Electricity: motive power systems – Switched reluctance motor commutation control
Reexamination Certificate
2002-04-22
2004-05-04
Nappi, Robert (Department: 2837)
Electricity: motive power systems
Switched reluctance motor commutation control
C318S135000, C318S128000
Reexamination Certificate
active
06731083
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
Aspects of the present invention relate to flux control systems and, more particularly, to flux control systems for use in motion control applications. Aspects of the present invention also relate to the control of electric machines, such as switched reluctance machines, permanent magnet machines and hybrids thereof.
2. Description of Related Art
In many electromagnetic systems, the transfer of energy from one component of the system to another is critical to proper operation of the system. In many electromagnetic systems, this transfer of energy is accomplished by appropriately energizing one component of the system to establish a magnetic flux that interacts with another component of the system to transfer energy from the energized component to the other component. Despite the fact that the energy transfer is accomplished by the flux, in known electromagnetic systems the flux of the system is not directly controlled. Instead, the current and/or voltage applied to the energized member is controlled and, based on assumed relationships between current, voltage and flux, it is assumed that the control of the current and/or voltage based on the assumed relationships will produce the appropriate flux. Control of current and/or voltage is typically implemented, at least in part, because the prior art has not provided an efficient, low-cost, and easily implemented system for directly controlling flux in an electromagnetic system.
One drawback of current and/or voltage control systems as described above is that the relationships between current, voltage and flux are not easily represented mathematically and vary in a non-linear manner depending on a variety of variables. For example, the particular characteristics of each piece of magnetic material in a system will result in voltage, current and flux relationships that vary from one system to another and, even within a given system, from one section of the system to another. Because of these differing voltage, current and flux relationships, it is difficult to accurately and properly control the currents and/or voltages to produce the desired flux and, thus, the desired energy transfer. As such, the prior art is limited in its ability to provide an electromagnetic system in which flux is directly controlled.
The lack of an appropriate flux control system in the prior art is particularly noticeable in electromagnetic systems where it is desired to finally control the force exerted by one component of the system on another component of the system. In such systems, the actual force produced by the system is related to the flux established by the energized component of the system. As described above, however, because the prior art cannot directly and finely control flux, it cannot, therefore, finely control the force produced by such systems. The inability of the prior art to finely control the forces established in an electromagnetic system is particularly acute in applications where the movement of at least one component of the system must be precisely controlled.
The typical switched reluctance machine comprises a rotor defining rotor poles, a stator defining stator poles, and a set of windings arranged in relation to the stator poles to define one or more phases. In a reluctance machine, energization of one or more phase windings sets up a magnetic flux in the associated stator poles, urging the rotor into a position of minimum reluctance. Timing the sequential energization of the windings according to rotor position induces rotor movement. Switched reluctance machines are well known. More detail is provided in the paper ‘The Characteristics, Design and Applications of Switched Reluctance Motors and Drives’ by Stephenson and Blake, presented at the PCIM '93 Conference and Exhibition at Nurnberg, Germany, Jun. 21-24, 1993, which is incorporated herein by reference. As is well known in the art, these machines can be operated as motors or generators simply by altering the timing of the application of the excitation to the phase windings.
As explained in the Stephenson and Blake paper, the method of torque production in a switched reluctance machine is quite different from that in conventional machines, e.g. induction or synchronous machines, which are operated by rotating waves of magneto-motive force (mmf) and in which the torque is produced by the interaction of a magnetic field with a current flowing in a conductor. Such machines are known as ‘electromagnetic’ machines and encompass, e.g., so-called brushless DC machines in which the current is in stator coils and the field is produced by permanent magnets on the rotor. These machines require the use of permanent magnets.
By contrast, switched reluctance machines are purely ‘magnetic’ machines, where the torque is produced solely by the magnetic field as the reluctance of the magnetic circuit changes. It follows that the methods of controlling the two types of machine are typically quite different, since the control is related to the method of torque production. In general, the control methods used for conventional, sinusoidally fed machines have been considered quite inappropriate for switched reluctance machines.
FIG. 1
shows a typical switched reluctance machine in cross section. In this example, the stator
10
has six stator poles
12
, and the rotor
14
has four rotor poles
16
. Each stator pole carries a coil
18
. The coils on diametrically opposite poles are connected in series to provide three phase windings. Only one phase winding is shown, for clarity. The control of the switched reluctance machine can be achieved in a variety of ways. The machine could be controlled in an open-loop fashion, i.e. as commonly used for stepping motors. In this regime, the phase windings in the machine are sent pulses in turn and it is assumed that the rotor lines up with each pair of stator poles in turn, i.e. the position of minimum reluctance for that phase which is excited. Of course, because the system is open-loop, there are no means of knowing if the rotor has moved or not. To remove this uncertainty, it is conventional to use a rotor position detection scheme of some sort which provides a signal representative of rotor position. The excitation can then be applied as a function of the position. Such machines are often referred to as “rotor position switched machines”.
Since current in the windings is relatively easy to measure, closed-loop current control is commonly accomplished by monitoring and controlling the energizing current in the windings. However, the desired output of the machine is usually torque, position or speed, and current has a highly non-linear relationship to all of these. The result is that current control techniques generally have inaccuracies in the output, such as torque ripple, position error or speed error.
A typical switched reluctance drive is shown in FIG.
2
. In this example, the machine
36
corresponds to that shown in FIG.
1
. The three phase windings A, B and C are switched onto a d.c. supply V by a set of power electronic switches
48
. The moments at which the switches operate are determined by the controller
38
, which may be implemented either in hardware or in the software of a microcontroller or digital signal processor. The firing signals are sent to the switches via a data bus
46
. Closed loop current feedback is provided by sensing the phase currents by one or more current sensors
44
and feeding back signals proportional to phase current. The control algorithms often include a proportional (P), proportional-plus-integral (P+I), time optimal, feedback linearized, proportional/integral/derivative (PID) function, or one of many others as is well understood in the art. It is also common for an outer control loop of position or speed to be provided by feeding back a rotor position signal from a position detector
40
.
In operation, a current demand i
D
on line
42
is provided to the controller and this regulates the current in the windings, according to the particular cont
Colon Santana Eduardo
Dicke Billig & Czaja, PLLC
Nappi Robert
Switched Reluctance Drives Ltd.
LandOfFree
Flux feedback control system does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Flux feedback control system, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Flux feedback control system will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3197148