Structure and method for distributing failure-induced transient currents in a multiphase electrical machine

An auxiliary stranded copper conductor carries the fault currents generated by a diode failure about the periphery of a brushless exciter diode wheel. The auxiliary conductor is constructed with a lower high-frequency impedance than the adjacent diode wheel, so that AC fault currents in particular are diverted to the auxiliary conductor. The auxiliary conductor carries the fault currents to equalize both AC and DC fault currents among the fuse-legs of the same phase. In a preferred embodiment, the auxiliary conductor is circular and is mounted between all of the diode fuse bases at one end of the diode wheel and the diode wheel itself.

FIELD OF THE INVENTION 
The present invention is directed generally to a structure and method for 
conducting and distributing transient currents which are present upon 
failure of a component in a multiphase electrical machine, and more 
particularly to an auxiliary peripheral current conductor for a brushless 
exciter diode wheel. 
BACKGROUND OF THE INVENTION 
There are many kinds of multiphase electrical machines having rotating 
members which carry electrical components and include a cylindrical 
conductor. One example of such a rotating member is a brushless exciter. 
The major structural component of a brushless exciter is typically a 
rotating cylindrical conductor made from magnetic steel and known as a 
diode wheel. The basic function of a brushless exciter is to generate an 
alternating current and convert that current into a direct current used to 
produce the rotating field for an electrical generator. Rectification is 
accomplished through the use of multiple diode circuits which are rigidly 
mounted to the diode wheel. For a three-phase wye system a minimum of six 
diodes is required. Typically, however, many more are used to provide 
spare legs, to accommodate multiple pole designs, and to provide high 
current carrying capacity. Where high current capabilities are needed, 
many diodes are connected in parallel to reduce the current flow through 
each diode. To protect such diodes from degradation due to overcurrent 
application, and to protect the exciter's operation from the effects of a 
shorted diode, designs typically include fuses connected in series with 
the diodes about the periphery of the diode wheel. 
In brushless exciter designs that include many diode-fuse combinations in 
parallel, the failure of a single diode should not have an adverse effect 
on the remaining diode-fuse combinations. Although each of the remaining 
parallel fuses will be required to carry an increased current load, this 
current is usually within the design capability of the fuse. In 
recognition of this fact, a number of systems have been proposed for 
detecting single diode failures which have not resulted in a shutdown of 
the generator. Such systems are illustrated in U.S. Pat. Nos. 4,635,045, 
and 4,952,915, assigned to the assignee of the present invention. 
In practice, however, it has been observed that a single fuse failure in a 
brushless exciter design which utilizes phase paralleling rings has the 
potential to cause a cascade failure of the other diode fuses, resulting 
in a forced outage of the exciter (and the generator). Therefore, there is 
a need for a design which prevents overload of adjacent diode fuses 
resulting from failure of another diode fuse, thus preventing unnecessary 
cascade failures and generator shutdown. 
SUMMARY OF THE INVENTION 
Therefore, it is a general object of the present invention to provide a 
system and method for equalizing high-frequency current flow in an 
electrical machine. 
Another general object of the present invention is to provide a system and 
method for preventing cascade failures of fuses in a brushless exciter. 
Another general object of the present invention is to provide a system and 
method for equalizing fault currents in a brushless exciter. 
A more specific object of the present invention is to provide a structure 
for connecting a plurality of current conductors to a rotating cylindrical 
direct current conductor in an electrical machine, including an auxiliary 
conductor with an impedance lower than that of the cylindrical current 
conductor. 
A further object of the present invention is to provide a circular 
conductor to distribute fault currents among connections to an electrical 
machine component. 
Yet another object of the present invention is to provide a structure for 
connecting a plurality of conductors to a cylindrical conductor, including 
an auxiliary conductor which is stranded to reduce high-frequency 
impedance. 
Another object of the present invention is to provide a structure for 
connecting a plurality of conductors to the cylindrical diode wheel of an 
electrical machine, including an auxiliary conductor which is connected 
between a group of diode fuses and the cylindrical diode wheel. 
A further object of the present invention is to provide a structure for 
connecting a plurality of conductors to the cylindrical diode wheel of an 
electrical machine, including an auxiliary conductor which is connected 
between a group of diode fuses and the cylindrical diode wheel and which 
is mounted in position using the mounting hardware of the diode fuses. 
These objects and others are achieved in the present invention by providing 
an auxiliary stranded copper conductor which carries the fault currents 
generated by a diode failure about the periphery of a brushless exciter 
diode wheel. The auxiliary conductor is constructed with a lower 
high-frequency impedance than the adjacent diode wheel, so that high 
frequency fault currents in particular are diverted to the auxiliary 
conductor. The auxiliary conductor carries the fault currents to equalize 
these high frequency fault currents among the fuse-legs of the same phase. 
