WO1997034365A1

WO1997034365A1 - System for controlling operation of a switched reluctance motor between a multi-phase operating mode and a reduced phase operating mode

Info

Publication number: WO1997034365A1
Application number: PCT/US1997/003963
Authority: WO
Inventors: Sergei F. Kolomeitsev; Scott E. Blackburn
Original assignee: Dana Corporation
Priority date: 1996-03-15
Filing date: 1997-03-13
Publication date: 1997-09-18
Also published as: GB9820111D0; DE19781648T1; GB2327160B; GB2327160A

Abstract

An apparatus for changing the operating modes of a switched reluctance motor is disclosed. The motor can be operated in either a multi-phase mode or a single-phase mode. When the motor is operated in a multi-phase mode, all stator windings are energized, however, when the motor is operated in a single-phase mode, only the windings corresponding to a single phase are energized. The apparatus includes a logic circuit (146) for sensing the motor speed, and a comparator circuit (172) for changing the motor operating modes. The mode change is based on the speed levels of the motor. Between the speed levels, there is a hysteresis operating band to prevent undesirable hunting between the single-phase mode and the multi-phase mode.

Description

SYSTEM FOR CONTROLLING OPERATION OF A

SWITCHED RELUCTANCE MOTOR BETWEEN A MTJLT -PHASE

OPERATING MODE AND A REDUCED PHASE OPERATING MOPE

BACKGROUND OF THE INVENTION

1. Technical Field

This invention relates generally to a system for cor.trolling a switched-reluctance (SR) motor, and more particularly, to u system for particularly controlling the operation of an SR motor between a multi-phase mode and a reduced phase operating mode.

2. Discussion of the Related Art Switched reluctance (SR) machines have been the subject of increased investigation due to their many advantages, which makes them suitable for use in a wide variety of situations. An SR machine operates on the basis of varying reluctance in its several magnetic circuits. In particular, such machines are generally doubly salient motors--that is, they have teeth or poles on both the stator and the rotor. The stator teeth have windings which form machine phases of the motor. In a common configuration, stator windings on diametrically opposite poles are connected in series to form one machine phase.

When a stator phase is energized, the closest rotor pole pair is attracted towards the stator pole pair having the energized stator winding, thus minimizing the reluctance of the magnetic path. By energizing consecutive stator windings (i.e., machine phases) in succession, in a cyclical fashion, it is possible to develop torque, and thus rotation of the rotor in either a clockwise, or counter-clockwise direction. As further background, the inductance of a stator winding associated with a stator pole pair varies as a function of rotor position. Specifically, the inductance varies from a lower level, when a rotor pole is unaligned with a corresponding stator pole, to an upper or maximum level when the rotor pole and stator pole are in alignment. Thus, when the rotor pole rotates and sweeps past a stator pole, the inductance of the stator winding varies through lower-upper-lower inductance levels. This inductance-versus-rotor position characteristic is particularly relevant for controlled operation of the motor. Specifically, current flowing through the stator winding must be switched on prior to (i.e., advanced), and maintained during the rising inductance period to develop positive torque. Since positive phase current during the decreasing inductance interval produces a negative or breaking torque, the phase current must be switched off before this interval occurs to avoid generating negative torque. Accordingly, rotor position sensing is an integral part of a closed-loop variable-reluctance motor drive system so as to appropriately control torque generation.

Further, such motors may be operated in a multi-phase mode of operation, which is desirable when a relatively large load is driven by the motor. However, in some instances, the motor may be operated for a period of time in a low load condition (e.g., no load, or lightly loaded--hereinafter a "Low Load Condition") . When this occurs, the speed of the motor may rise rapidly. Conventional control methods and devices have continued to operate the motor in a multi-phase mode in this low load condition (i.e., all of the machine phases being sequentially energized to effect rotor rotation) . This mode of operation, however, is less than optimally efficient. Particularly, since only a low load is being driven, energizing current in each of the multiple phases goes to a low level, which, for SR motors, may generate less torque per unit current than when energized at a higher current level. That is, it is inefficient to continue to supply the same amount of electrical current to all of the stator windings of the motor.

Accordingly, there is a need to provide an improved system for controlling a switched reluctance machine that minimizes or eliminates one or more of the problems as set forth above. Specifically, it would be desirable to provide an improved structure for an electric motor which is responsive to the operating conditions for selectively reducing the amount of electrical current supplied to the stator windings in order to converse power consumption.

SUMMARY OF THE INVENTION This invention relates to an improved structure for an electric motor which is responsive to the operating conditions for selectively reducing the amount of electrical current supplied to the stator windings in order to conserve power consumption in low-load or no-load conditions. The motor includes a hollow stator having a plurality of radially inwardly extending stator poles. A rotor having a plurality of radially outwardly extending rotor poles is supported for rotation relative to the stator. A winding of an electrical conductor is provided about each of the stator poles so as to provide a plurality of sequentially energized phases. Preferably, the windings are provided as a plurality of opposed pairs on the stator poles, wherein each of the opposed pairs of the stator windings constitutes one phase for the operation of the motor. When electrical current is supplied to the windings from a current pulse generating circuit or similar structure, both the stator and the rotor are magnetized to cause rotation of the rotor relative to the stator. A phase control circuit generates a phase control signal to the current pulse generating circuit to control the mode of operation thereof. The phase control signal is provided to selectively operate the current pulse generating circuit in either a single phase mode, wherein pulses of electrical current are fed only to one of the phases of the stator windings, or a multiple phase mode, wherein pulses of electrical current are fed sequentially to all of the phases of the stator windings. The phase control signal is responsive to an operating condition of the motor, such as phase current, torque load, rotor speed, and the like for selectively generating the phase control signal.

The present invention also provides an improved system for controlling operation of a switched reluctance machine. In particular, the present invention provides an apparatus for changing the operating mode of the switched reluctance machine between a multi-phase operating mode, wherein a first number of the machine phases are energized, and a reduced phase operating mode, wherein a second number less than said first number of machine phases are energized, according to a hysteresis loop operating map wherein two rotor speed references are used. One advantage of the present invention is that mode changes are made in a controlled fashion, thus eliminating the "hunting" or "oscillation" between modes that may otherwise occur if only a single rotor speed reference was used. Another advantage of the present invention is that the electrical energy consumed by the motor is minimized during operation in the reduced phase mode (relative to the multi-phase mode) .

