1. Field of the Invention
The present invention relates to dynamoelectric machines, and more particularly to dynamoelectric machines having a characteristic of decreased noise while operating.
2. Relation to Prior Art
Dynamoelectric machines are well known in the art. One such dynamoelectric machine is a reluctance machine. In general, a reluctance machine is an electric machine in which torque is produced by the tendency of a movable part to move to a position where the inductance of an excited winding is maximized (i.e., the reluctance is minimized).
In one type of reluctance machine, the phase windings are energized at a controlled frequency. This type of reluctance machine is generally referred to as a synchronous reluctance machine. In another type of reluctance machine, circuitry is provided to determine or estimate the position of the machine""s rotor, and the windings of a phase are energized as a function of rotor position. This type of reluctance machine is generally referred to as a switched reluctance machine. Although the description of the current invention is in the context of a switched reluctance motor, the present invention is applicable to all forms of reluctance machines, including synchronous and switched reluctance motors, reluctance generators, and to other machines that have phase winding arrangements similar to those of switched reluctance machines.
Generally, the stator of a switched reluctance motor includes a ring of inwardly extending stator poles about which are positioned one or more phase windings. The energization of such a phase winding tends to cause the rotor to move into a position where the inductance of an excited winding is maximized. The energization of such a phase winding will also tend to deform the stator by drawing certain stator poles associated with the energized phase winding towards the rotor poles. In general, as a rotor pole comes into alignment with a stator pole, the forces tending to draw the stator pole towards the rotor pole (i.e., the normal forces) will generally begin to increase, will reach a maximum at full alignment, and will decrease thereafter.
FIG. 1A generally illustrates an exemplary rotor pole 1 as it rotates into and then past alignment with a stator pole 3, surrounded by a phase coil 5. The current profile supplied to the stator pole 3 is shown in FIG. 1B. The current profile presented is idealized, and actual currents will have characteristics different from those represented in FIG.1B. Particular apparatus for generating the illustrated currents as a function of the angular position of the rotor is omitted, and the construction of such apparatus will be apparent to one of ordinary skill in the art.
The exemplary current in FIG. 1B supplied to coil 5 surrounding stator pole 1 is shown in relation to the angular position of a rotor pole 1. The current typically involves a ramp increase to energize the stator pole. Then, the current is maintained at a substantially constant level to bring the rotor pole into minimum reluctance relation to the stator pole. Once the rotor pole has aligned with approximately 50 % of the stator pole (corresponding to the position identified by the dashed line in FIGS. 1A-1C), the current undergoes a cutoff, and begins to decrease in accordance with traditional energization schemes. The rotor pole reaches minimum reluctance as it aligns 100% with the stator pole and then passes out of relation to the stator pole.
As the rotor pole 1 moves in relation to stator pole 3, the inductance between the poles changes. The change in inductance produces torque in the rotor, which causes angular displacement of the rotor. The torque has a tangential and normal component due to the magnet flux path that passes through the radially opposed pole pairs, the rotor and the stator. The tangential component will tend to cause the rotor to rotate. The normal component will tend to cause the stator pole to move towards the rotor pole.
FIG. 1C generally illustrates the normal forces exerted on the stator pole as a function of the angular position of the rotor for the illustrated current waveform. As illustrated, the normal forces will begin to increase near the point where the rotor pole begins to overlap the stator pole. In FIG. 1C, the normal force increases as the rotor pole aligns with stator pole until a maximum force is reached at the point where the rotor pole is fully aligned with the stator pole. This point of full alignment also generally corresponds to the minimum reluctance point. The normal force then decreases steadily as the poles pass out of alignment. As illustrated in FIGS. 1A-1C, for traditional reluctance machines energized in the traditional manner, the normal force curve has a continuous, uniform profile for the electrical interaction of the two uniform faced poles as they pass in relation to one another.
The establishment of the normal forces described above tends to result in an xe2x80x9covalizingxe2x80x9d of the stator as normal forces attempt to collapse the air gap between the rotor poles and the stator poles associated with the energized phase winding. These radially opposed normal forces tend to distort the stator yoke from its generally circular configuration and force it out of round. Upon de-energization of the stator poles, the stator returns to its original circular configuration. Even though the deflection during energization is extremely slight, under continuous operation the distortion produces a noticeable whining noise.
