Electricity: motive power systems – Synchronous motor systems – Hysteresis or reluctance motor systems
Reexamination Certificate
2001-05-25
2003-01-21
Fletcher, Marlon (Department: 2837)
Electricity: motive power systems
Synchronous motor systems
Hysteresis or reluctance motor systems
C318S254100, C318S721000
Reexamination Certificate
active
06509710
ABSTRACT:
TECHNICAL FIELD
The present invention relates to the driving of motors commonly referred to as “SRM”, an abbreviation for Switched Reluctance Motor.
BACKGROUND OF THE INVENTION
A motor of this type may essentially be regarded as a stepper motor in which the rotor and the stator are in the form of salient poles. Unlike a conventional stepper motor, however, an SRM has a rotor which does not possess the permanent magnets necessary for creating the magnetic rotor flux and the rotor essentially consists of a stack of iron laminations. Owing to this particular feature, the SRM is generally classified as a single-flux synchronous motor. Moreover, owing to the geometry of the stator and rotor pack, the reluctance of the magnetic circuit which is formed depends, at each instant, on the position of the rotor and the phase energized.
This results in a periodic modulation effect of the inductance of each stator winding which is reflected in the problem of polarization of a variable inductance.
The conventional techniques used for the driving of SRMs envisage the sequential energization of the stator phases in synchronism with the current position of the rotor. It is therefore of fundamental importance to know the position of the rotor. Although a large number of solutions which do not use sensors are mentioned in literature on the subject, the most common commercial solution remains hitherto that which envisages reading of the rotor position by means of optical or Hall-effect sensors keyed onto the rotor.
Owing to the information supplied by the sensors it is relatively simple to construct the phase energization sequence to be sent to the power switches which connect the stator windings to the continuous power supply of the power stage.
The exact knowledge of the positions in which the inductance has its minimum value allows the corresponding phase to be energized with a rapidly increasing current (when the winding is excited by a voltage generator), while knowledge of the points where the inductance has its maximum value allows the braking effect associated with the generation of negative torques to be avoided.
Consequently, the degree of efficiency of an SRM is closely associated with the capacity of the associated driving unit to provide phase currents which as far as possible have a rectangular waveform, i.e. have leading and trailing edges which are ideally vertical so as to be able to energize and de-energize the associated phase in as short a time as possible.
FIGS. 1 and 2
of the accompanying drawings illustrate the typical procedures for driving an SRM in conditions of low speed of rotation (
FIG. 1
) and high speed of rotation (FIG.
2
).
In particular, in both the figures, the uppermost diagram, indicated by a), illustrates the trend of the ideal inductance L.
In
FIG. 1
, the two lower diagrams, indicated by b
1
) and b
2
), illustrate respectively the ideal trend and the actual trend of the current I in conditions of low speed of rotation. Similarly, the diagram b) in
FIG. 2
illustrates the actual trend of the current in high-speed driving conditions.
Finally, the two diagrams indicated by c) illustrate the trend of the voltage V to be applied, again with reference, respectively, to the low-speed and high-speed driving conditions.
Observing
FIGS. 1 and 2
it can be understood that the electrical and mechanical time constants of the motor impose limits on the rising and falling speed of the current when the phase is energized using a constant-voltage generator. These time constants of the motor give rise to a phase energization delay or, in other words, conditions such that the winding current reaches its mean value with a rising time which is excessively long.
In particular, at high speeds, the electrical time constant L/R becomes comparable to the energization time Ton of the phase: since the phase must be de-energized even before the current has reached its rated value, it therefore becomes impossible to reach the desired mean current value (and therefore the desired torque level).
Substantially similar problems exist for middle-range speeds and high loads for which equally high mean currents are required.
In an attempt to avert these negative phenomena, the solution of introducing an advance in energization of the phase to be polarized is known in the art. In other words, with this solution, current starts to be injected a short time before the minimum inductance value of the phase. This is performed in order to reach the appropriate current level during the positive-torque inductance zone.
Introduction of an energization advance in principle may be achieved by displacing the position sensors a few mechanical degrees forwards or backwards (in the opposite direction to the direction of rotation of the rotor). However, with the sensors keyed onto the rotor in a permanent manner it is possible to achieve an optimum result only for a predefined and single advance value. The result is that an optimum current waveform is obtained only for predefined speeds, with a decrease in efficiency for the whole of the remainder of the motor-speed range.
Various solutions have been proposed, which solutions, based on the information supplied by the sensors, calculate the optimum energization instants so as to provide different advances for the various speeds of the rotor and the different load conditions.
These solutions, however, encounter problems associated with computational difficulties. The option of limiting the complexity of the calculation relating to the advance value results in an excessively simple operation which is therefore less than optimal. If a more refined solution is required, machine-calculated equations must be used, resulting in the need to use high performance microcontrollers (which are therefore costly) or DSPs.
For example, the patent U.S. Pat. No. 5,977,740 discloses a control system which energizes in a selective manner the winding of a motor so as to maximize the efficiency and the output torque, taking into account possible variations in the speed of the rotor, the temperature of the windings and the power supply voltage. More specifically, this document describes a vehicle equipped with a motor in which the advance is controlled in order to brake the vehicle. Control of the advance is performed using a DSP and complex mathematical calculations.
U.S. Pat. No. 5,942,865 describes a solution in which the advance of a motor is controlled by means of analog circuitry. Besides the constructional complexity, this solution has the drawback that the advance may be set only manually using potentiometers.
SUMMARY OF THE INVENTION
An embodiment of the present invention provides a solution for the control of motors of the SRM type which is able to reconcile the need for low-cost and easily implemented solutions (for example using an 8-bit microcontroller) with the need for a sophisticated driving diagram (preferably using fuzzy-type logic systems).
Essentially, the invention proposes a solution providing an alternative to solutions which involve calculation of the phase energization instants. This is performed in a preferred manner by means of characterization of the advance values using fuzzy-logic models.
In particular, a solution according to the invention allows control of the necessary phase energization advance in relation to the speed and load conditions of the motor. A solution according to the invention may be implemented in a particularly advantageous manner in three-phase motors which can be used, for example, in electric household appliances.
In any case, a solution according to the invention is applicable to any number of phases and may be used in widely varying areas of application.
In a preferred manner, a solution according to the invention is based on the use of fuzzy-type logic in order to provide an expert system capable of maximizing the electromechanical efficiency of the motor at every working point in the system.
Owing to this expert system, it is possible to optimize the energy consumption and the maximum torque which can be output by the sai
Grasso Giuseppe
Zichella Giovanni
Fletcher Marlon
Iannucci Robert
Jorgenson Lisa K.
Seed Intellectual Property Law Group PLLC
STMicroelectronics S.r.l.
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