The apparatus for controlling the switched reluctance machine includes means for sensing the speed of the rotor portion of the motor and generating a speed signal in response thereto, and an operating mode changing means. The speed signal is used, in a preferred embodiment, as a proxy for the load on the motor output shaft. The operating mode changing means is responsive to the speed signal for changing the operating mode of the machine from the multi¬ phase mode to the reduced phase mode when the rotor speed reaches a first predetermined level, and for changing the operating mode of the machine from the reduced phase mode to the multi-phase mode when the rotor speed reaches a second, predetermined level that is less than the first predetermined level. The relative magnitudes of the first and second predetermined levels are based on an assumption that when the speed of the motor rotor rises to reach the first predetermined level, the motor is operating under the above-mentioned low load condition. When this occurs, it is desirable to change the operational state of the motor from the multi-phase mode to the reduced phase mode. Furthermore, as a result, the operating condition of the motor follows a hysteresis-like track, defined by the first and second predetermined rotor speed levels, which prevents the hunting or oscillating between modes that might otherwise occur if only a single transition speed was provided and the motor was operated at or near that transition speed.

In a preferred embodiment, the multi-phase mode has a multi-phase current reference associated therewith, and the reduced phase mode has a reduced phase current reference associated therewith. The reduced phase current reference is larger in magnitude; thus, although fewer phases are energized, they are energized to a higher, more efficient current level (i.e., more torque production per unit current than at the current levels associated with no or low load area) . Only selected ones of the machine phases need be provided with the reduced phase current reference because the nonselected ones of the machine phases would be completely disabled during the reduced phase mode. In another preferred embodiment, a three-phase SR motor is controlled to operate in a single-phase mode. These and other features and objects of this invention will become apparent to one skilled in the art from the following detailed description and the accompanying drawings illustrating features of this invention by way of example, but not by way of limitation.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is an exploded perspective view of a portion of a variable reluctance electric motor in accordance with this invention.

Figure 2 is a sectional elevational view of the variable reluctance electric motor illustrated in Fig. 1 shown assembled, together with schematically illustrated portions of two of the opposed stator windings which constitute one phase for operating the motor.

Figure 3 is a schematic block diagram of all of the stator windings of the variable reluctance electric motor illustrated in Figs. 1 and 2 shown connected to a first embodiment of an electronic control circuit in accordance with this invention.

Figure 4 is a graph which illustrates the operation of the first embodiment of the electronic control circuit illustrated in Fig. 3.

Figure 5 is a schematic block diagram of all of the stator windings of the variable reluctance electric motor illustrated in Figs. 1 and 2 shown connected to a second embodiment of an electronic control circuit in accordance with this invention. Figure 6 is an exploded, perspective view of a portion of a switched reluctance electric motor suitable for use in connection with a third embodiment of the present invention.

Figure 7 is a diagrammatic, exaggerated, cross-sectional view of the switched reluctance electric motor of Figure 6 illustrating the relative positions of a stator, and rotor portions thereof.

Figure 8 is a simplified, rotor speed-versus-phase current diagram view illustrating a reduced phase (single phase) mode, and multi-phase mode current reference traces as a function of rotor speed.

Figure 9 is a simplified, block and schematic diagram view showing a third control apparatus embodiment in accordance with the present invention illustrating, particularly, a selected machine phase to be selectively operated in both the multi-phase mode, and the reduced phase (single-phase) mode, while non-selected machine phases being enabled/disabled according to a mode signal.

Figure 10 is a simplified, partial schematic and block diagram view showing, in greater detail, the controller portion of the embodiment illustrated in Figure 9.

Figure 11A is a simplified, speed-versus-voltage graph of the speed signal V_A generated by the speed signal generating circuit shown in Figure 10.

Figure 11B is a simplified, speed-versus-voltage graph illustrating the reduced phase (single-phase) current reference signal, and the multi-phase current reference signal, both displaced relative to an inverted version of the graph of the speed signal shown in Figure 11A.

Figure 12 is a simplified, schematic diagram view showing, in greater detail, the comparator with hysteresis illustrated in Figure 10.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, there is illustrated in Figs. 1 and 2 a portion of a variable reluctance electric motor, indicated generally at 10, in accordance with this invention. The motor 10 includes a stator 11 which is generally hollow and cylindrical in shape. A plurality of radially inwardly extending poles, indicated generally at 12, are formed on the stator 11 and extend longitudinally throughout the length thereof. The stator poles 12 are preferably provided in opposed pairs, such as shown in Fig. 2 at Al and A2, Bl and B2, Cl and C2, and Dl and D2. Thus, eight stator poles 12 are provided on the illustrated stator 11. However, it is known in the art to provide the stator 11 with either a greater or lesser number of stator poles 12.

Each of the stator poles 12 is generally rectangular in cross sectional shape. The radially innermost surfaces of the stator poles 12 are slightly curved so as to define an inner diameter. However, the stator poles 12 may be formed having any desired cross sectional shape. The stator 11 and the stator poles 12 are formed from a magnetically permeable material, such as iron. As will be explained below, each of the stator pole pairs Al and A2, Bl and B2, Cl and C2, and Dl and D2 represents one phase for energizing the variable reluctance motor 10 for operation. Thus, the illustrated motor 10 has four electrical phases for energization. However, the number of electrical phases may be greater or lesser than as illustrated.

A cylindrical rotor 13 is co-axially supported within the stator 11 for relative rotational movement. The rotor 13 has a plurality of radially outwardly extending poles, indicated generally at 14, formed thereon. As with the stator poles 12, the rotor poles 14 extend longitudinally throughout the length of the rotor 13 and are preferably provided in opposed pairs, such as shown at XI and X2, Yl and Y2, and Zl and Z2. Thus, six rotor poles 14 are provided on the illustrated rotor 13. However, it is known in the art to provide the rotor 13 with either a greater or lesser number of rotor poles 14. Generally, the number of rotor poles 14 is different from the number of stator poles 12.

