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Claims  |
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We claim:
1. 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, 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.
2. The apparatus of claim 1 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.
3. The apparatus of claim 1 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.
4. The apparatus of claim 3 wherein said mode signal generating means
includes a comparator having hysteresis.
5. 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.
6. The apparatus of claim 5 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.
7. The apparatus of claim 5 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.
8. 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, said apparatus 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.
9. The apparatus of claim 8 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.
10. The apparatus of claim 9 wherein said mode signal is applied to said
nonselected ones of said plurality of machines phases to thereby disable
energization thereof.
11. 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. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
1. Technical Field
This invention relates generally to a system for controlling a
switch-reluctance (SR) motor, and more particularly, to a 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.
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.
SUMMARY OF THE INVENTION
The present invention 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 one 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
FIG. 1 is an exploded, perspective view of a portion of a switched
reluctance electric motor suitable for use in connection with a preferred
embodiment of the present invention.
FIG. 2 is a diagrammatic, exaggerated, cross-sectional view of a switched
reluctance electric motor illustrating the relative positions of a stator,
and rotor portions thereof.
FIG. 3 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.
FIG. 4 is a simplified, block and schematic diagram view showing a
preferred 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.
FIG. 5 is a simplified, partial schematic and block diagram view showing,
in greater detail, the controller portion of the preferred embodiment
illustrated in FIG. 4.
FIG. 6A is a simplified, speed-versus-voltage graph of the speed signal
V.sub.A generated by the speed signal generating circuit shown in FIG. 5.
FIG. 6B 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 FIG. 6A.
FIG. 7 is a simplified, schematic diagram view showing, in greater detail,
the comparator with hysteresis illustrated in FIG. 5.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to the drawings wherein like reference numerals are used to
identify identical components in the various views, FIG. 1 shows the major
mechanical components of a switched reluctance (SR) electric motor 10,
which includes a stator assembly 12, and a rotor assembly 14.
Although the invention will be described and illustrated in the context of
a switched reluctance electric motor 10, it will be appreciated that this
invention may be used in conjunction with other well-known electric motor
structures. Stator assembly 12, in a preferred embodiment, comprises a
plurality of laminations 16. The laminations 16 are formed using a
magnetically permeable material, such as iron.
Stator 12 is generally hollow and cylindrical in shape. A plurality of
radially, inwardly extending poles 18 are formed on stator 12 (via
laminations 16) and extend throughout the length thereof. Poles 18 are
preferably provided in diametrically opposed pairs. In a constructed
embodiment (not shown for clarity), each of the six poles 18 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 18 may be
provided in any particular configuration.
Each of the poles 18 may have a generally rectangular shape, when taken in
cross-section. The radially innermost surfaces of the poles 18 are
slightly curved so as to define an inner diameter representing bore 20.
Bore 20 is adapted in size to receive rotor assembly 14.
Rotor assembly 14, when assembled into stator 12 (see FIG. 2) is coaxially
supported within stator 12 for relative rotational movement by
conventional means. For purposes of description only, rotor assembly 14
may be supported by conventional bearings (not illustrated) mounted in
conventional housings (not shown) secured to the longitudinal ends of
stator assembly 12. Rotor assembly 14 includes a generally cylindrical
shaft 22, and rotor 24. Shaft 22 may be hollow. Rotor 24 is secured to
shaft 22 for rotation therewith. For example, rotor 24 may be secured to
shaft 22 by means of a spline (not shown), or other conventional means
well-known in the art. Thus, it should be appreciated that shaft 22, and
rotor 24 rotate together as a unit.
Rotor 24 includes the plurality of poles 26 formed on an outer surface
thereof. Each pole 26 extends radially outwardly from the outer surface
thereof and is formed having a generally rectangular shape, when taken in
cross-section. Rotor poles 26 extend longitudinally throughout the entire
length of the outer surface of rotor 24. The radially outermost surfaces
of rotor poles 26 are curved so as to define an outer diameter, adapted in
size to be received within the inner diameter defining bore 20. That is,
the outer diameter formed by the poles 26 is slightly smaller than the
inner diameter defined by the radially innermost curved surfaces of stator
poles 18. Rotor poles 26 are also preferably provided in diametrically
opposed pairs. Four (4) rotor poles 26 are provided on the illustrated
rotor assembly 14. However, it should be appreciated that a greater or
lesser number of rotor poles 26 may be provided. For example, in a
constructed embodiment, fourteen (14) rotor poles are provided (not
shown). For SR motors, in general, the number of rotor poles 26 differs
from the number of stator poles 18, as is well-known. Rotor 24, including
poles 26, may be formed from a magnetically permeable material, such as
iron.
Referring now to FIG. 2, a diagrammatic view of a cross-section of an
assembled motor 10 is illustrated. In particular, as referred to above,
poles 18 occur in pairs: i.e., AA', BB', and CC'. The rotor poles 26 also
appear in pairs. Stator windings 28 (shown only on stator pole pair AA'
for clarity) of diametrically opposite poles (e.g., AA') associated with
stator 12 are connected in series to form one machine phase. Thus, the
windings 28 on poles AA' are referred to as "Machine Phase A" of SR motor
10. In the illustrated example, SR motor 10 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 24. 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 preceding to a detailed description of the apparatus and technique
for controlling a switched reluctance motor in accordance with the
preferred embodiment of the present invention, a basic overview of the
control established by the present invention will be set forth.
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