In a preferred embodiment, the auxiliary conductor is circular and is 
mounted between all of the diode fuse bases at one end of the diode wheel 
and the diode wheel itself.

DETAILED DESCRIPTION OF THE INVENTION 
In its broadest application, the present invention is applied to a 
multiphase electric machine having a rotating cylindrical conductor with a 
plurality of current-carrying conductors attached thereto, generally at 
spaced locations about the periphery of the cylindrical conductor. In the 
illustrative embodiment disclosed herein, the invention is described in 
conjunction with a brushless exciter. However, those skilled in the art 
will recognize that the principles of the present invention may be applied 
to any generally cylindrical conductor of an electric machine. 
A typical rotating portion 2 of a brushless exciter is shown in partial 
cross-section in FIG. 1. Although the construction of brushless exciters 
is well-known in the art, the rotating portion 2 will be described briefly 
herein to facilitate ready understanding of the present invention by those 
who are less familiar with the art. 
As shown in FIG. 1, the rotating portion 2 comprises a diode wheel 4, diode 
assemblies 6 and 8, diode fuses 10 and 12 corresponding to each diode 
assembly 6 and 8, fuse attachment bolts 18, 20, 24, and 26, and fuse 
mounting block 22. The diode fuses 10 and 12 include fuse bases 14 and 16 
respectively. The diode wheel 4 is a generally cylindrical wheel which 
rotates about its central longitudinal axis (not shown) and is constructed 
of magnetic steel. Diode assemblies 6 and 8 are mounted on the inside of 
diode wheel 4. Diode fuses 10 are mounted on one end of the diode wheel 4 
about its periphery, where they are held in place by fuse attachment bolts 
18 which mount the fuse bases 14 to the diode wheel 4. Diode fuses 12 are 
mounted at the other end of the diode wheel 4 about its periphery and are 
held in place by fuse attachment bolts 20 which mount their fuse bases 16 
to the fuse mounting block or blocks 22. Conductors 28 and 30 connect 
diode assemblies 6 and 8 to diode fuses 10 and 12 by means of fuse 
attachment bolts 24 and 26 respectively. The diode fuses 10 and 12 
interrupt the flow of current from a particular diode assembly 6 or 8 in 
case of failure of that diode assembly. 
Although shown here in cross-section, so that only one of each component is 
shown, it will be recognized that the diode wheel 4 is of generally 
cylindrical shape, and that a plurality of each of the other components 
shown are carried in spaced relationship about the inside of the diode 
wheel 4. Generally, the number of diode assemblies 6 and 8 depends on the 
number of phases and the current-carrying capacity provided in the design 
of the brushless exciter. Typically, in a three-phase design, there might 
be six groups of four diode assemblies 6 and 8 arranged in parallel within 
the groups. The components described may be arranged on one or both ends 
of the rotating portion 2. In the illustration diode assemblies 6, 8 and 
diode fuses 10, 12 are arrayed on both ends of the rotating portion 2. In 
this design, a plurality of diode assemblies 6 and diode fuses 10 are 
spaced about the periphery of diode wheel 4, located in respectively 
common vertical planes at the left end of the rotating portion 2 as shown. 
A plurality of diode assemblies 8 and diode fuses 12 are spaced about the 
periphery of diode wheel 4, located in respectively common vertical planes 
at the right end of the rotating portion 2 according to its orientation in 
the drawing figure. 
The diode assemblies 6 and 8 receive an alternating current input from 
windings (not shown) through phase leads 27. This current input is 
received at any one of the diode assemblies 6 and 8 only periodically 
during rotation of the machine in a manner which is well known in the art. 
In general, the length of the period during which a given diode conducts 
depends on the number of diode groups and the number of phases in the 
machine design. The timing of the current input depends on the diode 
location and on which phase is associated with the diode. The diode 
assemblies 6 and 8 rectify the alternating current input to produce a 
chopped alternating current output which is transmitted through conductors 
28 and 30 respectively to the diode fuses 10 and 12 respectively. This 
chopped alternating current output passes through fuse bases 14 and 16 to 
diode wheel 4 and fuse mounting block or blocks 22 respectively. The sum 
of all chopped alternating currents which lead to the diode wheel is a DC 
current, which flows axially through the wheel. The direct current output 
is then collected and transmitted to a field winding which is to be 
energized in a manner which is well-known. 