Traditional switched reluctance machines have rotor and stator constructions that result in the establishment of normal forces such that each of the stator poles in the machine experiences the same normal forces, although not necessarily at the same point in time.
FIGS. 2A-2B generally and schematically illustrate the types of xe2x80x9cdeflection modesxe2x80x9d that are established in reluctance machines having two and four normal forces acting on the stator. As used herein, the number of deflection modes that a reluctance machine may have corresponds to the number of localized areas where normal forces are generated in the machine during energization of one of the machine""s phase windings. Typically, the number of deflection modes for a given machine will correspond to the number of stator poles encircled by each phase winding in the machine. Although the present discussion describes reluctance machines with 2 and 4-modes of deflection, it is to be understood that additional modes exist beyond those depicted. Furthermore, 3-mode deflection (i.e. odd-mode deflection) cannot occur in an electromagnetically balanced motor.
Referring to FIGS. 2A-2B, the identified figures include arrows 8 representing the normal forces acting on the stator poles associated with an exemplary energized phase winding at a specific point in time. As noted in the Figures, the normal forces 8 acting on the various stator poles associated with the given energized phase winding are, at each point in time, substantially equal.
In FIG. 2A, the equal and opposite normal forces 8 tend to ovalize the stator 6. This represents a two-mode deflection since there are two localized areas where normal forces 8 are generated during energization of phase A windings. Also, FIG. 2B represents a four-mode deflection since there are four localized areas where normal forces 8 are generated during energization of phase A windings. Higher ordered modes of deflection are also possible beyond four modes.
In addition to establishing normal forces that are equal with respect to an energization of a given phase winding, traditional reluctance machines are constructed so that the normal forces established by the energization of another phase winding are the same in profile (although physically displaced) as the normal forces established upon energization of the given or another phase at a later point in time. In other words, in typical reluctance machines, the normal forces that are exerted on the stator poles associated with the energization of a phase winding at one time are the same as the normal forces associated with the energization of that phase winding at all other times andxe2x80x94in magnitude although not necessarily in alignmentxe2x80x94with the forces associated with the energization of all other phase windings.
To better illustrate the distribution of equal normal forces around a stator during phase energizations, FIGS. 3A and 3B are presented. FIG. 3A shows a reluctance machine 10 having eight stator poles, four rotor poles 22 and two phase windings A and B. FIG. 3A also illustrates the normal forces 14 established upon energization of a first phase winding A at an initial time. As may be noted, because the phase winding energized at this time surrounds four stator poles, there are four points where the deflection forces are substantially localized on the stator. As such, the machine of FIG. 3A has four deflection modes.
FIG. 3B illustrates the condition of the machine at a point in time later than that illustrated in FIG. 3A. Only the second of the phase windings B of the machine is energized. Because the second phase winding surrounds four stator poles, as with energization of the first phase winding, there are four points where the deflection forces 14xe2x80x2 are substantially localized on the stator. The forces 14xe2x80x2 developed at the four deflection points are substantially identical to those forces 14 developed during energization of the first phase winding A, although they are located at differing physical points on the stator. Due to the distortions resulting from such a distribution of normal forces about the stator during subsequent phase energizations, many switched reluctance motors have noise spikes at the commutation frequency and harmonics thereof.
One proposed solution in the prior art for reducing the noise produced by a reluctance machine includes increasing the number of stator modes. For example, the stator/rotor combination of 6/4 can be doubled to a 12/8 combination that provides four-mode deflection of the stator. The increase in modes, however, undesirably reduces the power density of the motor. It also requires the use of rotor and stator laminations with a large number of poles that may result in increased material or manufacturing costs.
Another proposed solution in the prior art to reduce noise involves controlling the energization currents applied to the phase windings in an effort to shape the normal forces established in the machine. Such xe2x80x9ctuningxe2x80x9d of the energization currents is often sensitive to the application to which the motor is applied.
Another solution in the prior art to reduce noise involves stiffening the stator yoke. A more robust stator yoke attempts to limit deflection caused by the normal forces and thereby reduces the noise spike. Unfortunately, adding material to the yoke increases weight and cost to the switched reluctance motor while producing diminished gains in performance.
