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1. (WO2018077718) METHOD AND APPARATUS FOR ADAPTING THE MAGNETIC CHARACTERISTICS OF A SYNCHRONOUS RELUCTANCE MOTOR
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METHOD AND APPARATUS FOR ADAPTING THE MAGNETIC CHARACTERISTICS OF A SYNCHRONOUS RELUCTANCE MOTOR

DESCRIPTION

Field of the invention

[0001] The present invention generally finds application to the field of electric machines, and particularly relates to a method of determining the magnetic characteristics of a synchronous reluctance motor and automatically adapting the motor to maximum efficiency conditions.

[0002] In a second aspect, the invention relates to an apparatus for implementing the aforementioned method on a synchronous reluctance motor.

Background art

[0003] In a reluctance motor, torque is known to be generated by magnetic anisotropy and maximization of this characteristic is known to be one of the main design specifications. Nevertheless, the anisotropic behavior of the machine is highly complex and depends on a great number of factors.

[0004] When the magnetomotive force applied to the stator is aligned with the maximum and minimum reluctance axes, the resulting flux characteristics λdo and λqo are those experimentally obtained by bench measurements, as shown in FIG. 1.

[0005] Nevertheless, these flux curves do not fully describe the magnetic behavior of the motor. Thus, when both components id and iq are other than zero, additional information is required. Generally, the fluxes λd are λq are functions of both currents, whereas the functions λdo and λqo simply represent two particular cases of the general functions

where


[0006] Methods are known for laboratory measurement of the magnetic characteristics of a synchronous reluctance motor. These methods usually require a test system, which comprises a constant-speed run motor for driving the motor to be tested, whereas current steps are applied to the motor to be tested, which generate electromotive forces (EMF) that can be measured using appropriate measurement instruments.

[0007] One method for laboratory measurement of the magnetic characteristics of a synchronous reluctance motor is known, for instance, from IT-A-1354129.

[0008] The main drawback of this known method is that a laboratory and the equipment for carrying out bench tests on the motor are not always available.

[0009] Furthermore, a very large number of current steps may be required to be applied to the run motor for the flux characteristics to be mapped with an appropriate accuracy, and this will considerably increase the costs of the test, which will lead to a dramatic increase of motor costs.

[0010] US2015/0226776 discloses a method of measuring the inductance of synchronous permanent-magnet motors, including the steps of applying a test voltage to the stator, measuring the response current in the stator while the motor is still, and determining the differential value of the response current using a digital filter.

[0011] No method is known in the art for adapting the magnetic

characteristics of a synchronous reluctance motor without using bench tests.

Technical Problem

[0012] In view of the prior art, the technical problem addressed by the present invention is deemed to consist in allowing adaptation of the magnetic characteristics of a synchronous reluctance motor without carrying out laboratory tests and in minimizing the number of characteristics to be determined while maintaining the same order of accuracy as would be obtained by experimental testing.

Disclosure of the invention

[0013] A general object of the present invention is to solve the aforementioned technical problem and obviate the above drawbacks, by providing a method of adapting the magnetic characteristics of a synchronous reluctance motor that is highly efficient and relatively cost-effective.

[0014] A particular object is to provide a method as defined above, that allows the characteristics of the motor to be mapped without requiring expensive laboratory testing.

[0015] A further object is to reduce the number of measurements of physical quantities taken by the system to reduce the complexity and costs of the method.

[0016] Yet another object is to ensure fully automated adaptation of the magnetic characteristics of the motor using the inverter and the electronic motor-control system.

[0017] These and other objects, as better explained hereafter, are fulfilled by a method of automatically adapting the magnetic characteristics of a synchronous reluctance motor as defined in claim 1.

[0018] According to another aspect, the invention provides an apparatus for carrying out the aforementioned method defined in the independent claim 15.

[0019] λdvantageous embodiments of the invention are defined in accordance with the dependent claims.

Brief description of the drawings

[0020] Further features and advantages of the invention will be more apparent from the detailed description of one preferred but non-exciusive embodiment of a method and an apparatus for adapting the magnetic characteristics of a synchronous reluctance motor according to the invention, which are described as a non-limiting example with the help of the annexed drawings, in which:

[0021] FIG. 1 shows the flux curves of a motor as experimentally measured in a laboratory according to the prior art.

[0022] FIG. 2 schematically shows an apparatus for adapting the magnetic characteristics of a synchronous reluctance motor of the invention;

[0023] FIG. 3 is a sectional view of the synchronous reluctance motor, with the axes of maximum and minimum reluctance being schematically arranged;

[0024] FIG. 4 is a functional diagram of the apparatus of FIG. 2;

[0025] FIG. 5 shows a flow chart of the inventive method;

[0026] FIG. 6 partially shows the current and voltage curves as experimentally measured;

[0027] FIG. 7 shows the diagrams of differential inductances detected with the inventive method, as compared with the corresponding diagrams that can be deduced from experimental testing;

[0028] FIG. 8 shows the flux curves as determined from the diagrams of differential inductances as shown in FIG. 6, also as compared with the corresponding flux curves deduced from experimental testing.

Detailed description of a preferred exemplary embodiment

[0029] The aforementioned figures show an apparatus for adapting the magnetic characteristics of a synchronous reluctance motor of the invention, designated by numeral 1.

[0030] Particularly, the apparatus 1 comprises a synchronous reluctance motor 2 having a stator, or a stationary portion 3, with a rotor 4 housed therein.

[0031] The motor 2 has at least one pair of axes D, Q of minimum magnetic reluctance and maximum magnetic reluctance respectively, as shown in FIG. 3 and a control unit 5 for controlling the power supply to the motor, preferably an inverter, which is connected to a three-phase supply network 6, with appropriate digital interfaces possibly associated therewith, such as a display 7 and a keypad 8 or a computer, not shown.

