Rotary electric machine with centrifugal filter

A centrifugal filter for a high speed electrical machine of the type having a fluid flow path through its rotor. The centrifugal filter includes an enlargement at the beginning of the flow path through the rotor and which extends radially outwardly of the axis of rotation. As a consequence, contaminants in an incoming fluid stream are subjected to centrifugal force during operation of the machine and moved thereby radially outwardly within the enlargement so as to be out of the fluid stream. Consequently, the fluid stream, after separation of the contaminants therefrom within the enlargement by centrifugal force, may be passed with safety through the rotor for cooling and/or lubricating purposes.

TECHNICAL FIELD 
This invention relates to rotary electric machines, and more specifically, 
to rotary electric machines having a centrifugal filter for removing 
particulate material from a circulated liquid acting as a coolant and/or 
lubricant. 
BACKGROUND ART 
Many rotary electric machines as, for example, generators, have a fluid 
flow path through the rotor. In most cases, a liquid such as oil is 
circulated through the flow path to provide the dual function of cooling 
rotor windings to thereby increase the capacity of the machine and to 
provide for lubrication of bearings journalling the rotor. In some 
instances, the construction will utilize but one or the other of the two 
above specified functions, while in others, the fluid may be discharged 
from the rotor to impinge on stator winding end turns to provide an 
additional cooling function. 
Invariably, impurities, most notably, very small particulates of metal or 
the like are found in the liquid being circulated. Where the liquid is 
serving a lubricating function, the presence of such particulates 
accelerate bearing wear resulting in increased maintenance and/or 
decreased machine life. Though not as obvious a problem, such foreign 
material will also shorten machine life where the liquid is employed as a 
coolant, particularly in those rotary electric machines wherein the 
coolant passages for winding are formed by the interstices between the 
windings themselves. 
Over a period of time, the particulates lodge in the interstices and impede 
coolant flow. As particle build-up continues, coolant flow is 
progressively lessened with the consequence that hot spots in the windings 
develop. Ultimately, the insulation on the windings will fail at the hot 
spot causing short circuiting and machine failure. 
It has thus been proposed to filter the coolant. See, for example, U.S. 
Pat. No. 3,242,360 issued Mar. 22, 1966 to Carle wherein what appears to 
be a conventional filter is installed in the coolant flow path of a 
submersible pump. 
While this approach may work well in many applications, it is not 
satisfactory in many instances where the rotary electric machine is one 
which operates at high speed. For example, aircraft generators are 
typically operated at 12,000 rpm or more and in such operation, extremely 
fine oil contaminants which may escape a conventional filter media are 
subjected to high gravitational loads during operation due to centrifugal 
force. The large forces tending to move such fine contaminants are 
disposed to move them radially outwardly and since, for convenience, most 
rotary electric machines locate coolant inlets and outlets to the rotor on 
the axis of rotation, and yet have coolant flow paths spaced from the axis 
of rotation, it is extremely difficult, if not impossible, to entrain such 
fine particles in the coolant stream sufficiently to move them radially 
inwardly within the rotor the fluid outlet therefrom. As a consequence, 
the coolant passages displaced from the axis of rotation slowly become 
plugged ultimately resulting in failure of the machine. 
The present invention is directed to overcoming one or more of the above 
problems. 
SUMMARY OF THE INVENTION 
It is the principal object of the invention to provide a new and improved 
rotary electric machine with means for filtering fluid being passed to the 
rotor thereof. 
According to the invention there is provided a rotary electric machine 
including a stator and a rotor journalled for rotation relative to the 
stator about an axis of rotation. A fluid flow path extends through the 
rotor and an inlet is located on the rotor to the fluid flow path, the 
inlet being on the axis of rotation of the rotor. 
According to the invention, there is provided an enlargement of the flow 
path adjacent the inlet which extends radially outwardly of the axis. By 
this structure, centrifugal separation of fine particulates from the fluid 
being passed to the flow path occurs in the enlargement. Consequently, 
fluid exiting the enlargement and traveling through the remainder of the 
flow path is relatively contaminant free such that the liquid may be 
advantageously employed in cooling windings and/or lubricating bearings 
without fear of the deliterious effects of entrained particulate 
contaminant in the fluid. 
