil length, a similar relationship results for the inertial energy density in the rotors

'INER - ■/__ M ^~ - = r _ _ SU>

where

E INER = inertial energy stores in rotor(s)

J = rotor moment of inertia v = rotor angular velocity r = rotor radius = 0.9 R.bore

= total rotor length = 0.9 bore = density of rotor material,

Substituting for r and ^~ r' and recalling that

'INER = 0.182 b v ² bore

Assuming a 50% energy transfer efficiency from the in¬ ertial store to the inductive store, the following relationship between the inertial and magnetic energy densities obtains,

(0.50) ^ElNER = .f^L

Vbore Vb.ore

Substituting and solving for B gives a relationship be- tween rotor tip speed and the maximum achievable core coil flux density B,

Since the flux in a self-excited air core HPG ranges from essentially zero to some peak value while in a comparable iron core _N .HPG it remains constant, the self- excited air core HPG must achieve a peak flux density of approximately twice that of the iron core machine in order to achieve comparable performance, i.e., approximately 4 T. In order to realize the primary advantage of the self-excited air core HPG, that is, the ability to operate at higher flux densities than iron core machines, such machines should operate at peak flux densities sub- stantially higher than 4 T.

The improved energy density concept of the present invention is the separation of the energy storage and voltage generation functions. Since the present day brush speed -capability limits rotor energy density, separation of functions allows the energy storage rotor(s) to operate at peak tip speeds for maximum energy density while the electrical generating section of the generator operates at surface speeds dictated by the brushes.

Still referring to Figure 1, this innovation allows design freedom in several directions. Since the energy storage flywheels 12,13 no longer must serve as electrical conductors they can be fabricated of fiber- reinforced epoxy resin composites which are capable of storing energy at much higher energy densities than metals. Graphite fiber-reinforced epoxy flywheels, oper¬ ating consistently on a production basis at 1200 m/s achieve energy densities more than twelve times greater than aluminum flywheels.

Turning now to Figure 2 where the superconducting HPG embodiment is shown, to provide the superconducting cryogenic temperatures for the field coils 16,17, each coil is contained within a structural Dewar container with appropriate cryogenic temperatures therein to achieve the superconducting operating conditions »for the coils. To. provide. support to £he rotating shaft, a pair of bearings 24 are disposed to opposite ends of the rotating shaft 18 to provide rotational support for the shaft within the magnetic field of the stator assembly. (Each embodiment shown in the figures contains some sort of bearing means for supportably receiving the rotor for rotation in the stator field. ) The rotating shaft 18 is constructed of a high strength, high conductivity material and functions as a single turn stator coil between the slip ring surfaces 19. An alternate embodiment of the HPG shown in Figure 2 would be to use the squirrel-cage armature design, as shown in Figure 3, in place of the solid shaft 18 as shown in Figure 2.

Furthermore, the separation of energy storage and electrical generation function means that as improvements are made in either area they can be incorporated into the HPG without being limited by the other function. That is, brush speed and rotor tip speed are permanently decoupled

in the present invention and may be adjusted relative to each other as future capability dictates merely by adjust¬ ing the relative diameters of the energy storage and electrical generator rotors.

The following Table 1 illustrates a comparison of the maximum possible surface speeds and the resulting energy densities that can be obtained for the energy storage wheels 12,13 made of different materials.

TABLE 1

Maximum ≤ Surface Ξnergy Density

Material Velocity (m/s) (kJ/kg) Flat Steel Disk 550 15 Shaped Steel Disk 670 21 Fiberglass/Epoxy Disk 760 41 Kevlar/Epoxy. Disk 960 65

In accordance with the present invention, if a 36 inch diameter, 12 ^' inch thick Kevlar/Epoxy flywheel is divided into two 6-inch thick energy storage wheels and operated at 20,000 rpm, a homopolar generator can be con¬ structed of approximately the same size as a typical prior art A-I-R HPG, but storing 90 MJ rather than the maximum obtainable 6.2 MJ for the A-I-R design.

The highest acceptable prior art HPG machine capacitance has been determined to be around 5,000 F (farads), although higher values are acceptable for a homopolar generator constructed in accordance with the principals of the present invention. Assuming, however, a machine capacitance of 5,000 F, the voltage of the gener¬ ator V would equal 190 Volts as determined from the following calculation:

V = ( (2 x E )/C) ^{1 2} = ( (2 x 90 x 10 ⁶ )/5 , 000 ) ^{1 2} = 190 V

Since

V =<j> >/2TT or φ = 2TT V/u) = 0.57 Wb (webers),

to produce a field strength of .57 Wb, an average magnetic field strength in the rotating shaft 18 of 12.7 T is required. Increasing the machine capacity to 10,000 F, which is acceptable in the case of the present invention, reduces the required field strength to 9 T.

In the case of superconducting coils 16,17, to produce a magnetic field density of 9 T would require a coil weighing approximately 1,100 pounds. This means that the generator in accordance with the present invention would weight less than 2,000 pounds and achieve a specific energy density of approximately 100 kJ/kg, a value which is substantially greater than the specific energy density of 6.2 kJ/kg which is theoretically possible in accordance with the principles of the prior art A-I-R HPG designs.