Each of the rotor poles 14 is generally rectangular in cross sectional shape. The radially outermost surfaces of the rotor poles 14 are slightly curved so as to define an outer diameter. However, the rotor poles 14 may also be formed having any desired cross sectional shape. The outer diameter defined by the rotor poles 14 is preferably only slightly smaller than the inner diameter defined by the stator poles 12. Thus, a small radial gap is defined between the inner ends of the stator poles 12 and the outer ends of the rotor poles 14 when they are radially aligned, such as shown in Fig. 2 with the poles Al and XI and with the opposed poles A2 and X2. The rotor 13 and the rotor poles 14 are also formed from a magnetically permeable material, such as iron. If desired, the inner ends of the stator poles 12 and the outer ends of the rotor poles 14 may be formed having pluralities of relatively small teeth (not shown) formed thereon. An electrical conductor is wound about each of the stator poles 12. As schematically shown in Fig. 2, a first pair of windings 20 and 21 are provided on the opposed stator poles Al and A2, respectively. Similarly, a second pair of windings 22 and 23 (see Fig. 5 3) are provided on the opposed stator poles Bl and B2, respectively. Also, a third pair of windings 24 and 25 (see Fig. 3) are provided on the opposed stator poles Cl and C2, respectively. Lastly, a fourth pair of windings 26 and 27 (see Fig. 3) are provided on the opposed stator poles Dl and D2, respectively. As will be explained in detail 10 below, electrical current is selectively supplied through the windings 20 through 27 to cause the rotor 13 to rotate relative to the stator 11.

When electrical current is supplied to the windings 20 and

15 21 by the current pulse generating circuit 30, both the stator 11 and the rotor 13 become magnetized. The windings 20 and 21 are oppositely wound such that stator pole Al (upon which the winding 20 is disposed) is energized to become a magnetic north pole, while the stator pole A2

(upon which the winding 21 is disposed) is energized become a magnetic

20 south pole. As a result, a magnetic circuit is created between these opposed stator poles Al and A2. Consequently, magnetic flux (lines of force) are created between the north stator pole Al and the south stator pole A2. The magnetic flux passes from the north stator pole

Al through the nearest rotor pole XI, through the body of the rotor 13,

25 and from the opposed rotor pole X2 to the south stator pole A2. The magnetic circuit between the north and south stator poles Al and A2 is completed through the outer annular portion of the stator 11.

The resistance to the passage of magnetic flux from the

30 north stator pole Al to the nearest rotor pole XI (and similarly from the south stator pole A2 to the nearest rotor pole X2) is referred to as reluctance, as discussed above. The magnitude of this reluctance changes with the rotational position of the rotor poles 14 relative to the stator poles 12. Reluctance is at a minimum when the rotor poles 14 are radially aligned with the stator poles 12, as with Al, XI and 5 A2, X2 in Fig. 2. Consequently, the generation of the magnetic circuit described above produces a torque which tends to align the opposed rotor poles XI and X2 with the energized opposed stator poles Al and A2, as shown in Fig. 2.

10 To effect rotation of the rotor 13 relative to the stator

11, the flow of electrical current to the first pair of windings 20 and 21 on the stator poles Al and A2 is turned off, and a flow of electrical current to the second pair of windings 22 and 23 on the stator poles Bl and B2 is turned on. As a result, Bl is energized to

15 become a magnetic north pole, and B2 is energized to become a magnetic south pole. Such energization attracts the nearest rotor poles Yl and Y2 to become aligned with the energized stator poles Bl and B2. Consequently, the rotor 13 is rotated relative to the stator 11. To continue such rotation of the rotor 13, the stator poles Bl and B2 are

20 de-energized, and the stator poles Cl and C2 are energized by passing electrical current through the windings 24 and 25. Thus, the rotor poles Zl and Z2 are attracted to the stator poles Cl and C2. By sequentially energizing the stator poles 12 in this manner, the rotor poles 14 are sequentially attracted thereto. As a result, the rotor

25 13 rotates relative to the stator 11 in a direction (counterclockwise in the illustrated embodiment) which is opposite to the direction (clockwise in the illustrated embodiment) in which the stator pole pairs Al and A2, Bl and B2, Cl and C2, and Dl and D2 are energized.

30 In the illustrated embodiment, the stator 11 is provided with eight stator poles 12, while the rotor 13 is provided with six rotor poles 14. From the above discussion, it can be seen that each time one of the stator pole pairs Al and A2, Bl and B2, Cl and C2, and Dl and D2 is energized, the rotor 13 will be rotated fifteen degrees relative to the stator 11. Thus, the stator pole pairs Al and A2, Bl and B2 , Cl and C2, and Dl and D2 must be energized in sequence six times in order to rotate the rotor 13 throughout one complete revolution. Because the rotational speed of the rotor 13 is directly related to the frequency of the current pulses supplied to the stator poles 12, the motor 10 operates as a synchronous motor. By varying the number of stator poles 12 and rotor poles 14, the rotational speed of the rotor 12 can be varied with respect to the frequency of the current pulses supplied to the stator 11. The structure and operation of the variable reluctance motor 10 described thus far is conventional in the art.

Referring now to Fig. 3, there is illustrated a schematic block diagram which shows all of the stator windings 20 through 27 of the variable reluctance electric motor 10 illustrated in Figs. 1 and 2. These stator windings 20 through 27 are connected to a first embodiment of an electronic control circuit, indicated generally at 30, in accordance with this invention. The electronic control circuit 30 includes a current pulse generating circuit 31 which is adapted to selectively generate pulses of electrical current through each of the pairs of stator windings 20 through 27 in the general manner described above so as to cause the rotor 13 to rotate relative to the stator 11. The timing, magnitude, and polarity of the electrical current pulses generated by the current pulse generating circuit 31 may be determined, at least in part, by the rotational position of the rotor 13 relative to the stator 11. To accomplish this, a conventional sensor 32 is provided which generates an electrical signal which is representative of the rotational position of the rotor 13 relative to the stator 11. The current pulse generating circuit 31 is responsive to this rotor position signal for controlling the timing and magnitude of the current pulses generated to the various pairs of stator windings 20 through 27 of the motor 10.

Typically, this means that the current pulses generated to a first stator winding on a particular stator pole 12 will be initially increased to a maximum magnitude, then substantially discontinued before the attracted rotor pole 14 is fully aligned therewith. For example, the current pulse generated to the first winding 20 can be gradually decreased as the current pulse generated to a second winding 22 is gradually increased. This is done to permit a smooth transition of a rotor pole 14 past an attracting stator pole 12. As a result, the rotor 13 will rotate at a relatively constant speed without stuttering movement from phase to phase. Alternatively, the rotor position sensor 32 may be omitted, and the current pulse generating circuit 3 1 may be activated by a conventional phase sequencer (not shown) which operates independently of the rotational position of the rotor 13.