[0032] The motor 2 has rated current I and rated voltage V values that are set during design and can be experimentally verified on the bench. The supply current I has components id, iq along the axes of minimum and maximum reluctance.

[0033] Generally, the method comprises the steps of applying a predetermined number of voltage and current variations dV, dl to the stator with a predetermined frequency, determining differential inductances (Ldiff) in response to voltage and current variations, calculating the flux curves λd, λq of the motor by integrating the differential inductances Ldiff and mapping the curves in the control unit 5 for automatically adapting the magnetic characteristics of the motor with substantially the same accuracy as that of an experimental test.

[0034] Conveniently, the number of voltage and current variations may be kept to a relatively low level as compared with the measurements that are experimentally taken on the bench, for the method to be quicker and more cost-effective.

[0035] Particularly, as shown in FIG. 5, a step a) is initially provided, in which the rotor is aligned with the axis D of minimum reluctance.

[0036] Such alignment may be achieved by either of the following modes: a') applying a current pulse of predetermined magnitude to the stator, such magnitude being about equal to the rated current I, to cause the rotor to rotate through a predetermined angle a to a stationary position in which it is aligned with the axis D of minimum reluctance, or

a") identifying the angular position a after voltage or current injection, and using such position a as a landmark for later processing in the axes D and Q, while avoiding the rotation of the rotor.

[0037] The step a in which the rotor is angularly aligned is followed by a step b in which a first number m of first current variations Aid are injected along the axis of minimum reluctance, each having a value of about Idmax/m.

[0038] The variations Aid have stepwise increasing average values, with a first maximum value Idmax above the rated current I to cause saturation of the stator along the axis D of minimum reluctance, while maintaining a zero current component iq along the axis Q of maximum reluctance.

[0039] As an example the aforementioned first maximum current value Idmax is about 150% the rated current I.

[0040] The current injection step b corresponds to a step c of measuring the voltage Vd and the current Id along the axis of maximum reluctance Q in response to stepwise increasing current values to obtain first detected magnitudes Vd1 , Id1.

[0041] In principle, a voltage variation (dV) applied to the motor at a given current (I) and at a frequency (f), will lead to a variation (dl) and an average value (V) that will allow the differential inductance to be calculated with the function:


[0042] A second injection step d follows, in which a second number m of current variations Alq are injected, with second current values lqmax/n, along the axis of maximum reluctance Q, where the variations Iq have stepwise increasing average values Alq with a second predetermined maximum value Iqmax smaller than the rated current I to maintain the rotor in the initial stationary alignment position, and maintain a zero current component id along the axis of minimum reluctance D.

[0043] As an example the aforementioned second maximum current value Idmax is about 15% the rated current I.

[0044] Then, a second measuring step e follows, in which the voltage Vq

and the current Iq are measured along the axis of minimum reluctance Q in response to the second stepwise increasing current variation values injected in step d to obtain second detected magnitudes Vq2, Iq2.

[0045] The first Vd1 , Id1 and second Vq2, Iq2 detected magnitudes undergo a processing step f which is designed to determine appropriate motor-specific parameters LdO, IdSat, LqO, IqSat, LSat, XCross.

[0046] Now, the method includes a step g in which the differential values Ldiff are determined from the specific parameters LdO, IdSat, LqO, IqSat, LSat, XCross whose meaning is explained below.

[0047] LdO represents the differentia] inductance along the axis D with the ideal value Id being 0.

[0048] IdSat represents the current that ideally corresponds to a zero differential inductance along the axis D.

[0049] LqO represents the differential inductance along the axis Q with the ideal value Iq being 0.

[0050] IqSat represents the current that ideally corresponds to a zero differential inductance along the axis Q.

[0051] LSat represents the differential inductance at saturation, along the axes D and Q.

[0052] XCross represents the percent values that accounts for the crossed saturation factor.

[0053] The derivative of Ldiffd is calculated to detect the change of the characteristic slope. Then the trend line is plotted in the first linear section, as a first-order equation

resulting in:


[0054] The above discussed procedure is repeated with Ldiffq.

[0055] LqSat may be deemed to be equal to LdSat.

[0056] LSat is determined from the saturation value equal to the Ldiffd value obtained at the maximum current injection point.

[0057] XCross defines how anisotropy affects the differential inductance along the axis D.

[0058] This parameter may be approximated as:


and is saturated to the limit value


[0059] The aforementioned differential inductance values are obtained using the following functions [1], [2], [3] and [4]:

[0060] The use of differential inductances affords linearization of characteristics, which will afford limitation of the specific parameters to small number, for example and without limitation 6.

[0061] Steps a to g are controlled by an appropriate algorithm, which is initialized in the inverter 5 to process the differential inductances and map flux curves λd, λq of the motor with an accuracy close to that obtained experimentally.

[0062] Finally, the maps of flux curves λd, λq are used in the algorithm for automatic adaptation of the magnetic characteristics of the motor.

[0063] The maps obtained by the invention have such an accuracy as to afford sensorless motor control, although the method also applies to motor control using sensors.

[0064] The method requires no further hardware, in addition to the inverter that is typically associated with the motor.

[0065] While the method and apparatus have been described with particular reference to the accompanying figures, the numerals are only used for the sake of a better intelligibility of the invention and shall not be intended to limit the claimed scope in any manner.

Industrial applicability

[0066] The present invention may find application in industry, because it can be produced on an industrial scale in factories for manufacturing drives for three-phase synchronous reluctance motors.