Yet another solution in the prior art to reduce noise involves the use of wide and narrow rotor poles. In this solution, a first set of opposed rotor poles is provided with a narrow width, while a second set of opposed rotor poles is much wider. The ratio of widths for the wide to narrow poles may be as great as 2:1. The solution is detailed in U.S. Pat. Nos. 5,582,334; 6,028,385 and 6,051,903 to Pengov. The solution attempts to limit the deflections of the stator by offsetting a first set of radial forces by a second set of radial forces that are 90xc2x0 therefrom. In other words, during the first half of the phase energization a 2-mode deflection acts on the stator. This 2-mode deflection acts to stiffen or resist the subsequent deflection of the quadrature stator poles which are fluxed in the second half of the phase current.
There are several disadvantages to the above approach. First, the solution requires that, due to the wide rotor poles, attractive forces on the stator poles for a first phase need to be maintained while attractive forces 6 on the stator are initiated for a second phase. Thus, significant attraction is required between the wide rotor poles with energized stator poles after the poles have passed 50% alignment. As a result, overly adverse normal forces and corresponding deflections occur when the wide rotor poles completely overlaps the energized stator poles.
Second, the use of wide poles as in Pengov creates a situation where a positive change in inductance with respect to a change in angular orientation   (            ⅆ      L              ⅆ      θ        )
does not occurs on all poles for a given phase energization. To illustrate the case where a positive   (            ⅆ      L              ⅆ      θ        )
does not occur for a given phase energization, FIGS. 4A-4D are presented.
The 8/4 reluctance machine 10 of FIG. 4A has wide rotor poles a and c and narrow rotor poles b and d. There are 8 stator poles, but focus will be on stator poles A, B, C and D corresponding to a single phase. The narrow rotor poles b and d have a width for their face that is substantially similar to that of the stator poles. As the current in FIG. 4B energizes the phase windings, the rotor moves relative to the stator.
FIGS. 4C and 4D show the inductance in the stator poles as a function of the angular orientation of the corresponding rotor poles. FIG. 4C depicts the change in inductance for stator poles A and C as the wide rotor poles a and c pass in relation. There is a point as the wide rotor poles pass in relation to the stator poles where the slope   (            ⅆ      L              ⅆ      θ        )
is not changing (i.e. the slope of the curve is essentially zero).
FIG. 4C depicts the change in inductance for stator poles B and D as the narrow rotor poles b and d pass in relation. In contrast to wide rotor poles passing in relation to the stator poles, the stator poles B and D experience a positive   (            ⅆ      L              ⅆ      θ        )
as they are energized and the narrow, like-sized rotor poles b and d pass in relation.
Another disadvantage of the Pengov solution relates to the magnetic flux paths that are used in the above solution. Long flux paths around the stator characterize the magnetic flux for the reluctance machine according to the Pengov solution to reduce noise in a reluctance machine. FIGS. 4E-F depict a reluctance machine having alternating wide and narrow rotor poles according to the Pengov solution. Phase A is shown energized so that magnetic flux lines pass through the energized stator poles, the rotor and the stator yoke.
FIG. 4E shows a reluctance machine having alternating wide and narrow rotor poles during the first 50% alignment of the rotor with the energized stator poles. The polarities of the four energized stator poles provides that each stator pole of one polarity (i.e. S) is adjacent to a stator pole of the same phase with the same polarity (S) and also adjacent to a stator pole of the same phase having an opposite polarity (N). In other words, stator pole 12 is an S-pole. Adjacent pole 12xe2x80x2 is also S-pole, while adjacent pole 14xe2x80x2 is N-pole. This is referred to as a NNSS orientation of the polarities.
There are essentially two, long pole paths that the flux travels in this first 50% portion of alignment. The first path has three, long flux paths 20 which pass from stator pole 12 with S-polarity, through the wide rotor poles, to stator pole 14 with N-polarity, and around the stator yoke. Thus, three long flux paths 20 traverse the side 1 of the stator yoke. Likewise, a second path has three, long flux paths 20xe2x80x2 which pass from stator pole 12 with S-polarity, through the wide rotor poles, to stator pole 14 with N-polarity, and around the stator yoke. Because the narrow rotor poles have not reached relation with stator poles 12xe2x80x2 and 14xe2x80x2, no flux travels by way of these stator poles.
