Permanent magnet rotor with saturable flux bridges

A rotor structure for a permanent magnet generator, or PMG, includes a plurality of spaced apart magnets and magnetic circuit elements in series with the magnets on the rotor to act as flux limitation means to thereby limit the output voltage of the permanent magnet generator at low current levels.

DESCRIPTION 
1. Technical Field 
The present invention relates generally to dynamoelectric machines and more 
particularly to a structure for a permanent magnet machine or generator. 
2. Background Art 
Permanent magnet generators, or PMG's, are inherently more efficient than 
wound rotor machines due to the fact that the PMG has only one winding 
which experiences copper losses as opposed to wound rotor machines which 
have multiple windings experiencing such losses. 
The recent development of high energy permanent magnet materials has aided 
in the design of permanent magnet generators, particularly those which are 
to be used in environments requiring low weight and high performance. 
However, the inherent nature of these high energy materials is that a 
large voltage drop occurs as load is applied. For example, if the PMG is 
run at a constant speed and is proportioned to operate near the maximum 
energy product of the magnets at maximum power, the output voltage at no 
load (i.e. the open circuit voltage) may be approximately 170% of rated 
voltage. 
Hence, PMG's have typically been utilized only as an auxiliary power 
source, for example, in a brushless alternator to supply field current to 
an exciter which in turn develops main field current for a generator. Even 
when used as an auxiliary power source, a voltage regulator is required to 
ensure that the PMG output is maintained at a substantially constant 
level. 
There have been various attempts to design improved rotor assemblies for 
permanent magnet generators. For example, each of McCarty et al U.S. Pat. 
No. 4,242,610, Silver U.S. Pat. No. 4,260,921 and Burgmeier et al U.S. 
Pat. Nos. 4,296,544 and 4,302,693 discloses a rotor assembly wherein 
tangentially magnetized magnets are separated by support members. The 
support members have a plurality of generally equally spaced holes 
disposed near the periphery thereof to reduce the rotor mass and 
presumably allow the rotor to operate at high speeds. It is noted in these 
patents that the size and location of the holes are established with 
regard to magnetic and structural considerations. However, there is not 
even a recognition in these patents of the problem of variations in output 
voltage with changes in load current, and hence, no solution to this 
problem is proposed in any of these patents. 
DISCLOSURE OF INVENTION 
In accordance with the present invention, a rotor structure for a permanent 
magnet generator, or PMG, includes means for limiting the voltage at low 
current below a desired level. 
In a first embodiment of the invention, a series of tangentially magnetized 
magnets are equally spaced about the periphery of a nonmagnetic hub. 
Disposed in the space between magnets is means for limiting the flux 
developed by the magnets. In this first embodiment, flux is limited by 
placing a ferromagnetic material having a region of relatively small 
cross-sectional area between the magnets so that flux density is increased 
to the saturation point of the below ferromagnetic material at all current 
levels up to a maximum current level below which voltage must be 
maintained at an approximately constant value. Accordingly, flux is 
limited below this maximum current level, and since voltage is 
proportional to flux, the output voltage of the PMG is also limited. 
In a second embodiment of the invention, a series of equally spaced 
radially polarized or magnetized magnets are disposed on a ferromagnetic 
yoke which is in turn mounted on a shaft. The yoke itself comprises flux 
limiting means in that it is made sufficiently thin at least in those 
portions between adjacent magnets to increase flux density to the 
saturation point for the yoke material. The result is that flux, and hence 
output voltage, are limited at desired levels. 
The ferromagnetic material is selected to exhibit an abrupt change in 
permeability at the maximum design load so that flux, and hence output 
voltage, is controlled to no more than a particular value at current 
levels below design rating. 
While in the above embodiments, flux limiting is accomplished by suitable 
dimensioning of ferromagnetic material in a magnetic circuit, it should be 
understood that such flux limiting can be accomplished without dimensional 
change by utilizing materials such as iron or iron-nickel alloys which 
saturate at a particular flux level.

