Electricity: motive power systems – Synchronous motor systems – Hysteresis or reluctance motor systems
Reexamination Certificate
2000-02-14
2002-02-26
Fletcher, Marlon T. (Department: 2837)
Electricity: motive power systems
Synchronous motor systems
Hysteresis or reluctance motor systems
C318S254100, C318S700000, C318S434000
Reexamination Certificate
active
06351094
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the control of switched reluctance machines, particularly those machines which are operated without a sensor to measure rotor position.
2. Description of Related Art
In general, a reluctance machine is an electrical machine in which torque is produced by the tendency of its movable part to move into a position where the reluctance of a magnetic circuit is minimized, i.e. where the inductance of the exciting winding is maximized. In one type of reluctance machine, the energization of the phase windings occurs at a controlled frequency. This type is generally referred to as a synchronous reluctance machine, and it may be operated as a motor or a generator. In a second type of reluctance machine, circuitry is provided for detecting the angular position of the rotor and energizing the phase windings as a function of the rotor position. This second type of reluctance machine is generally known as a switched reluctance machine and it may also be operated as a motor or a generator. The characteristics of such switched reluctance machines are well known and are described in, for example, “The characteristics, design and application of switched reluctance motors and drives” by Stephenson and Blake, PCIM'93, Nürnberg, Jun. 21-24, 1993, incorporated herein by reference. That paper describes in some detail the features of the switched reluctance machine which together produce the characteristic cyclically varying inductance of the phase windings.
FIG. 1
shows the principal components of a typical switched reluctance drive system. The input DC power supply
11
can be either a battery or a rectified and filtered AC supply and can be fixed or variable in magnitude. In some known drives, the power supply
11
includes a resonant circuit which produces a DC voltage which rapidly varies between zero and a predetermined value to allow zero voltage switching of the power switches. The DC voltage provided by the power supply
11
is switched across the phase windings
16
of the motor
12
by a power converter
13
under the control of the electronic control unit
14
. The switching must be correctly synchronized to the angle of rotation of the rotor for proper operation of the drive. A rotor position detector
15
is typically employed to supply signals indicating the angular position of the rotor. The output of the rotor position detector
15
may also be used to generate a speed feedback signal.
The rotor position detector
15
may take many forms, for example it may take the form of hardware, as shown schematically in FIG.
1
. In other systems, the position detector can be a software algorithm which calculates or estimates the position from other monitored parameters of the drive system. These systems are often called “sensorless position detector systems” since they do not use a physical transducer associated with the rotor which measures the position. As is well known in the art, many different approaches have been adopted in the quest for a reliable sensorless system. Some of these approaches are discussed below.
The energization of the phase windings in a switched reluctance machine depends on detection of the angular position of the rotor. This may be explained by reference to
FIGS. 2 and 3
, which illustrate the switching of a reluctance machine operating as a motor.
FIG. 2
generally shows a rotor
24
with a rotor pole
20
approaching a stator pole
21
of a stator
25
according to arrow
22
. As illustrated in
FIGS. 2 and 3
, a portion
23
of a complete phase winding
16
is wound around the stator pole
21
. When the portion
23
of the phase winding
16
around stator pole
21
is energized, a force will be exerted on the rotor, tending to pull rotor pole
20
into alignment with stator pole
21
.
FIG. 3
generally shows typical switching circuitry in the power converter
13
that controls the energization of the phase winding
16
, including the portion
23
around stator pole
21
. When switches
31
and
32
are closed, the phase winding is coupled to the source of DC power and is energized. Many other configurations of lamination geometry, winding topology and switching circuitry are known in the art: some of these are discussed in the Stephenson & Blake paper cited above. When the phase winding of a switched reluctance machine is energized in the manner described above, the magnetic field set up by the flux in the magnetic circuit gives rise to the circumferential forces which, as described, act to pull the rotor poles into line with the stator poles.
In general, the phase winding is energized to effect the rotation of the rotor as follows. At a first angular position of the rotor (called the “turn-on angle”, &thgr;
ON
), the controller
14
provides switching signals to turn on both switching devices
31
and
32
. When the switching devices
31
and
32
are on, the phase winding is coupled to the DC bus, causing an increasing magnetic flux to be established in the machine. The magnetic flux produces a magnetic field in the air
10
gap which acts on the rotor poles to produce the motoring torque. The magnetic flux in the machine is supported by the magneto-motive force (mmf) which is provided by a current flowing from the DC supply through the switches
31
and
32
and the phase winding
23
. Current feedback is generally employed and the magnitude of the phase current is controlled by chopping the current by rapidly switching one or both of switching devices
31
and/or
32
on and off. FIG.
4
(
a
) shows a typical current waveform in the chopping mode of operation, where the current is chopped between two fixed levels. In motoring operation, the turn-on angle &thgr;
ON
is often chosen to be the rotor position where the center-line of an inter-polar space on the rotor is aligned with the center-line of a stator pole, but may be some other angle.
In many systems, the phase winding remains connected to the DC bus (or connected intermittently if chopping is employed) until the rotor rotates such that it reaches what is referred to as the “freewheeling angle”, &thgr;
FW
. When the rotor reaches an angular position corresponding to the freewheeling angle (e.g., the position shown in
FIG. 2
) one of the switches, for example
31
, is turned off. Consequently, the current flowing through the phase winding will continue to flow, but will now flow through only one of the switches (in this example
32
) and through only one of the diodes
33
/
34
(in this example
34
). During the freewheeling period, the voltage drop across the phase winding is small, and the flux remains substantially constant. The circuit remains in this freewheeling condition until the rotor rotates to an angular position known as the “turn-off angle”, &thgr;
OFF
, (e.g. when the center-line of the rotor pole is aligned with that of the stator pole). When the rotor reaches the turn-off angle, both switches
31
and
32
are turned off and the current in phase winding
23
begins to flow through diodes
33
and
34
. The diodes
33
and
34
then apply the DC voltage from the DC bus in the opposite sense, causing the magnetic flux in the machine (and therefore the phase current) to decrease. It is known in the art to use other switching angles and other current control regimes.
As the speed of the machine rises, there is less time for the current to rise to the chopping level, and the drive is normally run in a “single-pulse” mode of operation. In this mode, the turn-on, freewheel and turn-off angles are chosen as a function of, for example, speed and load torque. FIG.
4
(
b
) shows a typical such single-pulse current waveform where the freewheel angle is zero. It is well known that the values of turn-on, freewheel and turn-off angles can be predetermined and stored in some suitable format for retrieval by the control system as required, or can be calculated or deduced in real time.
Many sensorless position detection systems are reviewed and categorized in “Sensorless methods for determining the rotor posit
Dicke, Billig & Czaja P.A.
Duda Rina I.
Fletcher Marlon T.
Switched Reluctance Drives Ltd.
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