The electronic control circuit 30 also includes a phase control circuit 33 which is connected to the current pulse generating circuit 31. The phase control circuit 33 is provided to generate a phase control signal to the current pulse generating circuit 31 to control the mode of operation thereof. Specifically, the phase control signal is provided to selectively operate the current pulse generating circuit 33 in either a single phase mode or a multiple phase mode. When the current pulse generating circuit 33 is operated in the single phase mode, pulses of electrical current are fed only to one of the opposed pairs of stator windings 20 through 27. For example, pulses of electrical current may be fed only to the stator windings 20 and 21 provided on the stator pole pairs Al and A2, and no current pulses are fed to the other stator windings 22 through 27 provided on the other stator pole pairs Bl and B2 , Cl and C2, and Dl and D2. When the current pulse generating circuit 33 is operated in the multiple phase mode, pulses of electrical current are fed sequentially to all of the opposed pairs of stator windings 20 through 27 as described above.

The nature of the phase control signal generated by the phase control circuit 33 is controlled in response to an operating condition of the motor 10. In the first embodiment of the electronic control circuit 30, the phase control signal generated by the phase control circuit 33 is controlled in response to the magnitude of the electrical current being passed through one of the opposed pairs of stator windings. To accomplish this, a conventional current sensor 34 is provided. In the illustrated embodiment, the current sensor 34 is responsive to the electrical current being fed through the pair of stator windings 20 and 21 provided on the stator pole pairs Al and A2 for generating a signal which is representative of the magnitude thereof. It will be appreciated, however, that the current sensor 34 may be responsive to the electrical current being fed through any one (or more than one) of the other pairs of the stator windings 22 through 27 for generating the current magnitude signal.

This current magnitude signal is fed to the phase control circuit 33, which compares it with a predetermined reference level. If the magnitude of the sensed electrical current is greater than (or alternatively greater than or equal to) the predetermined reference level, the phase control circuit 33 will generate a first phase control signal to the current pulse generating circuit 31. In response thereto, the current pulse generating circuit 31 will be operated in the multiple phase mode, wherein pulses of electrical current are fed sequentially to all of the opposed pairs of stator windings 20 through 27 for normal operation as described above. If the magnitude of the sensed electrical current is less than (or alternatively less than or equal to) the predetermined reference level, the phase control circuit 33 will generate a second phase control signal to the current pulse generating circuit 31. In response thereto, the current pulse generating circuit 31 will be operated in the single phase mode, wherein pulses of electrical current are fed only to one of the opposed pairs of stator windings (such as the stator windings 20 and 21, for example) for single phase operation as described above.

Fig. 4 graphically illustrates the mode of operation of the motor 10 as described above. As shown therein, the magnitude of the electrical current being passed through the stator windings 20 through 27 is related to the magnitude of the torque being generated by the motor 10. This relationship is valid when, for example, the motor 10 is being operated at a constant speed. As additional loads are placed upon the motor 10, additional torque is required to be generated to maintain the constant speed. This additional torque is provided by increasing the magnitude of the electrical current supplied to the stator windings 20 through 27. In this instance, the predetermined reference level is identified as I_{LOW IΛAD}, which is representative of the magnitude of electrical current required to maintain the constant speed of the rotor 13 when there is only a small load is placed thereon. The corresponding torque required to maintain the speed of the rotor at this constant speed under this small load is identified as T _{L0W L}^. When the magnitude of the sensed electrical current is greater than i^, _LOAD/ the phase control circuit 33 will cause the current pulse generating circuit 31 to operate in the multiple phase mode. When the magnitude of the sensed electrical current is less than i^ _L0AD, the phase control circuit 33 will cause the current pulse generating circuit 31 to operate in the single phase mode. Single phase operation of the motor 10 in this manner under low-load or no-load conditions is desirable because it significantly reduces the amount of electrical power which is consumed. As a result, the overall efficiency of the motor 10 is enhanced. Single phase operation of the motor 10 in this manner is also desirable because it reduces the number of commutations of the motor 10. This is significant because it reduces the amount of acoustic noise which would otherwise be generated when the motor 10 is operated in this condition. Of course, single phase operation of the motor 10 can result in a relatively high amount of torque ripple. However, it has been found that in the low-load condition or no-load condition (which is the only time when single phase operation is experienced) , the mechanical inertia of the rotor 13 is sufficiently large as to minimize the adverse effects of the torque ripple.

Referring now to Fig. 5, there is illustrated a schematic block diagram which shows the stator windings 20 through 27 connected to a second embodiment of an electronic control circuit, indicated generally at 40, in accordance with this invention. The electronic control circuit 40 also includes a current pulse generating circuit 41 which is adapted to selectively generate pulses of electrical current through each of the pairs of stator windings 20 through 27 in the general manner described above so as to cause the rotor 13 to rotate relative to the stator 11. The electronic control circuit 40 may also include a sensor 42 for generating an electrical signal to the current pulse generating circuit 41 which is representative of the rotational position of the rotor 13 relative to the stator 11. The electronic control circuit 40 also includes a phase control circuit 43 which is connected to the current pulse generating circuit 41 for controlling the mode of operation thereof between the single phase mode or the multiple phase mode. In the second embodiment of the electronic control circuit 40, the phase control signal generated by the phase control circuit 43 is controlled in response to the rotational speed of the rotor 13 5 relative to the stator 11. To accomplish this, a conventional rotor speed sensor 44 is provided. The rotor speed sensor 44 is responsive to the rotational speed of the rotor 13 for generating a signal which is representative of the magnitude thereof. Alternatively, the rotor speed signal can be developed in other manners. For example, the

10 frequency at which pulses are generated by the current pulse generating circuit 41 is representative of the rotational speed of the rotor 13, and the rotor speed signal may be generated therefrom. Therefore, the rotor speed sensor 44 is intended to encompass any means for generating a signal which is representative of the rotational speed of the rotor

15 13.

This rotor speed signal is fed to the phase control circuit 43, which compares it with a predetermined reference level. If the magnitude of the sensed rotor speed is greater than (or alternatively

20 greater than or equal to) the predetermined reference level, the phase control circuit 43 will generate a first phase control signal to the current pulse generating circuit 41. In response thereto, the current pulse generating circuit 31 will be operated in the multiple phase mode, wherein pulses of electrical current are fed sequentially to all

25 of the opposed pairs of stator windings 20 through 27 for normal operation as described above. If the magnitude of the sensed rotor speed is less than (or alternatively less than or equal to) the predetermined reference level, the phase control circuit 43 will generate a second phase control signal to the current pulse generating

30 circuit 41. In response thereto, the current pulse generating circuit 41 will be operated in the single phase mode, wherein pulses of electrical current are fed only to one of the opposed pairs of stator windings (such as the stator windings 20 and 21, for example) for single phase operation as described above.