BEST MODE FOR CARRYING OUT THE INVENTION 
Referring now to FIGS. 1A and 1B, there is schematically illustrated a pair 
of prior art permanent magnet generators, or PMG's 12,14, respectively. In 
FIGS. 1A and 1B, common elements are designated by like reference 
numerals. 
Each PMG 12,14 includes a stationary stator 15 and a rotor 16 which is 
rotatable with respect to the stator 15, as is conventional. 
The rotor 16 includes either a plurality of tangentially polarized or 
magnetized magnets 17a in the PMG 12 or a plurality of radially polarized 
or magnetized magnets 17b in the PMG 14. In the PMG's illustrated in FIGS. 
1A and 1B, the magnets are the high energy type having high coercive force 
and low flux density, such as samarium cobalt. 
The magnets 17a of the PMG 12, FIG. 1A, are mounted on a hub 18 of 
nonmagnetic material which is in turn secured to a shaft 19 driven by a 
prime mover (not shown). Disposed in the spaces between magnets 17a in the 
PMG 12 are bodies or spacers 22 of ferromagnetic material such as soft 
iron laminations, or the like. 
The magnets 17a, spacers 22 and stator 15 together comprise a magnetic 
circuit to establish flux linkages, one of which is shown by the dotted 
lines in FIG. 1A. 
The magnets 17b of the PMG 14 are mounted on a hub 20 which is fabricated 
of ferromagnetic material such as soft iron or the like to form a yoke. 
The hub 20 is secured to the shaft and is driven by a prime mover, as 
noted in connection with FIG. 1A. Spacers 23 of nonmagnetic material are 
disposed between the magnets 17b. The magnets 17b, hub or yoke 20 and 
stator 15 together comprise a magnetic circuit similar to that noted in 
connection with FIG. 1A. 
Referring now to FIG. 2, the voltage/current characteristic for each of the 
PMG's 12,14 is illustrated. As shown by the solid line of FIG. 2, the 
voltage at the terminals of the PMG decreases approximately linearly as a 
function of increasing current since the magnet flux decreases with 
increasing load current. Likewise, the "air gap" voltage, i.e. the voltage 
developed by the generator without regard to the internal resistance and 
reactance of the stator windings also varies approximately linearly with 
changes in current. It can be seen from the graph of FIG. 2 that the 
terminal and air gap voltages increase substantially above rated voltage 
when the current approaches zero. This is disadvantageous since the load 
circuits connected to the output of the PMG must be capable of tolerating 
this wide fluctuation in voltage with changes in current and/or a voltage 
regulator must be used. 
In order to minimize the dependence of output voltage with output current, 
the magnetic structure of the present invention effectively imposes an air 
gap in the magnetic circuit by causing the permeability of the 
ferromagnetic material in the circuit to approach that of air at a 
particular flux level. This is accomplished by appropriate dimensioning of 
the ferromagnetic material in the magnetic circuit and/or by utilizing 
ferromagnetic materials which inherently saturate at a given flux density 
as noted more specifically below. 
Referring now to FIGS. 3 and 4, a magnetic structure for the rotor of a PMG 
utilizing tangential magnets is illustrated, only a portion of the 
magnetic structure being shown for purposes of clarity. The tangential 
magnetic structure shown in FIGS. 3 and 4 may be utilized in the PMG 12 in 
place of the rotor 16 shown in FIG. 1A. 
As seen in FIGS. 3 and 4, a plurality of tangentially magnetized magnets 30 
are secured to a nonmagnetic hub 32 which is in turn secured to a shaft 
36. The magnets 30 may be secured to the hub 32 by any suitable means such 
that an interpole volume 38 is created between adjacent poles 30. Secured 
to the hub 32 in the interpole volume 38 is flux limiting means in the 
form of a U-shaped interpole mass 40 having a region of decreased 
cross-sectional area 41. 