The present invention is a method and structure which channels fault 
currents developing upon the failure of a diode assembly 6 or 8 to prevent 
damage to other components and undesired shutdown of the machine. Under 
normal operation, currents in the diode wheel 4 are DC currents, and the 
magnetic steel material of the diode wheel 4 does not develop an inductive 
reactance. The diode assemblies 6 and 8 each have essentially the same 
impedance, and no current imbalance exists between phases. 
When a brushless exciter diode such as diode assemblies 6 and 8 fails, it 
short-circuits, causing a current from all active diode-fuse legs of 
another phase to flow circumferentially around the periphery of the diode 
wheel 4 and thus feed the short circuit. The excess current condition 
thereafter actuates the diode fuse 10 or 12 associated with that diode 
assembly 6 or 8. The changes in current paths and reversals in current 
flow generate transient fault currents in diode wheel 4 that are 
high-frequency AC. 
These AC fault currents create an inductive reactance in the highly 
magnetic material of the diode wheel 4, which as noted previously is 
acting as the field current conductor. Because of these inductive 
backvoltages which retard sudden distributions of current in the magnetic 
steel diode wheel 4, diode fuse-legs (comprising a diode assembly 6 or 8 
and a diode fuse 10 or 12) located farther away from the faulted diode 
assembly 6 or 8 experience less intense surges than those nearer the 
fault. This effect is only significant on design which utilize phase 
paralleling rings between the diode modules and the armature winding. This 
current imbalance may result in a single diode fuse 10 or 12 adjacent to 
the failed diode instantaneously carrying nearly 50% of the total fault 
current. Thus, the AC fault currents will no longer distribute evenly per 
the balanced DC current paths which are axial, but will instead distribute 
per the unbalanced high frequency AC current paths, which are 
circumferential. Further, instantaneous high-frequency current components 
transmitted to the diode wheel 4 at the instant of failure tend to flow on 
the surface of the diode wheel 4 rather than being distributed uniformly 
throughout its thickness. This phenomenon adds uncertainty to transient 
current flows that are already difficult to analyze. 
The current flow asymmetry which has been discussed affects proper 
operation of diode fuses 10 and 12, in some cases causing the diode fuse 
10 or 12 associated with the failed diode assembly 6 or 8 to operate more 
slowly, and in other cases causing the diode fuses 10 or 12 of nearby 
unfailed diodes to operate spuriously. When a large percentage of the 
total fault current is conducted through a single diode fuse 10 or 12, the 
fuse may blow. A "cascade failure" of the diode fuses 10 or 12 may result 
if the number of remaining fuses is less than the minimum amount required 
to carry the maximum field current. 
The present invention solves this problem by providing a means for carrying 
fault currents more efficiently about the periphery of the diode wheel 4 
so that upon a failure of a diode assembly 6 or 8, the resulting high 
frequency AC fault currents are more quickly equalized among the fuse-legs 
of the same phase. FIGS. 2 and 3 show a preferred embodiment of the 
present invention. In FIGS. 2 and 3, components with like names and 
functions to the components described in FIG. 1 are designated with the 
same numbers and also using the prime symbol, so that, for example, a 
diode wheel 4' is provided in the present invention, the diode wheel 4' 
being in most respects similar to the diode wheel 4 known in the prior 
art. 
Referring now to FIG. 2, there is provided a copper stranded conductor 32 
formed in a generally circular ring shape and attached peripherally about 
the diode wheel 4'. In this first preferred embodiment, the conductor 32 
is attached to the inner perimeter of the diode wheel 4' at the edge of 
diode wheel 4' so as to carry fault currents about the periphery of diode 
wheel 4'. In the event of a diode fault, the high frequency fault currents 
will flow through the low impedance copper conductor more easily than 
through the steel diode wheel. This auxiliary current path in the 
peripheral, as opposed to axial, direction relative to the cylindrical 
diode wheel 4' carries fault currents and tends to equalize the fault 
currents among the diode fuses 10' which are associated with the same 
phase of the machine. 
