Continued advances in the design and manufacture of electric machines have enabled an increase in the types and number of applications which may take advantage of this technology. In particular, the use of electric machines, particularly electric motors and generators, in critical safety systems and medical and life support systems has been growing as a direct result of these advances. As can well be imagined, use of electric machines in these types of environments require a high level of reliability and, in many systems, a level of redundancy as well to prevent a single failure within the machine from causing a catastrophic failure in the overall system. In the past, it has not been uncommon for these systems to employ the use of multiple electric machines to ensure that the loss of any one machine would not result in a failure of the output and thus a catastrophic loss of the critical system.
However, as with all other areas of technology, there is a drive to reduce the size of the equipment used in these types of systems. Such a driving force no longer tolerates the crude systems of redundancy of the past which utilized multiple redundant machines to protect against a possible failure of one of them. In addition to requiring a larger physical size, these old systems of redundancy also were very expensive due to the requirement of having two or more physically separate electric machines.
In one area of technology of electric machines, that relating to electric motors and generators, a solution to the redundancy requirement, which has in the past been solved by having two separate electric motors or generators, has been found which does not require redundant machines to be used. In an electric motor or generator, the stator of the machine can be wound in such a manner to include multiple sets of stator windings. These multiple stator windings provide the redundant drive required to drive a given load, or the redundant source of electric power to supply multiple electric loads in the case of a generator configuration. Such a winding configuration allows for the use of a "single" electric motor or generator having a single rotor, a single stator, and a single housing, but utilizing multiple sets of stator windings to provide the required redundancy for the critical systems.
One such electric motor utilizing redundant windings is described in U.S. Pat. No. 4,434,389 issued to Langley et al. on Feb. 28, 1984, entitled MOTOR WITH REDUNDANT WINDINGS. In this patent, an electric motor is described which is wound with redundant sets of stator windings which are energized by independent electric circuits. This enables the operation of the motor even in the presence of a failure of a winding or any particular energization circuit. The electric machine described in this reference utilizes a distributed phase winding system for each set of stator windings. These sets of stator windings are physically separated, allowing for no overlapping of the windings between each separate set. The description of this patent distinguishes its redundant motor design from a conventional motor in which adjacent winding sets are allowed to overlap.
Another electric machine utilizing redundant stator windings to increase its fault tolerance is described in U.S. Pat. No. 4,550,267 issued to Vaidya on Oct. 29, 1995, for REDUNDANT MULTIPLE CHANNEL ELECTRIC MOTORS AND GENERATORS. As with the Langley et al. '389 patent, this patent also teaches the use of multiple redundant stator windings wound in a distributed winding configuration and having non-overlapping regions for the stator windings themselves. However, unlike the Langley et al. '389 patent, the redundant electric machine described in Vaidya '267 allows for a configuration where electromagnetic isolation may be compromised by allowing the windings within the two regions to overlap, although no description of such a configuration is included other than to mention is acceptability.
While the electric machines of each of the two above-referenced patents are directed at overcoming the requirement for separate electric machines to satisfy the redundancy requirements of critical systems, neither design is appropriate for the most highly critical applications, nor are they appropriate for use in the medical field. Specifically, each of the above referenced disclosures recognizes the problem of short circuits occurring within the stator windings as one of the failure conditions which necessitates the use of redundant stator windings. However, each of these references describe the use of a distributed wiring winding configuration used for each set of the distributed stator windings. Unfortunately, such a distributed wiring winding configuration significantly increases the probability of a phase-to-phase short circuit as will be described below with reference to FIG. 1.
FIG. 1 illustrates schematically a redundant electric machine 10 constructed in accordance with the teachings of the prior art utilizing a conventional distributed wiring winding configuration. This machine 10 requires a 24 slot stator 12, which is wound, in its simplest configuration, with two sets of stator windings 14, 16. The two halves are divided in FIG. 1 by the line 18 which has been included only to aid the understanding of this configuration, and does not represent any physical device actually included in this design. As may be seen even from this simplified schematic diagram, this distributed wiring winding configuration results in a significant number of phase wire crossings 20.
To determine the actual number of times that a phase wire crosses the wire from another phase, the types and number of wire crossings must be accounted for. The first type of wire crossing is the coil-to-coil crossing. In a three phase distributed wiring system there are six (6) wire crossings of this type. The number of crossings for each one of these crossings is the number of turns per coil for one phase times the number of turns per coil for the other phase. In a typical machine there will be 35 turns per coil. Therefore, the total number of coil-to-coil wire crossings is equal to 6(T.sub.c *T.sub.c) or 6(35*35) which equals 7,350 crossings.
The next type of wire crossing is that of an interpole loop to a coil. As with the coil-to-coil crossings, there are six (6) of these crossings. For each one of this type of crossing, the number of actual wire crossings is the number of turns per coil times the number of wires comprising the inter-pole loop. This is typically one (1). Therefore, the total number of inter-pole loop to coil wire crossings is equal to 6(T.sub.c *T.sub.ipl) or 6(35*1) which equals 210 crossings.
The final type of wire crossing resulting from the distributed wiring system used in the prior art is the inter-pole to inter-pole wire crossings. Unlike the above types of crossings, there are only three (3) crossings of this type. Also, since typically only one wire is used to construct the inter-pole loop, the number of actual wire crossings for each of these is only one (1). Therefore, the total number of inter-pole to inter-pole wire crossings is equal to 3(T.sub.ipl) or 3(1) which equals three (3) crossings.
After having calculated each of the component types of wire crossings resulting from the distributed wiring system of the prior art, these numbers must be added together and multiplied by the number of redundant windings utilized and by the number of ends on the stator. The simplest example uses only two sets of stator windings, and has only two ends. Therefore, the final calculation of the number of wire crossings resulting from a distributed winding system for a redundant machine is (7,350+210+3)(2)(2), which equals 30,252 individual wire crossings.
Unfortunately, each one of these 30,252 wire crossings presents an opportunity for the development of a short circuit of these wires. While such probability may be reduced by increasing the insulation on these wires, such serves to unacceptably increase the cost of manufacture of such a machine. Additionally, the number of wire crossings increases substantially as additional sets of redundant stator windings are added, further increasing the probability of an inter-phase short circuit. This is an unacceptable result in a system which demands increased redundancy to prevent catastrophic failure of the system.