Electricity: motive power systems – Synchronous motor systems – Hysteresis or reluctance motor systems
Reexamination Certificate
2002-03-06
2003-07-01
Masih, Karen (Department: 2837)
Electricity: motive power systems
Synchronous motor systems
Hysteresis or reluctance motor systems
C318S717000, C318S723000, C318S798000, C318S815000, C388S907500, C388S902000, C388S815000
Reexamination Certificate
active
06586904
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATION
The subject matter of this application is related to the subject matter of British Patent Application No. GB 0105502.9, priority to which is claimed under 35 U.S.C. §119 and which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
Embodiments of the present invention generally relate to an apparatus and method of compensating for DC link voltage variations in an electric drive system. More particularly, embodiments of the present invention relate to the operation of a switched reluctance drive that periodically reads a value corresponding to a DC link voltage and adjusts a speed indication signal to compensate for changes in the DC link voltage. The compensated speed signal is transmitted to a controller where it is used to compensate the energization of the windings of the switched reluctance drive.
2. Description of Related Art
The general theory of the design and operation of switched reluctance machines is well known and is discussed, for example, in “The Characteristics, Design and Applications of Switched Reluctance Motors and Drives”, by Stephenson and Blake and presented at the PCIM '93 Conference and Exhibition at Nuremberg, Germany, Jun. 21-24, 1993 and incorporated herein by reference. The general construction and operation of controllers for switched reluctance drives is generally understood and is described herein for background purposes only.
The switched reluctance motor is generally constructed without windings or permanent magnets on the moving part of the motor (called the rotor). The stationary part of most switched reluctance motors (called the stator) includes coils wound around stator poles that carry unidirectional current. In one type of switched reluctance motor, coils around opposing stator poles are connected in series or parallel to form one phase winding of a potentially multi-phase switched reluctance motor. Motoring torque is produced by applying a voltage across each of the phase windings in a predetermined sequence that is synchronized with the angular position of the rotor so that a magnetic force of attraction results between poles of the rotor and stator as they approach each other. Similarly, generating action is produced by positioning the pulse of voltage in the part of the cycle where the poles are moving away from each other. In typical operation, each time a phase winding of the switched reluctance machine is energized, magnetic flux is produced by the phase winding, thereby causing a force of attraction on the rotor poles.
A typical prior art drive is shown schematically in
FIG. 1
driving a load
19
. The arrangement includes a DC power supply
11
that can be either a battery or rectified and filtered AC mains. The DC voltage provided by the power supply
11
is supplied across a DC link, represented as line
10
, and switched across phase windings
16
of the motor
12
by a power converter
13
under the control of the electronic control unit
14
. A current transducer
18
is used to provide current feedback signals to the controller. In the present application, the DC voltage provided to the switched reluctance machine (whether from a battery, rectifier or otherwise) is referred to as the “DC link voltage”.
For proper operation of the drive, the switching must be correctly synchronized to the angle of rotation of the rotor. The performance of a switched reluctance machine depends, in part, on the accurate timing of phase energization with respect to rotor position. Detection of rotor position is conventionally achieved by using a transducer
15
, shown schematically in
FIG. 1
, such as a rotating toothed disk mounted on the machine rotor, which cooperates with an optical or magnetic sensor mounted on the stator. A pulse train indicative of rotor position relative to the stator is generated and supplied to control circuitry
14
, allowing accurate phase energization.
In order to maintain the speed and related torque developed by a switched reluctance machine, it is desirable to control carefully the instants at which voltage is applied to the phase windings. However, changes in the voltage of the AC supply and changes in the electrical environment in which the drive system operates often result in a DC link voltage that varies over time. Because the supply voltage can vary significantly, a control scheme that ignores changes in supply voltage may experience a significant reduction in ability to control the machine as demanded by the user. This is because the flux produced by the phase windings is directly related to the applied voltage. Accordingly, a change in the supply voltage may result in more or less flux produced by the phase windings than would otherwise be desired. This undesirable change in the flux in the motor can result in degraded performance.
In most known switched reluctance drives, the relationship between the speed of the motor, the desired torque and the time for which voltage is applied to the phase windings is determined empirically through a process referred to as characterization, in which the operating parameters of the motor are determined for a wide variety of operating conditions. These operating parameters are then stored in an analog or digital circuit called a control law table.
Typically, a digital control law table is simply a series of storage locations in a memory of some sort, in which each location corresponds to a point on a control map. For that point, typical systems store an on-angle, an off-angle and possibly a free-wheel angle which will be appropriate for that operating point. These parameters may be stored in adjacent locations in the table. Alternatively, the table may be sub-divided and be thought of as a series of tables, each holding values for one of the parameters.
An example of a control law table is shown in FIG.
2
. In this example, the torque is discretized into 128 values at any speed, i.e. at any chosen speed, 100% torque at that speed corresponds to the 128
th
storage location and 50% torque corresponds to the 64
th
location, etc. Likewise, the speed is discretized, but in this case into 256 values. In each location in the table are stored control parameters A for controlling the machine to provide the corresponding speed and torque. As mentioned above, each of the parameters A could be a series of control parameters or could be a single control parameter, with each location of the table containing a pointer to other tables to enable other parameters to be found. Other methods of storing the data are known in the art, for example, a sparse matrix of 16×16 locations could be used and some method of interpolation used in real-time to determine the appropriate parameter values.
FIGS. 3 and 4
show examples of control maps that correspond to control law tables. In these, the horizontal axis represents speed and extends up to a maximum of 256 units, which is representative of the maximum speed of the drive. The solid line represents the maximum torque available at any speed. Any one point on the map corresponds to a location in the control law table holding the parameter(s) required to operate at that point. The example of
FIG. 3
is representative of a system with a relatively constant torque output, whereas the example of
FIG. 4
is representative of a system in which the torque output changes significantly with speed.
During operation of the drive, the control system provides the control law table with signals representing the speed of the motor and the desired torque and thus finds the firing angles appropriate to that operating condition. These firing angles are used to control the switches and thus control the energization of the phase windings. The use of motor controllers with control law tables is generally understood and is not discussed herein.
In order to minimize the effort involved in the characterizing process, it is common practice to keep the DC link voltage constant during the process. Realizing that the DC link voltage will typically vary when the drive system is
McClelland Michael Leo
Quinn Jonathan Richard
Dicke Billig & Czaja, PLLC
Masih Karen
Switched Reluctance Drives Ltd.
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