FIG. 3 is a partial cross-sectional view of the diode wheel 4' of FIG. 2 
showing this first preferred installation of the conductor 32. As can be 
seen in the drawing figure, conductor 32 is located on the inner perimeter 
of diode wheel 4' flush with the end edge of diode wheel 4'. An extended 
fuse base 14' is provided for mechanically holding the conductor 32 in 
electrical connection with the diode wheel 4'. The extended fuse base 14' 
is comprised of first and second portions 34 and 36 joined at right 
angles. The first portion 34 of extended fuse base 14' has a hole to 
accommodate fuse attachment bolt 18', and the inner side of this portion 
is mounted in electrical contact with the diode wheel 4'. The second 
portion 36 of extended fuse base 14' is connected on its outer side to the 
diode fuse 10' and is held in electrical connection with the conductor 32 
on its inner side. The extended fuse base 14' differs in significant 
regard from the prior-art fuse base 14 (shown in FIG. 1) in that the first 
portion 34 is lengthened to accommodate the conductor 32 which is placed 
between the extended fuse base 14' and the diode wheel 4'. If desired, 
additional means may be provided to ensure a good electrical path between 
each diode fuse 10' and the conductor 32, such as additional fasteners, 
welding or soldering, coatings, etc. 
A second preferred option for installation of the conductor 32 is 
illustrated in FIG. 4. In this embodiment, conductor 32 is mounted on the 
end edge of diode wheel 4' between first portion 34 of diode fuse base 14' 
and the diode wheel 4'. The fuse attachment bolt 18' passes through a hole 
in conductor 32 to fix both the diode fuse 10' (including diode fuse base 
14') and the conductor 32 firmly to the diode wheel 4'. Rather than first 
portion 34 of diode fuse base 14' being extended to accommodate conductor 
32, the second portion 36 of diode fuse base 14' is extended in this 
embodiment. 
It should be noted that the preferred embodiments of FIGS. 2, 3, and 4 are 
merely illustrative and that, as long as the necessary electrical 
connections between conductor 32 and diode fuse bases 14' are made, the 
conductor 32 can be attached in any position in the specified region, such 
as about the outside perimeter of the cylindrical diode wheel 4'. It is 
believed desirable for structural support and ease of assembly to 
incorporate means for attaching conductor 32 to the diode wheel 4' into 
diode fuse base 14', but those skilled in the art will recognize that 
attaching means not associated with the diode fuse base 14' could be used. 
In any of the embodiments, the conductor 32 might be a solid block 
conductor, but a stranded conductor is preferred for its increased surface 
area which decreases high-frequency impedance. Copper is preferred for its 
properties of low impedance and low cost. The choice of a stranded copper 
conductor ensures that the high-frequency impedance of the conductor 32 
will be lower than the high-frequency impedance of the magnetic steel 
diode wheel 4. It is desirable that the high-frequency impedance of the 
conductor 32 be lower than the high-frequency impedance of diode wheel 4 
so that, upon failure of a diode assembly 6 or 8 (shown in FIG. 1) AC 
fault currents will follow a path within the conductor 32 rather than 
being transmitted to the diode wheel 4 where they would generate 
significantly larger inductive reactances. Also, although in the 
embodiment disclosed a single conductor 32 is provided, any desired number 
of conductors 32 could be installed. For example, there might be a 
separate conductor 32 provided for each phase of the machine, with the 
fuse legs associated with each phase all connected together by means of 
the associated conductor 32. To minimize weight and structural complexity, 
the embodiment having a single conductor 32 for an end of the diode wheel 
4' is preferred. Finally, although a conductor 32 has been specified 
herein at only one end of the diode wheel 4', it may also be desirable to 
install another such conductor 32 at the other end of the diode wheel 4', 
to be installed in similar fashion between diode fuse base 16 and fuse 
mounting block 22 (shown in FIG. 1). 
In the method of the present invention, it is first necessary to predict 
the magnitude of the fault currents expected, and the conductor 32 should 
then be sized to carry the expected short-term fault currents. In a 
typical machine, such instantaneous high-frequency fault currents might be 
in the range of 10,000 to 40,000 Amperes. The conductor 32 should not, 
however, be made so large that it produces an imbalance under normal DC 
operation. Depending on the size (and therefore weight) of the conductor 
32 selected and the mechanical structure of diode wheel 4', mechanical 
strengthening of the diode wheel 4' might be necessary. The conductor or 
conductors 32 are then constructed and installed on the diode wheel 4' 
according to the procedure outlined above. 
As those skilled in the art will appreciate, the present invention provides 
a number of benefits. First, the invention increases reliability of a 
machine on which it is installed by preventing cascade diode fuse 
failures, thus decreasing the forced outage rate of the brushless exciter. 
Second, by promoting more uniform fault current distribution in the 
brushless exciter, the present invention provides a brushless exciter that 
behaves in a more predictable manner. Unpredictable conditions can be 
eliminated so that failures due to unexpected interactions can be 
prevented. Finally, the present invention helps assure that diode fuse 
protection will operate as designed and permits sizing of diode fuses 
according to the desired current carrying capacity rather than to 
accommodate transient fault currents.