In the preferred embodiment, the magnets are of the samarium cobalt type 
which has high coercive force and low flux density. The material of the 
interpole mass 40 is selected so that the magnetics saturate the interpole 
mass at the desired level to achieve the proper voltage limiting. The 
interpole mass 40 is preferably fabricated from a solid piece of 
magnetically saturable ferromagnetic material, preferably a ferromagnetic 
alloy of iron, cobalt and vanadium with trace elements marketed by 
Alleghany Ludlum Steel Corporation of Pittsburgh, Pa. under the name 
Vanadium Permendur. The interpole mass 40 may alternatively comprise a 
series of laminations, as described more specifically below. 
A retaining ring (not shown) may extend about the periphery of the rotor 
structure to maintain the placement of the various parts, if desired. 
The U-shaped interpole mass 40 cross-sectional area decreases from a 
cross-sectional area A.sub.1 adjacent one of the magnets 30a to a 
cross-sectional area A.sub.2 midway between the magnet 30a and a second 
magnet 30b. This decrease in cross-sectional area results in a greater 
flux density in the region about area A.sub.2 than in the region of area 
A.sub.1. At a particular flux density, the ferromagnetic material of the 
interpole mass 40 saturates and thereby limits the flux in the series 
magnetic circuit comprising the magnets 30, the interpole mass 40 and the 
stator (not shown). As seen in FIG. 5, since output voltage is 
proportional to flux, the voltage of the device is likewise maintained 
near a predetermined level for current levels less than 100% of the rated 
design point. This limiting of output voltage at low current levels means 
that the circuitry connected to the output of the resulting PMG need not 
be capable of tolerating wide voltage swings as was necessary with the 
prior art devices. Furthermore, particularly where high energy magnetics 
are used such as samarium cobalt, the resulting PMG can, with less weight 
and size, replace the heretofore known PMG-exciter-generator arrangement. 
As an example of the rotor structure shown in FIGS. 3 and 4, assume that 
the flux density at the maximum power point for the magnets is 4500 gauss 
and that Vanadium Permendur is the ferromagnetic material in the interpole 
volume 38. The cross-sectional areas A.sub.1 and A.sub.2 can be selected 
so that the flux density approaches 23.5 kilogauss as load current in the 
stator windings of the PMG drops below 100% of rated current. As load 
current continues to decrease and the voltage or flux increases, the 
Vanadium Permendur material saturates at a flux density slightly greater 
than 23.5 kilogauss and appear as a very long air gap in series in the 
magnetic circuit. 
It is desirable to have the break or "knee" 42 of the curve representing 
air gap voltage to be as close as possible to the maximum load point. This 
point is a function of PMG speed and magnet volume and is also related to 
the saturation parameters of the ferromagnetic material of the flux 
limiting means. 
It is possible to limit flux in the stator of the PMG rather than in the 
rotor; however, controlling flux density in the stator is somewhat less 
efficient than rotor flux control since the stator flux density is 
changing at a high frequency and the magnetics are driven relatively hard 
into saturation leading to high losses in the core material. Owing to the 
inherent nature of the PMG, it is generally more desirable to control flux 
in the rotor, where the flux is continuous and saturation has no effect on 
efficiency. 
The embodiment shown in FIGS. 3 and 4 is preferred for the tangential 
magnet arrangement since the outer surface of the magnets and interpole 
mass is cylindrical, thereby reducing windage and unbalance problems. 
Other configurations can be utilized for the interpole mass 40, such as 
those shown in FIGS. 6 and 7. For example, as seen in FIG. 6, the 
interpole mass 40 may comprise a series or stack of laminations 46 of 
varying length which when assembled together and placed in the interpole 
volume 38 act as flux limiting means in the series magnetic circuit 
including the magnets 30, the stator of the PMG and the laminations 46. 
