Patent Application
20130270828
The present application relates to gas turbine engines, and more particularly to a housing for an electrical starter and/or motor for use with gas turbine engines.
Current starting systems for gas turbine engines include electrical, pneumatic, and hydraulic based starting systems. Electrical based starter systems offer several advantages over pneumatic or hydraulic based starter systems in that they tend to be more compact, have lower cost, and higher reliability. Additionally, electrical starter systems offer improved speed/torque control for starting, re-engagement, and motoring.
A conventional electrical starter includes a central rotor shaft surrounded by a stationary winding (stator). The rotor is supported at each end by bearings. The stator is supported within an outer housing of the motor. Typically, stators are constructed from a lamination stack of metallic elements, which are configured to abut one another and are arranged axially along the length of the rotor shaft outward of the rotor.
One factor in the effectiveness of electrical starter systems is the ability of the motor to convert electromagnetic forces into useful output torque. Useful output torque is maximized when the stator-armature gap provides a flux path having a minimal reluctance. In addition, the magnetization characteristics including the magnetic field (B) and the magnetizing field (H) of the stator lamination material are maximized.
In an effort to minimize reluctance and maximize the B-H material coefficients, the interface between the stator lamination stack and an inner surface of the outer housing is precision press fit. However, such precision press fitting can have unintended consequences. For example, under certain operating conditions, such as a cold start operation, the hoop stress of the laminations is significantly increased because of the generation of a contact stress due to differences between the thermal coefficient of expansion of the outer housing material and the laminations. Increased hoop stress reduces the performance of the starter. Thus, a larger starter than is desirable may be required in order to develop sufficient torque to actuate the gas turbine engine. The larger starter contributes undesirable properties such as additional size and weight to the gas turbine engine and the starter system (including the motor, feeders, and motor controller).
A housing for an electrical device includes an interior surface and one or more stress reducing notches. The interior surface is configured to interface with one or more internal components of the motor. The one or more stress reducing notches are located along the interior surface of the housing.
In another aspect, a starter motor includes a housing and a plurality of stator laminations. The housing has one or more stress reducing notches therein located along an interior surface of the housing. The stator laminations are disposed within the housing and interface with at least a portion of the interior surface.
In yet another aspect, an assembly for a gas turbine engine includes an accessory gearbox and a starter motor. The starter motor is mounted to the accessory gearbox and includes a plurality of stator laminations and a housing having one or more stress reducing notches therein. The notches are located along an interior surface of the housing. The stator laminations are disposed within the housing and interface with at least a portion of the interior surface.
Accessory gearbox 14 is mounted to gas turbine engine 12 to initiate rotation thereof. As described in U.S. Pat. No. 7,131,275, which is incorporated herein by reference, electric starter motor 16 operates to drive accessory gearbox 14. In the embodiment shown in
Shaft 20 is mounted to inner portions 36 of outer housing 30 via bearings 22a and 22b. Bearings 22a and 22b are located adjacent rotor 24. That is, the bearings 22a and 22b are disposed radially interior (with respect to centerline CL axis of shaft 20) of rotor 24. Clutch 18 is mounted on shaft 20 within a cavity formed adjacent rotor 24 and end mount housing 28. End mount housing 28 can have various configurations as desired in order to mount on accessory gearbox 14 (
In the embodiment shown in
Stator 26 is mounted radially outward of rotor 24 with respect to the center line axis CL of shaft 20. Thus, stator 26 is disposed between rotor 24 and outer housing 30. More particularly, windings 32 of stator 26 are disposed adjacent rotor 24 while laminations 34 abut portions of outer housing 30. Housing 30 comprises an enclosure which interfaces with laminations 34 of stator 26 and also extends to enclose other interior components of starter motor 16.
Windings 32 are disposed within laminations 34 and can include copper wire or other conductive filaments. In the embodiment shown, laminations 34 comprise a plurality of generally annular segments which are disposed to abut one another. Laminations 34 can be constructed from suitable magnetic materials. In one embodiment, the laminations 34 comprise an iron-cobalt-vanadium soft magnetic alloy.
During operation rotor 24 rotates with shaft 20 within stator 26. In particular, rotor 24 is configured to rotate about the center line axis CL relative to the stator 16, so that a gap (not shown) is maintained between the two components to form part of a magnetic flux path. An excitation current creates a magnetomotive force to drive magnetic flux in the closed magnetic flux path. The magnetic flux attracts rotor 24 toward the stator 26, according to well known principles of magnetism, and tends thus to urge the rotor 24 either clockwise or counterclockwise to create a working torque output.
Notches 42 are formed in outer housing 30 adjacent laminations 34 along an interior surface 44 of outer housing 30. As illustrated in
Therefore, the depth, circumferential position along the interior surface 44, and the circumferential extent and shape of notches 42 depends upon the size and shape of outer housing 30. Stress analysis can be performed utilizing finite element method on commercially available software to determine the location, shape, and size of notches 42 within outer housing 30.
Notches 42 reduce contact stresses between laminations 34 and outer housing 30 due to differences between the thermal coefficient of expansion of laminations 34 and outer housing 30. Thus, the hoop stress within the laminations 34 (which results from the contact stresses between the laminations 34 and outer housing 30) is reduced. This allows the size of the starter motor 16 to be maintained at a smaller size (even with the starter motor 16 subjected to cold start operations) than would otherwise be achievable. Thus, the starter motor 16 does not contribute an inordinate amount to the size and weight of the gas turbine engine assembly 10 (
While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.