Turning now to Figure 3, there is illustrated a self-excited air core homopolar generator similar to that shown in Figure ^* 1. The HPG shown in Figure 3 illustrates yet a further feature of the present invention. One of the problems in self excited HPGs is the flux trapped in the rotor's conductive material during discharge of the inductive store which results inefficient energy conversion. Because of the division between the energy store and current collection functions in the present invention, a squirrel-cage armature construction is possible for the rotor shaft 18, as opposed to a solid shaft. Since a squirrel-cage armature can be made highly conductive in the axial direction and highly resistive in the circumferential direction, the armature bars 23 from

which the armature is constructed can be close to the field coil for maximum voltage generation and yet not act as a flux trapping shorted turn when the inductor is discharged.

The field coils 16,17 are wound in opposite directions so they produce opposite magnetic fields in the armature. Although the opposing (counterwound) field coil pair 16,17 may not appear to be optimal from an energy storage aspect, such an arrangement provides the highest ratio of useful flux density (flux cut by rotor armature) to peak flux density (limited by strength of inductor structure). Decoupling the inertial energy in the fly¬ wheels from the inductor (by placing the storage wheels outside the stator field) allows several operational modes which were not possible with prior art HPGs. In a typical

- self-excited air core HPG, the rotor(s) is brought up to speed, some initial ^* source of exc-itation flux is provided

(usually to 10% of peak flux) and the inertial .energy in the rotor(s) is transferred to inductive energy in the series connected field coil, the field coil flux rising with the generator current. When the current in the field coil peaks, a switch is opened in the circuit transferring the energy stored in the inductor into the load.

However, if the flywheel energy is increased sub¬ stantially over that required to excite the field coil(s) (which can be accomplished simply by making the flywheels thicker in the axial direction), a different operating mode is possible. The discharge proceeds in the classical manner until peak current is reached in the field coil. Since substantial inertial energy will remain in the rotor at this point, the field coil can be crowbarred (short circuited) and energy extracted from the rotor as a high voltage HPG. Field coil current will decay with the L/R time of the inductor, but this time can be made long com-

pared to the discharge time of the HPG. One extremely attractive aspect of this operating mode is that it does not require an opening switch capable of interrupting the current in the inductor.

Other operating modes include (1) operation as a self-excited continuous duty generator driven by a high power turbine, (2) replacing the self-excited field coil with a superconducting coil set (see Figure 2), or (3) using a superconducting coil to supplement the self- excited coil to provide the initial flux.

Still referring to Figure 3, the self-excited air core HPG uses two 1.0 m diameter, 0.2 m thick graphite fiber-reinforced epoxy flywheels operating at 1200 m/s (23,000 rp ) and storing 113 MJ each.

The squirrel-cage HPG armature 18 is 0.25 m in diameter with an active length of 0.45 m. With an average flux density at the armature of 6.7 T the generator will develop 450 volts at half speed.

The field coil consists of two multiconductor 4 turn liquid nitrogen (LN2) cooled aluminum inductors 16, 17 reinforced either with a fiberglass or Kevlar overwrap or with internal boron filaments. They will operate at a peak flux density of approximately 20 T at a current of 3.2 MA (see Figure 4). The coils will be insulated for 10,000 V working volts.

The ends of the squirrel-cage armature conductors 23 will be joined circumferentially by highly resistive metal spacers 43 to provide a continuous slip-ring surface for actively cooled copper finger brushes 28,29 operating at 300 M/S and a slip-ring current density of 20,000 ^' a/in . The resistive spacers 43 prevent the armature

conductors 23 from trapping field coil flux during the inductor discharge. The armature bars 23 are insulated from the material of the rotor shaft 18 by fiberglass epoxy-wrapped pieces 41. Accordingly, only the conductive bars 23 conduct current as a result of rotation of the bars through the magnetic field of the stator coils 16, 17.

The HPG is enclosed in a fiberglass reinforced epoxy structure 42 which is 1.1 m x 1.1 m x 1.5 m long. This enclosure will provide structural support, act as a vacuum vessel (necessary for high speed rotors), a cryo-

2 stat for the LN coil coolant, and will house the auxili¬ ary components necessary for machine operation. The weight of the HPG shown in Figure 3 is 2500 kg. Operating parameters of the HPG shown in Figure 3 are summarized in the following Table 2., and the discharge performance is plotted in Figure 4.

TABLE 2

Overall Volume 1.8 m 3 Overall Weight 2500 kg Inertial Energy Storage _- 230 MJ (92 kJ/kg) Inductive Energy Storaggee 136 MJ (54 kJ/kg) Peak Current 3.2 MA Peak HPG Voltage 450 V Peak Inductor Voltage 10000 V Peak HPG Power 1.4 GW (560 kW/kg) Peak Output Power 32 GW (13 MW/kg)

While a particular embodiment of the present invention has been shown and described, it will be under¬ stood that the invention is not limited thereto, since many modifications may be made and will become apparent to those skilled in the art. For example, rather than using self-

-20-

excited stator coils, the coils could be excited from an external source, with the current from the brushes con¬ ducted directly to the output terminals.