Patent Application
20080315730
The present invention is in the field of electrical generators and, more particularly, electrical generators operating at very high rotational speeds.
In certain applications of generators such as those employed in aircraft, there is a requirement to produce a high power density with a generator that is small in size and light in weight. In these applications, a desired high power density may be achieved with relatively small generators which operate at very high rotational speeds. A typical aircraft generator may operate at rotational speeds of 12,000 to 24,000 rpm.
When a generator is operated at such high rotational speeds, rotatable components of the generator may be subjected to correspondingly high centrifugal forces. Some rotatable components may be particularly vulnerable to damage from centrifugal forces. Examples of these vulnerable components are electrical leads that are external to field coils of the generators.
In a typical aircraft high-speed generator, a field coil is interconnected with rectifier diodes using leads that are external to the field coil. In prior art constructions of such generators, these external leads may be secured to a shaft that extends from the field coil. When the leads are secured, with an epoxy-coated tape wrap, to the shaft, the leads may be constrained from radial displacement when the shaft and field coil are rotated.
This prior art method of lead constraint has limited capability. The epoxy-coated tape wrap must have sufficient strength to resist being destroyed by high centrifugal forces. Consequently, prior art generators have employed massive epoxy-coated tape wraps to constrain external leads. These prior art tape wraps are disadvantageous from at least two points of view. First of all, tape wraps which are strong enough to resist 24,000 rpm centrifugal forces are undesirably heavy and therefore adversely contribute to the weight of an aircraft generator. Secondly, fabrication of a generator with epoxy-coated tape wrap requires a number of cumbersome and expensive manufacturing steps. Typically, tape must be wrapped around the leads and the shaft in an expensive manual operation. Also an epoxy coating step must be performed after the tape is in place. These steps are costly and they may contribute to product defects if the steps are not performed correctly.
In an alternate prior art construction these leads may be embedded in axially-oriented grooves cut in a shaft that extends from the field coil. The leads may be sleeved in insulating material and embedded in epoxy. These prior art grooves are disadvantageous from several points of view. First of all, the grooves may reduce the shaft stiffness with a consequent detrimental effect on rotor dynamics. Secondly fabrication of a generator with epoxy embedded grooves may require expensive additional machining and a molding step after the leads are in place.
In such a prior-art generator, a bobbin may be used to interconnect the field coil to diodes. This prior-art bobbin may consist of a body using castellations for alignment to an exciter, a wound resistor and a set of bus rings for interconnection. This prior-art bobbin may be disadvantageous in several ways. Firstly the bobbin may not provide for any mechanical support for diode leads to resist centrifugal forces. Secondly the wound resistor may use expensive bus rings for interconnection of the diode leads to the two flat wire field leads. This may add considerable cost and complexity. Finally, the wound resistor may be located inside the bobbin leaving no radial room for the field leads to route axially through the bobbin. This means that flat wire in a grooved shaft may be required for the interconnection of the field winding to the diodes.
In the field of high-speed generators it is desirable to construct field coils from tightly wound flat conductors. In the prior art, an end portion of the flat conductor was utilized to make electrical interconnection between the field coil and other components of the generator. Construction of such prior-art generators required introduction of a multiple bends in the flat conductor. These bends are required because portions of a flat conductor used for interconnection purposes required a planar orientation parallel to an axis of rotation of the generator. Conversely, portions of the flat conductor that comprised the wound field coil required a planar orientation that was circumferential to the axis of rotation of the generator. Introducing bends in a flat conductor is a cumbersome task. Typically, it must be performed manually by experienced craftsmen so as not to introduce potential defects in the generator.
As can be seen, it would be desirable to construct a high-speed generator in which vulnerable rotatable components are restrained from radial displacement without use of massive and expensive epoxy-coated tape wraps. Additionally, it would be desirable to provide a method for constructing such generators without using portions of flat field coil conductors as electrical interconnection elements outside the field coil.
In one aspect of the present invention a generator comprises a field coil, a shaft extending axially from the field coil, field leads extending axially from the field coil and an annular member positioned on the shaft which constrains the field leads from moving radially during rotation of the field coil without employing an epoxy-coated tape wrap.
In another aspect of the present invention a generator comprises a field coil wound from flat conductor and round field leads. Electrical interconnection between the flat conductor and the round field lead is made on a brazing clip.
In still another aspect of the present invention a method for producing electrical current with a high-speed electrical generator comprises the steps of constraining field leads in a resistor bobbin positioned on a shaft of the generator; and rotating the resistor bobbin with the shaft whereby the field leads are constrained from radial displacement during the rotation.
These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims.
The following detailed description is of the best currently contemplated modes of carrying out the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention, since the scope of the invention is best defined by the appended claims.
Broadly, the present invention may be useful in improving high-speed electrical generators. More particularly, the present invention may provide a simple expedient to constrain rotatable components such as electrical leads from radial displacement and damage resulting from centrifugal forces. The present invention may be particularly useful in aircraft generators which operate at high rotational speeds of up to about 24,000 rpm.
In contrast to prior-art generators, among other things, the present invention may provide a generator that is constructed without epoxy-coated wraps or grooved shaft for constraint of electrical leads. The present invention may, instead of epoxy coated wraps or grooved shaft, use a unique resistor bobbin that may be provided with passages in which field leads are radially constrained. The present invention may also incorporate a unique transition device or brazing clip that expediently provides for use of round field leads in a generator. These desirable improvements of a high-speed generator may be achieved by constructing a generator in an inventive configuration illustrated in
Referring now to
The field coil 12 may be comprised of a tightly wound conductor 30 which may have a generally flat configuration such as that illustrated in
Referring now to
In a typical prior-art high speed generator, a resistor bobbin is positioned between a field coil and an exciter coil. In the present invention, the resistor bobbin 28 uniquely serves a dual purpose. The resistor bobbin 28 performs its prior art role of supporting a resistor coil, but it serves an additional purpose of radially constraining the field leads 26 during high speed rotation of the generator 10. Thus the inventive resistor bobbin 28 may replace a massive coated tape constraining system of the prior art with the simple expedient of utilizing the resistor bobbin 28 for a dual purpose. This expedient may allow for radial constraint of the field leads 26 without introduction of any additional mass into the generator 10. This configuration allows the field leads 26 to be routed axially and positively constrained without compromising the stiffness and balance of the shaft 16 with shaft grooves.
Referring now to
During high speed rotation of the generator 10, the tabs 54 may be forced into contact with the laminated core 20 along their respective outer surfaces 54a. The laminated core 20 may be a metallic annular member. The laminated core 20 may be thus resistant to radial expansion during rapid rotation. Because the outer diametrical dimension of the surfaces 54a may be only slightly smaller than the inner diametrical dimension of the laminated core 20, there may be only a very small radial expansion of the resistor bobbin 28. Thus the bobbin 28 may be constructed from a relatively light weight material such as the plastics described hereinabove. It may be seen that the castellations 52 and the tabs 54 of the bobbin 28 may facilitate its radial constraint functionality while permitting its mass to be desirably low.
By continued reference to
By referring to
In one embodiment of the present invention, a method is provided for constructing a high speed generator (e.g. the generator 10). In that regard the method may be understood by referring to
In another embodiment of the present invention, a method is provided for producing electrical current with a high speed generator (e.g. the generator 10). In that regard the method may be understood by referring to
It should be understood, of course, that the foregoing relates to exemplary embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims.