Figure 6 shows the major mechanical components of a switched reluctance (SR) electric motor 110, which includes a stator assembly 112, and a rotor assembly 114 used in connection with a third embodiment of a control apparatus.

Although the invention will be described and illustrated in the context of a switched reluctance electric motor 110, it will be appreciated that this invention may be used in conjunction with other well-known electric motor structures. Stator assembly 112, in a preferred, embodiment comprises a plurality of laminations 116. The laminations 116 are formed using a magnetically permeable material, such as iron.

Stator 112 is generally hollow and cylindrical in shape. A plurality of radially, inwardly extending poles 118 are formed on stator 112 (via laminations 116) and extend throughout the length thereof. Poles 118 are preferably provided in diametrically opposed pairs. In a constructed embodiment (not shown for clarity) of motor 110, each of the six poles 118 includes a respective pair of teeth for a total of 12 stator teeth. It should be appreciated, however, that a greater or lesser number of poles 118 may be provided in any particular configuration.

Each of the poles 118 may have a generally rectangular shape, when taken in cross-section. The radially innermost surfaces of the poles 118 are slightly curved so as to define an inner diameter representing bore 120. Bore 120 is adapted in size to receive rotor assembly 11 .

Rotor assembly 114, when assembled into stator 112 (see Figure 7) is coaxially supported within stator 112 for relative rotational movement by conventional means. For purposes of description only, rotor assembly 114 may be supported by conventional bearings (not illustrated) mounted in conventional housings (not shown) secured to the longitudinal ends of stator assembly 112. Rotor assembly 114 includes a generally cylindrical shaft 122, and rotor 124. Shaft 122 may be hollow. Rotor 124 is secured to shaft 122 for rotation therewith. For example, rotor 124 may be secured to shaft 122 by means of a spline (not shown) , or other conventional means well-known in the art. Thus, it should be appreciated that shaft 122, and rotor 124 rotate together as a unit.

Rotor 124 includes the plurality of poles 126 formed on an outer surface thereof. Each pole 126 extends radially outwardly from the outer surface thereof and is formed having a generally rectangular shape, when taken in cross-section. Rotor poles 126 extend longitudinally throughout the entire length of the outer surface of rotor 124. The radially outermost surfaces of rotor poles 126 are curved so as to define an outer diameter, adapted in size to be received within the inner diameter defining bore 120. That is, the outer diameter formed by the poles 126 is slightly smaller than the inner diameter defined by the radially innermost curved surfaces of stator poles 118. Rotor poles 126 are also preferably provided in diametrically opposed pairs. Four (4) rotor poles 126 are provided on the illustrated rotor assembly 114. However, it should be appreciated that a greater or lesser number of rotor poles 126 may be provided. For example, in a constructed embodiment of motor no, fourteen (14) rotor poles are provided (not shown) . For SR motors, in general, the number of rotor poles 126 differs from the number of stator poles 118, as is well-known. Rotor 124, including poles 126, may be formed from a magnetically permeable material, such as iron.

Referring now to Figure 7, a diagrammatic view of a cross-section of an assembled motor 110 is illustrated. In particular, as referred to above, poles 118 occur in pairs: i.e., AA' , BB' , and CC . The rotor poles 126 also appear in pairs. Stator windings 128 (shown only on stator pole pair AA^' for clarity) of diametrically opposite poles (e.g., AA^') associated with stator 112 are connected in series to form one machine phase. Thus, the windings 128 on poles AA' are referred to as "Machine Phase A" of SR motor 110. In the illustrated example, SR motor 110 also has a machine phase B, and a machine phase C. Each of these three machine phases may be energized individually, which, when done in a controlled manner, provides for rotation of rotor 124. Although a three-phase machine is described and illustrated, any machine having at least two phases (i.e., a selected machine phase to be switched between modes, and a nonselected machine phase, to be operated only during a multi-phase mode) is contemplated as falling within the spirit and scope of the present invention. For example, four-phase motors are contemplated as within the spirit and scope of the invention.

Before proceeding to a detailed description of the apparatus and technique for controlling a switched reluctance motor in accordance with the third, preferred embodiment of the present invention, a basic overview of the control established by the present invention will be set forth. Referring now to Figure 8, an underlying assumption for operation of an embodiment of the present invention is that when SR motor 110 is loaded, a relatively lower rotor speed will result, while, in contrast, when such load is lightened or removed from SR motor 110 (i.e., operation in the low load area), the rotor speed will rise relatively quickly. When SR motor 110 is loaded, and thus requires greater output power, the motor operates in a multi-phase mode wherein a first number of machine phases are energized to develop torque, and thus rotor rotation. When the load is lightened or removed from SR motor 110, however, the motor is controlled to operate in a reduced phase mode, wherein a second number less than the first number of the machine phases are energized to thereby operate the now-less-in-number machine phases at a higher current--i.e. , at a more efficient level to reduce the amount of power that would otherwise be consumed. In the preferred embodiment, all three phases of three-phase SR motor 110 are energized in the multi-phase mode, while only a single phase

(e.g., machine phase A) is energized in the reduced phase mode. It should be understood that illustration and description of this preferred embodiment does not diminish the generality of the principles of this invention, any reduced-in-number phase operation being with the spirit and scope of this invention. The remainder of this disclosure shall be made with reference to the preferred embodiment, without any loss in generality.

Figure 8 shows a hysteresis operating band employed by the present invention to eliminate undesirable "hunting" or "oscillation" between the reduced phase mode and the multi-phase mode. In particular, two separate current control references are used for the energization of the stator windings associated with the selected machine phase; that is, a first reference is used for the single-phase (i.e., reduced phase) mode of operation, and a second reference for the multi-phase mode of operation. The desired current through the selected machine phase is a function of speed, which is, in turn, a function of the load on the motor.

To illustrate how this map is applied, first assume operation of motor 110 in a steady-state loaded area of a torque-versus-phase current graph. Further assume that, when loaded, SR motor is controlled in a multi-phase mode. In the multi-phase mode, in the preferred embodiment, each of the machine phases are energized to a desired multi-phase current reference 130, which varies inversely as a function of rotor speed. When the load is removed (i.e., low load condition) , the rotor speed increases relatively rapidly until a first, high, predetermined speed level S_H, is reached. While in the multi- phase mode, each of the machine phases are energized sequentially to effect rotation of rotor 124.