The laminations 46 are of differing lengths or circumferential extent 
46a-46d with the lamination 46a being the shortest and the lamination 46d 
being the longest. The laminations are bonded together in the pattern 
shown in FIG. 6 wherein the length or circumferential extent of successive 
laminations, starting at one axial end 47 of the rotor, increases up to 
the lamination 46d which traverses the entire distance between adjacent 
magnets 30a,30b. The length of successive laminations then decreases down 
to the shortest lamination 46a and the sequence then repeats. The 
laminations are preferably constructed of the Vanadium Permendur material 
previously described. 
The lamination stack effects flux limitation due to the restriction in 
cross-sectional area between adjacent magnets. The number and size of 
laminations may be varied to achieve the proper flux saturation level. 
As seen in FIG. 7, the interpole mass 40 may alternatively comprise a solid 
piece of ferromagnetic material, such as Vanadium Permendur, having a 
series of indentations or dimples 50 which accomplish a reduction in 
cross-sectional area which in turn limits flux and therefore output 
voltage at low current levels. The indentations 50 may be present on one 
or both of the radially inner and outer walls 51,52 respectively of the 
interpole mass 40, as desired. 
Other means or methods of reducing magnetic flux may be utilized, such as 
by drilling holes, or otherwise selectively removing material to 
accomplish a cross-sectional area reduction. 
It should also be noted that flux limitation can be accomplished without a 
dimensional change in cross-sectional area by utilizing materials, such as 
iron or iron-nickel alloys, which saturate at a particular level. In such 
a case, the interpole mass 40 may be configured to minimize windage losses 
while at the same time providing the desired voltage limitation at low 
current levels. 
Referring now to FIG. 8, there is shown an alternative rotor assembly of 
the present invention for use in PMG's having radially magnetized magnets 
or poles 52. As seen in FIG. 8, two of the magnets 52a,52b are secured to 
a yoke 54. The yoke 54 is in turn connected to a nonmagnetic hub 55 which 
is in turn secured to a shaft (not shown). 
Disposed between the magnets 52a,52b is a nonmagnetic spacer 56. A 
retaining ring (not shown) may be used, as before, to maintain the 
placement of the various parts. 
The rotor assembly may be used in the PMG 14 shown in FIG. 1B in place of 
the rotor 16. 
In this embodiment of the invention, the yoke 54 acts as the means in which 
flux limiting is accomplished in the series magnetic circuit comprising 
the magnets 52, the yoke 54 and the stator of the PMG. The yoke material 
may be Vanadium Permendur, or any other suitable material. In this case, 
the thickness of the yoke 54 is selected to cause an increase of flux 
density therein to the saturation point of the yoke material so that flux, 
and hence output voltage, are limited. Typically, this would require that 
the yoke thickness be substantially less than the thickness of yokes 
disclosed in the prior art, such as the magnetic hub shown in FIG. 1B. 
This reduction in thickness may lead to a reduction in strength for the 
rotor assembly, and hence additional means may be necessary to strengthen 
the rotor assembly to prevent deformation of the yoke 54 during use. 
Referring now to FIG. 9, there is shown an alternative embodiment of a 
rotor structure for a PMG utilizing radially magnetized magnets wherein 
the yoke 54 shown in FIG. 8 is replaced by a yoke 60 of varying thickness. 
In FIGS. 8 and 9, common elements have like reference numerals. The yoke 
60 includes relatively thick portions 62 and thin portions 64. The thin 
portion 64 limits flux, and hence output voltage, as was noted with 
respect to the embodiment shown in FIG. 8. The thick portions 62 provide 
additional support for the magnetic structure on the yoke 60 to increase 
the strength and hence structural integrity of the rotor assembly. In this 
case, the additional strengthening means which may be necessary with the 
embodiment of FIG. 8 may be dispensed with in certain applications. 
Analogous to the previous embodiments, the flux limitation may be 
accomplished in various other ways in the yoke 54 or 60, such as by 
drilling holes or otherwise removing material at appropriate spaces in the 
series magnetic circuit, or by utilizing a material which saturates at a 
particular level, as was noted with respect to the embodiments utilizing 
tangential magnets noted above.