FIG. 4F shows the reluctance after the rotor has surpassed 50% alignment with the energized stator poles. Due to the arrangement of the polarities for the radially opposed pairs of stator poles, the flux lines take both short and long paths after 50% alignment. Short flux paths 22 and 22xe2x80x2 have three flux lines and pass around stator poles 12 and 14. Long flux paths 20 and 20xe2x80x2 have two flux lines each and pass around stator poles 12xe2x80x2 and 14xe2x80x2.
As a result, portions 1 and 1xe2x80x2 of the stator yoke accommodate 2 flux lines, while portions 2 and 2xe2x80x2 accommodate 1 flux line. The flux paths can over saturate the inductance capacity of portions of the stator yoke and thus decrease the efficiency of the reluctance machine.
In contrast, a conventional reluctance machine, as depicted in FIG. 4G, has all coil sides contributing to useful flux by facilitating short flux paths around the stator and through the rotor. The polarization of a conventional reluctance machine sets stator pole 10 to a N-pole. The adjacent pole of the same phase, pole 12 is polarized to an S-pole, as is the other adjacent pole of the same phase 14. This is referred to as a NSNS orientation of the polarities and facilitates short magnetic flux paths. The orientation presents a more efficient arrangement of flux paths.
As indicated above, existing techniques for reducing noise in switched reluctance machines typically are ineffective on a large-scale basis, costly to implement, or adversely impact the performance of the motor. The present invention provides an improved, low-noise reluctance machine that does not suffer from the drawback described above, and other drawbacks associated with such existing techniques.
In accordance with one aspect of the present invention, there is provided a linear electromagnetic machine a movable member and a stationary member. The stationary member defines at least one stationary pole. A phase winding is positioned such that, when current is flowing in the phase winding, the at least one stationary pole is energized. Also provided is a circuit for energizing the phase winding over a plurality of energization cycles to produce a given force tending to cause linear movement of the movable member with respect to the stationary member. The energizing of the phase winding also produces a normal force tending to cause movement of the movable and stationary members in a direction normal to the desired linear movement. The normal force profile experienced by the at least one stationary pole over a first energization cycle is different from the normal force profile experienced by the at least one pole stationary over a subsequent energization cycle.
In accordance with one aspect of the present invention, there is provided an electromagnetic machine. The electromagnetic machine includes a rotor defining a plurality of rotor poles. Each rotor pole has a pole face defining an angular width. The angular width of the rotor pole with the widest width is: (a) substantially equal to or greater than the angular width of the rotor pole with the narrowest width, and (b) less than 1.5 times the angular width of the rotor pole with the narrowest width. The electromagnetic machine includes a stator defining at least two stator poles that are radially opposed to one another. There is provided a phase winding positioned such that, when current is flowing in the phase winding, the at least two stator poles are energized. There is also provided a circuit for energizing the phase winding over a plurality of energization cycles to produce a given desired output on the rotor. The energizing of the phase winding also produces a normal force tending to cause movement of the at least two stator poles towards the rotor. The normal force profile experienced by the at least two stator poles over a first energization cycle is different from the normal force profile experienced by the at least two stator poles over a subsequent energization cycle.
In accordance with one aspect of the present invention, there in provided an electromagnetic machine including a rotor defining a plurality of rotor poles. Each rotor pole has a pole face defining an angular width. The angular widths of each of the rotor poles are substantially the same. The electromagnetic machine also includes a stator defining a first set of opposing stator poles and a second set of opposing stator poles. Each of the stator poles is associated with at least one current carrying member such that a stator pole is energized when current is flowing through a current carrying member associated with the stator pole. The electromagnetic machine also includes a circuit for energizing the at least one current carrying member over a given interval so as to simultaneously energize the first and second sets of opposing stator poles. The energizing of the current carrying member also produces normal forces tending to cause movement of the energized stator poles towards the rotor. The normal force profile experienced by the first pair of opposing stator poles over the given interval is substantially different from the normal force profile experienced by the second pair of opposing stator poles over the given interval.
Other aspects and advantages of the present invention will become apparent upon reading the following detailed description and upon reference to the drawings.