When the rotor speed reaches S_H, the operating mode of the SR motor 110 changes from the multi-phase mode to the single-phase mode (i.e., reduced phase mode) , wherein the current reference for the selected, single phase to be energized transitions along path 132 to a single-phase current reference level 134. Single-phase current reference level 134 also varies inversely with rotor speed, and is greater than the multi-phase current reference. The magnitude of the reference 134 is greater since only a single machine phase is being energized; however, the total power consumed is less than if all the phases were energized according to the lower, multi-phase reference. The relative magnitudes of reference 130, and reference 134 are selected to maximize operating efficiency (i.e., energize a machine phase in an efficient region of the torque-versus-phase current graph) .

When the load is again applied to SR motor 110, the rotor speed drops quickly, along trace 134. Thus, the motor operates in the single-phase mode down to a second lower predetermined speed level, S_L. At this point, the motor 110 switches to multi-phase mode again for low-speed (i.e., loaded) operation. As the motor returns to high speed operation (via lightening or removal of the load) , the switch from multi-phase mode to single-phase mode again occurs at speed S_H. A hysteresis band is therefore utilized to prevent oscillation between modes of operation when the load is changing at a very slow rate. Only one phase of the motor is controlled to have separate current control references that toggles between the single-phase and the multi-phase modes according to rotor speed (i.e., existence of a load) as described in Figure 8. The remaining, non-selected machine phases require only a multi-phase current reference, since, in the single-phase mode, the non-selected machine phases are disabled (i.e., remain deenergized) . It should be appreciated that this hysteresis loop technique can be applied to other situations, such as, for example, switches between energization of three (3) machine phases and two (2) machine phases, as well as between energization of two (2) machine phases and one (1) machine phase (for a three phase motor) . Other "step" transitions are within the spirit and scope of this invention.

Figure 9 shows an apparatus 140 for controlling SR motor 110 to change operating modes between the single-phase operating mode, and the multi-phase operating mode. Apparatus 140 includes a controller 142, and a plurality of drive circuits 144^ 144₂, . . ., 144_N. Controller 142 is responsive to a plurality of machine phase commutation signals _n for generating a plurality of output current reference signals I_re£1, l_τei2 , • • •/ I_re._m nd a MODE signal.

The machine phase commutation signals Φ_n comprise a plurality of individual signals, one for each machine phase of SR motor 110, wherein each one of the plurality of signals is indicative of whether the respective machine phase is commanded to be energized. Collectively, the machine phase commutation signals may be processed to provide an indication of the magnitude of the speed of rotor 124.

Each one of the output current reference signals I_refi_/ I_ref2, . . . , I_reιn, has associated therewith a magnitude corresponding to a desired current through the stator windings 128_^ of the machine phase corresponding thereto. The MODE signal has a first state indicative of the multi-phase mode of operation, and a second state indicative of the single-phase (i.e., reduced phase) mode of operation.

Drive circuits 144₁₍ 144₂, . . ., 144_N_ are respectively coupled to the current reference signals I_refl/ I_r--2/ • ■ • , I_ret_n. , and are provided for energizing the corresponding machine phase to the desired energization current defined by the magnitude of the reference signals. The MODE signal is coupled to drive circuits 144_: corresponding to nonselected machine phases, and, operates to disable energization of the stator windings 128i associated with the drive circuits when the MODE signal assumes the second state. Otherwise, when the MODE signal is in the first state, the nonselected machine phases are energized by way of the corresponding drive circuits

144₂, . . . 144_N. Referring now to Figure 10, controller 142 is shown in greater detail, and includes means, such as speed sensing circuit 146, for sensing a speed of the rotor and generating a first speed signal V'_A in response thereto, and an amplifier 148, means, such as multi-phase reference circuit 150, for generating a multi-phase current reference signal, means, such as single-phase reference circuit 152, for generating a single-phase current reference signal, and means, such as circuit 154, for changing the operating mode of the SR motor 110 from the multi-phase mode, when the rotor speed reaches a first predetermined level, and for changing the operating mode of motor 110 from the single-phase mode to the multi-phase mode when the rotor speed reaches a second predetermined level that is less than the first predetermined level to thereby define a hysteresis operating band.

Speed sensing circuit 146 is responsive to the above-mentioned machine phase commutation signals and is provided for generating the first speed signal. Circuit 146 includes logic means 156, responsive to the machine phase commutation signals, for generating an output signal having a frequency which is indicative of the rotor speed, and a frequency-to-voltage converter 158 responsive to the logic means output signal for generating the first speed signal, wherein the first speed signal has a voltage magnitude corresponding to the rotor speed.

Logic means 156 includes a first exclusive-OR (XOR) gate

160, and a second exclusive-OR gate 162 connected in series to generate an output pulse train whose frequency of transition corresponds to the rotor speed. It should be appreciated that other circuit arrangements performing the same logical functions as described above fall within the spirit and scope of the present invention. Frequency-to-voltage converter 158 is conventional in the art and performs its well-known function.

Amplifier 148 provides a voltage level, and impedance interface function to translate signal V'_A to a signal having a power level sufficient to drive subsequent stages; namely, second speed signal V_A.

Multi-phase reference circuit 150 is responsive to second speed signal V_A for generating a multi-phase input current reference signal I_3Φ. Circuit 150 includes a differential amplifier 164, which generates signal V_B, and a summing amplifier 166 responsive to signal

V_B, for generating the multi-phase input current reference signal I_3Φ.

Differential amplifier 164 is well-known and conventional in the art, and may comprise, for example, an operational amplifier appropriately configured. Likewise, summing amplifier 166 may include an operational amplifier, and functions to add the magnitude of each signal appearing on its two inputs; namely, signal V_B and the voltage generated by the voltage divider established by adjustable resistor R_x Summing amplifier 166 is well-known and conventional.

Figures 11A and 11B illustrate the relationships of signals at various nodes of circuit 142, wherein Figure 11A shows the second speed signal V_A is plotted versus rotor speed. It should be appreciated that the magnitude of speed signal V_A increases and is proportional to, the rotor speed.

Referring now to Figure 11B, signal V_B, appearing at the output of differential amplifier 164, is illustrated as trace 176. It should be appreciated that the particular amplifier design for amplifier 164 may take one of a plurality of forms well-known in the art for which the trace of signal V_A (Figure 11A) is translated to trace 176 (Figure 11B) . Furthermore, it should also be appreciated that the particular design of summing amplifier 166 may take one of a plurality of forms well-known in the art in order to translate trace 176 by way of displacement 178 to the trace indicated at I_3Φ (Figure 11B) .