According to a preferred embodiment of the invention, the enlargement is 
provided with a particle retaining means to prevent the particles from 
exiting the enlargement when the machine is quiescent. 
In a highly preferred embodiment, the retaining means is a fine wire mesh. 
The invention contemplates that the enlargement has a fluid exit located 
radially inwardly of the radially outermost part of the enlargement. Thus, 
due to centrifugal separation of fine contaminants from the fluid in the 
enlargement, and the fact that centrifugal force will tend to maintain the 
contaminants radially outwardly of the fluid due to differences in 
density, this structure assures that fluid exiting the enlargement will be 
free of fine contaminants. 
Other objects and advantages will become apparent from the following 
specification taken in conjunction with the accompanying drawings.

BEST MODE FOR CARRYING OUT THE INVENTION 
An exemplary embodiment of a rotary electric machine made according to the 
invention is illustrated in FIG. 1 in the form of a high speed generator. 
Specifically, a two pole, 24,000 rpm generator for aircraft use is shown. 
However, it should be understood that the invention can be employed with 
efficacy in other types of rotary electric machines wherein rotor cooling 
and/or bearing lubrication involves the provision of a fluid flow path 
through the rotor. 
The machine includes a stator armature, generally designated 10, having a 
steel core 12, windings 14 (only the end turns of which are shown), and a 
rotor/stator air gap 16. Bearings such as those shown at 18 mounted on a 
housing component 20 serve to journal a rotor, generally designated 22, 
for rotation relative to the stator 10 within the gap 16 about the 
longitudinal axis of the rotor 22. 
In the particular form of machine illustrated, the generator is of the 
so-called brushless variety and includes windings 24 extending from end to 
end of the rotor 22 as illustrated. The windings receive a direct current 
to generate a magnetic field which in turn rotates upon rotation of the 
rotor to induce current in the windings 14 of the stator 10. 
The rotor 22 includes a fluid inlet, generally designated 26, at one end 
and a fluid outlet, generally designated 28, at the other. A fluid system 
including a pump 30 and an air-oil separator 32, if desired, is operable 
to recirculate a liquid coolant/lubricant from the outlet 28 to the inlet 
26 via a transfer tube 33 disposed within the inlet 26. 
Within the rotor 22, the inlet 26 and outlet 28 are connected by a fluid 
flow path 34 displaced from the axis of rotation and defined by the 
interstices between the windings 24. However, it is to be understood that 
if desired, the fluid flow path 34 could be defined by passages formed 
within the rotor. The essential characteristic of the fluid flow path 34 
is that a portion of the same be in heat exchange relation with the 
windings 24 so that the latter may be cooled by the passage of a suitable 
fluid, such as oil, through the flow path 34. 
Where the fluid being passed through the rotor 22 is being utilized for 
lubrication purposes, the outlet 28 may include an annulus 36 and ports 
such as shown at 38 and 40 extending from the annulus to the bearings 18. 
Consequently, lubricating fluid will be directed to the bearings 18 by 
this structure. 
Immediately downstream of the inlet 26 is a centrifugal filter, generally 
designated 42. The filter 42 is carried by the rotor 22 and is operative 
to separate fine contaminants from the incoming fluid stream before the 
stream is passed to the flow path 34 for cooling and/or lubricating 
purposes. 
The filter 42 is formed by a casting 44 suitably machined for receipt 
within a bore 46 in an end shaft 47 forming part of the rotor 22. 
As seen in FIGS. 1 and 4, the casting 44 is provided with a peripheral, 
radially outwardly opening groove 48 which in turn receives an O-ring seal 
50 whereby the casting 44 may be sealed against the interior of the bore 
46. 
Centrally of the casting 44 is a sleeve-like formation 52 which receives 
the transfer tube 33 and defines the inlet 26. As seen in FIG. 1, the 
interior of the sleeve-like formation 52 is in fluid communication with a 
plurality of generally radial ports 56 which extend to enlargements or 
cavities 58 within the casting 44. The cavities 58 are located 
substantially radially outwardly of the rotor axis such that the contents 
thereof will be subjected to the high gravitational forces induced by 
centrifugal force during rotation of the rotor 22. 
