Electricity: motive power systems – Synchronous motor systems – Hysteresis or reluctance motor systems
Reexamination Certificate
2002-03-29
2004-09-07
Nappi, Robert (Department: 2837)
Electricity: motive power systems
Synchronous motor systems
Hysteresis or reluctance motor systems
C318S705000, C318S703000
Reexamination Certificate
active
06788021
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to switched reluctance motors for motor vehicles in general, and more particularly to an automatic control of the turn-on angle used to excite the switched-reluctance motor.
2. Description of the Prior Art
In the automotive industry there is rapid expansion in the incorporation of electronic and electrical systems for vehicle control, passenger comfort and safety, pollution reduction and economy of production, running and maintenance. The modern road vehicle relies heavily on electric motors (and drives); this dependency will inevitably increase, but requires an alternative to the convenient but costly a/c or d/c electrical motors. The current trend in electric motor control is to design low cost, highly energy efficient and highly (time) reliable systems.
Automotive electronic systems, particularly those associated with the engine compartment, must operate under difficult environmental conditions (temperature extremes, vibration, EMI, dirt mixed with oil, moisture and gas). It is necessary to cope with these conditions using good engineering practice and design.
The switched-reluctance motor (SRM) produces torque through excitation that is synchronized to rotor position. The simplest excitation strategy for the SRM is generally described by three excitation parameters: the turn-on angle &thgr;
on
, the turn-off angle &thgr;
off
, and the reference current I
ref
. A control algorithm would typically use the same excitation parameters for each phase, implemented with the spatial shift consistent with the symmetrically displaced phase structure. Control of the excitation angles results in either positive net torque for motoring, or negative net torque for generating. Basic operation of the SRM is given in several articles, such as “Variable-speed switched reluctance motors”, P. J. Lawrenson, et al., IEE Proc., Vol. 127, pt. B, no. 4, pp. 253-265, 1980; “Switched Reluctance Motors and Their Control”, T. J. E. Miller, Oxford, 1993; and “Switched Reluctance Motor Drives”, R. Krishnan, CRC Press, 2001, which are hereby incorporated by reference.
Efficient operation of the SRM, or any motor drive, is always of importance. Inefficiency leads to larger size, increased weight, and increased energy consumption. In order to maximize SRM efficiency, the need exists to maximize the ratio of the average torque to RMS phase current, T
avg
/I
phrms
. This ratio captures the intended goal of providing the required electromechanical output with the minimum electrical input. This approach is valid for both drive applications that are tolerant of SRM torque ripple and applications that require extremely smooth torque production, though smooth torque production may require current shaping that cannot be characterized by the single parameter I
ref
.
While the self-tuning approach to optimization of excitation parameters, such as described by “Self-tuning control of switched reluctance motors for optimized torque per Ampere at all operating points” B. Fahimi, et al., Proc. Of the IEEE Applied Power Electronics Conf., pp. 778-783, 1998; and “A self-tuning controller for switched reluctance motors”, K. Russa, et al., IEEE Trans. On Power Electronics, Vol. 15, pp. 545-552, 2000; or the approach based on extensive lookup tables is know, this invention seeks to provide an automatic excitation angle control algorithm that supports efficient operation of the SRM over its entire speed region.
Thus, the need exists for a means to control the SRM that is simple, compact, inexpensive, providing better performance and efficiency, and greater reliability over the entire speed region of the SRM.
SUMMARY OF THE INVENTION
The present invention provides an improved control apparatus and method for a switched reluctance motor (SRM) for use with automobiles or other devices utilizing a switched reluctance motor.
Switched reluctance machines (SRM) are brushless d.c. machines, having neither brushes or permanent magnets, thereby minimizing potential maintenance and wear issues. The SRMs are durable and long lasting, with bearing life being the primary wear determinant. The SRMs are typically less expensive to manufacture due to fewer parts and less labor. The overall motor and drive system cost is largely a function of the cost of the electronic drive controller, depending on the level of sophistication required by the application.
Switched reluctance machines operate on the principle that a magnetic field that is created about a component formed from a magnetically permeable material will exert a mechanical force on that component. This mechanical force will urge the component to become aligned with the magnetic flux (lines of force) generated by the magnetic field. Thus, by using the stator to establish and rotate a magnetic field about a rotor formed from magnetically permeable material, the rotor can be driven to rotate relative to the stator. The resistance to the passage of this magnetic flux from the stator to the rotor is referred to as reluctance. The magnitude of this reluctance changes with the rotational position of the rotor relative to the stator. Thus, electric motors of this type are commonly referred to as variable reluctance motors.
Typically, the SRM conventionally comprises a generally hollow cylindrical stator having a plurality of radially inwardly extending poles formed thereon, and a rotor rotatably supported concentrically within the stator and provided with a plurality of radially outwardly extending poles, i.e., SRM is doubly salient. Windings of an electrically conductive wire are provided about each stator pole. However, no electrical conductor windings or permanent magnets are provided on the rotor that consists only of iron laminations. Interconnecting the stator windings forms phase windings. For an SRM with q phases, the coil around every qth stator pole would be connected with alternating magnetic polarity whereby magnetic flux is alternately directed toward the rotor and away from the rotor. SRM phase windings are made with both series and parallel connections of stator coils, according to the intentions of the designer.
Torque is produced by switching current into 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 the rotor and stator poles that are approaching each other. Thus, electric machines of this type are commonly referred to as switched reluctance machines. The current is switched off in each phase before the rotor poles nearest the stator poles of the phase rotate past the aligned position. Otherwise, the magnetic force of attraction would produce a negative or braking torque. The torque developed is independent of the direction of current flow so that unidirectional current pulses synchronized with rotor movement can be applied to develop torque in either direction. These pulses are generated by a converter using current switching elements such as thyristors or transistors.
In operation, each time a phase of the switched reluctance motor is switched on by closing a switch in a converter, current flows in the stator winding of that phase providing energy from a direct current (DC) supply to the motor. The energy drawn from the supply is converted partly into mechanical energy by causing the rotor to rotate toward a minimum reluctance configuration and partly in stored energy associated with the magnetic field. After the switch is opened, part of the stored magnetic energy is converted to mechanical output and part of the energy is returned to the DC source.
The switched reluctance machine can be utilized also as a generator. When operated as a generator, the SRM produces current rather than voltage. Braking torque is produced when winding current continues to flow after a rotor pole has passed alignment with an associated stator pole. Because the SRM has no rotor excitation, it is necessary to first draw electric power from a DC bus in order to cause current to begin flowing
Mese Erkan
Sozer Yilmaz
Torrey David A.
Dana Corporation
Liniak Berenato & White
Miller Patrick
Nappi Robert
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