Referring to Figure 10, single-phase (i.e., reduced phase) reference circuit 152 includes second differential amplifier 168, and second summing amplifier 170 and is provided for generating single- phase input current reference signal I_lφ. Differential amplifier 168 is responsive to the second speed signal V_A, and generates a signal that is similar to trace 176 is Figure 11B. The particular design of differential amplifier 168 may take any one of a plurality of forms well-known and conventional in the art. Summing amplifier 170 is provided for summing the magnitudes of the output of differential amplifier 168 (V_B) , and an offset voltage established by the voltage divider network comprising variable resistor R₂. This offset is indicated at 180 in Figure 11B. The multi-phase input current reference signal I_3Φ, and the single-phase input current reference signal I_lφ, have a voltage magnitude corresponding to a desired current through the stator windings when energized according to the multi-phase mode, and the single-phase mode, respectively.

Circuit 154 is provided for changing the operating mode of motor 110 from the multi-phase mode, to the single-phase mode when the rotor speed of rotor 124 reaches a first predetermined level (indicated at S_H in Figure 8) , and for changing the operating mode of electric SR motor 110 from the single-phase mode to the multi-phase mode when the rotor speed of rotor 124 reaches a second predetermined level (indicated at S_L in Figure 8) . S_L is less than S_H and 1_1Φ is greater than I_3Φ, wherein a hysteresis operating band is defined to prevent undesirable oscillation between the single-phase mode, and the multi- phase mode. Circuit 154 includes a comparator 172 having hysteresis, and an analog switch 174 connected thereto.

The comparator circuit 172 is operative to generate the MODE signal as a function of the first rotor speed signal V'_A, and internal reference signals corresponding to the first, high-speed rotor speed signal S_H, and, the second, low-speed rotor signal S_L (both as shown in

Figure 8) .

Analog switch 174 includes a pair of inputs coupled to receive the multi-phase input reference signal I_3Φ, and the single-phase input current reference signal I_1Φ, and an output for generating the output current reference signal I_REF1 corresponding to the desired current through the selected machine phase. In particular, switch 174 is operative to select as an output on the switch output terminal, one of I_3Φ and I_lφ according to whether the mode signal is in a first state, or a second state, respectively. The output current reference signals for the non-selected machine phases of electric motor

110, namely, I_REF2, . . ., I_REFΠ- may be generated either directly from the I_3Φ tap of summing amplifier 166, or, alternatively, may be generated by circuitry similar to circuits 146, 148 and 150 shown in

Figure 10.

Figure 12 shows the comparator 172 of Figure 10 in greater detail. Comparator with hysteresis 172 includes a first operational amplifier 182, a comparator 184, and resistors R₃, R₄, R₅, R_β (variable; potentiometer), and R₇ (variable; potentiometer) . Operational amplifier 182, and voltage comparator 184 are conventional, and well-known components in the art and take the form of commercially available part numbers LM2904, and LM2901, respectively, from National Semiconductor. Resistors R₃-R₇ are also conventional, and whose values and tolerance ratings may be selected in accordance with desired operating characteristics, and is within the reach of one of ordinary in the art.

In the constructed embodiment, resistor R₃ assumes a value of 100k ohms, resistor R, 120k ohms, resistor R₅ Ik ohm, while resistors R₆ and R₇ have a variable resistance up to approximately 100K ohms, and 10K ohms, respectively.

Circuit 172 provides for a small amount of positive feedback and hysteresis relative to op amp 184. Op amp 182, resistor R₆ and R₇ are provided for generating a reference voltage V_REF at the inverting input of op amp 184. The values of resistors R₆ and R₇ define the magnitude of the voltage reference V_REF in accordance with well-known design principles.

To describe the operation of circuit 172, assume that the non-inverting input, which is coupled to the speed signal from frequency-to-voltage converter 158 by way of resistor R₃, goes above the reference input. This situation will drive the output, the MODE signal, towards ground, which in turn pulls the reference voltage V_REF down through resistor R„. The trip voltage (i.e., the reference V_REF) is now defined at a lower voltage level. When the speed signal decreases below this new, now-lower, trip signal, the output mode signal will again switch high, thus driving the voltage reference to a higher level (as pulled up through R₄) , and thus providing the needed Δ V between the upper and lower trip points (i.e., corresponding to S_H and S_L, respectively) .

The control apparatus in accordance with the present invention (preferably, the third embodiment) accomplishes an improved control of the operation of a switched reluctance electric motor, by virtue of having the capability of controlling an electric motor in either a reduced phase operating mode, wherein less than all of the plurality of machine phases are energized, and a multi-phase operating mode wherein ϋ of the machine phases are energized. The ability to operate a switched reluctance machine in a reduced phase mode, when a low load condition occurs, permits a substantially increased operating efficiency, since the aggregate power drawn from a power supply is less (i.e., although the phases that are energized are operated at a higher current level, there are less phases on, and, in addition, each phase is being operated in a more efficient region) . The hysteresis loop control feature employed in the basic multi-to-reduced phase operating transition provides for smooth and reliable operation by substantially eliminating "hunting" or "oscillating" between modes of operation.

This invention has been described and illustrated in the context of a variable reluctance electric motor. However, it will be appreciated that the concept and structure of this invention may be applied to other types of brushless DC electric motors. Also, this invention has been described and illustrated in the context of a multiple phase motor which is selectively operated in only a single phase mode. However, it will be appreciated that this invention may be used to selectively operate a multiple phase motor in a mode wherein less than all of the multiple phases, but more than only a single phase, are energized. For example, the illustrated four phase motor for the use in connection with the first and second embodiments may be operated in a two phase mode (wherein the stator windings 20 and 21 on the stator poles Al and A2 and the stator windings 24 and 25 on the stator poles Cl and C2) are operated when the sensed phase current or rotor speed drops below the predetermined reference level.

The preceding description is exemplary rather than limiting in nature. A preferred embodiment of this invention has been disclosed to enable one skilled in the art to practice the invention. Variations and modifications are possible without departing from the purview and spirit of this invention,- the scope of which is limited only by the appended claims.

Claims

CLAIMSWe claim:

1. An electric motor comprising: a stator including a plurality of stator poles having stator windings provided thereon representing a plurality of phases; a rotor supported for rotation relative to said stator and including a plurality of rotor poles; a current generating circuit for selectively supplying electrical current to said stator windings, said current generating circuit being operable in either a first mode, wherein electrical current is supplied to a first number of said stator windings, and a second mode, wherein electrical current is supplied to a second number of said stator windings greater than said first number; and a phase control circuit responsive to an operating condition of said motor for controlling said current generating circuit to operate in one of said first and second modes.