The center of the casting 44 is provided with a central solid section 60 in 
which the ports 56 are formed such that the solid section 60 serves as a 
baffle to assure that incoming fluid is directed radially outwardly to the 
cavities 58. 
As seen in FIGS. 1 and 4, the solid section 60 does not extend the full 
axial extent of the casting 44 but is recessed therefrom so as to allow 
radially inwardly directed fluid flow from the cavities 58 toward the 
rotational axis of the rotor as shown by the solid line arrows in FIG. 1. 
The cavities 58 are filled with a means for retaining particulate material. 
In a preferred form of the invention, such means are constituted by fine 
wire mesh 62 formed from stainless steel or the like. 
The cavities 58, with the mesh 62 in place, are closed by a closure 64 
having a central opening 66 which is, as illustrated in FIG. 1, in fluid 
communication with the windings 24, and thus the flow path 34. 
As seen in FIGS. 2 and 4, between adjacent ends of the cavities 58 there 
may be located stepped bores 70. Though not shown, the bores 70 receive 
electric leads from a source of direct current extending to the winding 24 
for energization purposes. 
Just radially inwardly thereof may be oppositely directed stepped bores 72 
for receipt of fasteners (not shown) which could be employed for holding 
the casting 44 axially in the location illustrated in FIG. 1 and/or for 
securing the closure 64 thereto. Bosses 74 may be located on the casting 
44 for receipt of fasteners for mounting other components (not shown) of 
the generator. 
In operation, an incoming fluid for both cooling the windings 24 and 
lubricating the bearings 18 enters the rotor on the rotational axis 
thereof via the inlet 26. The solid section 60 together with the ports 56 
directs the incoming flow in the radially outward direction and as the 
same moves progressively radially outwardly, it is subjected to higher and 
higher gravitational loads due to centrifugal force when the generator is 
in operation. The fluid enters the mesh 62 within the cavities 58 and 
ultimately, the pressure of incoming fluid will drive the same radially 
inwardly as illustrated by the arrows such that it passes through the 
opening 66 to the fluid flow path 34 and ultimately to the outlet 28 
and/or the bearings 18. 
The contaminants, typically being metal, will have a higher density than 
the incoming coolant and thus will be subjected to greater centrifugal 
loading during operation of the machine when they are displaced from the 
rotational axis of the rotor 22, that is, when they are in the cavities 
58. As a consequence, they will tend to move radially outwardly through 
the mesh 62 to the radially outer side of the corresponding cavity 58 and 
lodge thereagainst as shown by the dotted arrows in FIG. 1. Because the 
outlet defined by the opening 66 from the cavities 58 is on the rotational 
axis but, in any event, is located radially inwardly of the radial outer 
extent of the cavities 58, the contaminants will not move thereto during 
operation because of the centrifugal loading. 
When the machine is quiescent, i.e., the rotor is not being rotated, the 
contaminants might be expected to move under the influence of gravity or 
other forces that might be encountered in a given environment toward the 
rotational axis of the rotor such that they could ultimately enter the 
flow path 34. Such movement is, however, prevented by the presence of the 
mesh 62 within the cavity which, while allowing the contaminants to pass 
when they are subjected to high loads during operation, is of sufficiently 
small size to prevent free movement at all other times. 
From the foregoing, it will be appreciated that a centrifugal filter made 
according to the invention is ideally suited for use in high speed rotary 
electric machines and prevents the entry of fine contaminants into coolant 
passages which could be plugged thereby and/or bearings which could be 
abraded thereby. It should be recognized that the configuration of the 
cavities 58 illustrated in the drawings is not a critical aspect of the 
invention and that in some instances, a single peripheral cavity, or a 
plurality greater than two of the cavities can be employed. The 
configuration illustrated is utilized in the present invention to 
accommodate the passage of fasteners, electric leads, etc. whose location 
may be varied dependent upon the location of other rotor components as 
desired.