2. The electric motor defined in claim 1, wherein said stator and said rotor are formed from a magnetically permeable material.

3. The electric motor defined in claim 1, wherein said stator is hollow and includes a plurality of radially inwardly extending stator poles, and wherein said rotor is supported within said stator for relative rotational movement.

4. The electric motor defined in claim 1, wherein said stator is formed having a plurality of opposed stator poles, and wherein said windings are provided on said stator poles in opposed pairs.

5. The electric motor defined in claim 1, wherein said operating condition of said motor is phase current.

6. The electric motor defined in claim 1, wherein said operating condition of said motor is torque.

7. The electric motor defined in claim 1, wherein said operating condition of said motor is rotor speed.

8. The electric motor defined in claim 1, wherein electrical current is supplied to only one of said stator windings when said current generating circuit is operated in said first mode.

9. The electric motor defined in claim 1, wherein electrical current is supplied to only one of said phases of said electric motor when said current generating circuit is operated in said first mode.

10. An apparatus for controlling a switched reluctance machine machine to change operating modes between a multi-phase operating mode and a reduced phase operating mode wherein the machine includes a rotatable rotor, a stator, and a plurality of stator windings defining a corresponding plurality of machine phases, said apparatus comprising: means for sensing a speed of the rotor and generating a speed signal in response thereto; means responsive to said speed signal for changing the operating mode of the machine from the multi-phase mode, wherein a first number of machine phases are energized, to the reduced phase mode, wherein a second number less than said first number of machine phases are energized, when said rotor speed reaches a first predetermined level, and for changing the operating mode of the machine from the reduced phase mode to the multi-phase mode when said rotor speed reaches a second predetermined level that is less than said first predetermined level to thereby define a hysteresis operating band to prevent undesirable oscillation between the reduced phase mode and the multi-phase mode.

11. The apparatus of claim 10 wherein said sensing means includes logic means responsive to machine phase commutation signals for generating an output signal having a frequency which is indicative of said rotor speed, said sensing means further including frequency-to-voltage converter means responsive to said logic means output signal for generating said speed signal wherein said speed signal has a voltage magnitude corresponding to said rotor speed.

12. The apparatus of claim 10 wherein said changing means includes means responsive to said speed signal for generating a mode signal having a first state indicative of the multi-phase mode of operation, and a second state indicative of the reduced phase mode of operation, said mode signal being operative, when in said second state, to disable energization of machine phases selected for nonoperation during the reduced phase mode.

13. The apparatus of claim 12 wherein said mode signal generating means includes a comparator having hysteresis.

14. An apparatus for controlling a switched reluctance machine to change operating modes between a multi-phase operating mode and a reduced phase operating mode wherein the machine includes a rotatable rotor, a stator, and a plurality of stator windings defining a corresponding plurality of machine phases, comprising: a speed sensing circuit for sensing a speed of the rotor and generating a speed signal in response thereto; a comparator circuit responsive to said speed signal for changing the operating mode of the machine from the multi-phase mode, wherein a first number of machine phases are energized according to a predetermined operating strategy, to the reduced phase operating mode, wherein a second number less than said first number of machine phases are energized according to said strategy, when said rotor speed reaches a first predetermined level, and for changing the operating mode of the machine from the reduced phase mode to the multi-phase mode when said rotor speed reaches a second predetermined level that is less than said first predetermined level to thereby define a hysteresis operating band to prevent undesirable oscillation between the reduced phase mode and the multi-phase mode.

15. The apparatus of claim 14 wherein said sensing circuit includes a logic circuit responsive to machine phase commutation signals for generating an output signal having a frequency which is indicative of said rotor speed, said sensing circuit further including a frequency-to-voltage converter circuit responsive to said logic circuit output signal for generating said speed signal wherein said speed signal has a voltage magnitude corresponding to said rotor speed.

16. The apparatus of claim 14 wherein said comparator circuit is operative to generate a mode signal having a first state indicative of the multi-phase mode of operation, and a second state indicative of the reduced phase operating mode of operation, said mode signal being operative, when in said second state, to disable energization of predetermined ones of said machine phases selected for non-operation during the reduced phase mode.

17. The apparatus of claim 14 further comprising a multi¬ phase reference circuit responsive to said speed signal for generating a multi-phase current reference signal having a magnitude corresponding to a desired multi-phase current level through each one of the first number of machine phases, and a reduced phase reference circuit responsive to said speed signal for generating a reduced phase current reference signal having a magnitude corresponding to a desired reduced phase current level through each one of the second number of machine phases, said apparatus further comprising an analog switch having an output and a pair of inputs coupled to said multi-phase reference circuit and said reduced phase reference circuit, respectively, said analog switch being operative for selecting and outputting on said switch output one of said multi-phase and reduced phase current reference signals in accordance with said mode signal.

18. The apparatus of claim 17 further comprising a plurality of machine phase driver circuits coupled to a respective one of said plurality of machine phases for energization thereof wherein said analog switch output is connected to at least one machine phase driver circuit to thereby vary a current level of the machine phase associated therewith.

19. The apparatus of claim 18 wherein said mode signal is applied to said nonselected ones of said plurality of machines phases to thereby disable energization thereof.

20. An electric motor comprising: a stator including a plurality of stator poles having stator windings provided thereon defining a plurality of motor phases; a rotor supported for rotation relative to said stator, and including a plurality of rotor poles; a speed sensing circuit for sensing a speed of said rotor and generating a speed signal in response thereto; a multiple-phase reference circuit responsive to said speed signal for generating a multi-phase current reference signal; a reduced phase reference circuit responsive to said speed signal for generating a reduced phase current reference signal; a comparator circuit responsive to said speed signal for generating a mode signal having a first state indicative of the multi-phase mode of operation, wherein a first number of machine phases are energized, and a second state indicative of the reduced phase mode of operation, wherein a second number less than said first number of machine phases are energized, said mode signal transitioning from said first state to said second state when said rotor speed reaches a first predetermined level, said mode signal transitioning from said second state to said first state when said rotor speed reaches a second predetermined level that is less than said first predetermined level to thereby define a hysteresis operating band to prevent undesirable oscillation between the single-phase mode and the multi-phase mode; an analog switch having an output for selecting and outputting on said switch output one of said multi-phase current reference signal and said reduced phase current reference signal in accordance with the state of said mode signal; a plurality of phase drive circuits for energizing said plurality of machine phases, said analog switch output being connected to at least one of said phase drive circuits for varying a current level through the machine phase connected thereto according to one of said multi-phase current reference and said reduced phase current reference signals.