Method for controlling a reluctance motor

Abstract

A method for controlling a reluctance motor with a rotor and a stator, wherein the rotor has a plurality of rotor segments and the stator has a number of coils dependent thereon, for example, eight coils in the case of six rotor segments, with a control device which applies a voltage to a coil of a respective phase of the stator, wherein at a time when a first coil is already excited, namely when a rotor segment moves towards the coil, the second coil following in the direction of rotation of the rotor can already be additionally excited, and wherein a position detection of the rotor is carried out by means of sensors, characterized in that the excitation of the first coil is carried out in accordance with a predetermined, but changeable time window and, within the time window, in accordance with the position detection of the rotor, the following coil is excited in the sense of a phase overlap,wherein the phase overlap is maintained until the expiry of the time window, and wherein, to avoid a single tone, a fixed rotational frequency of the rotor is composed of rotor steps of different movement intensities, whereby the rotor steps recurring after a rotation of 360° also each differ in their movement intensity.

Description

[0001] The invention relates to a method for controlling a reluctance motor with a rotor and a stator, wherein the rotor has a plurality of rotor segments and the stator has a number of coils dependent thereon, for example eight coils in the case of six rotor segments, with a control device which applies a voltage to a coil of a respective phase of the stator, wherein at a time when a first coil is already excited, namely when a rotor segment moves towards the coil, the second coil following in the direction of rotation of the rotor can already be additionally excited and wherein a position detection of the rotor is carried out by means of sensors.

[0002] In switched reluctance motors, which are well known, the smooth running and starting of the rotor depend on its position relative to the stator field. Depending on the rotor's position relative to the stator field, the torque can be so small, especially at low speeds, that the rotor stops.

[0003] This disadvantage can be counteracted by additionally exciting the coil following the rotor's direction of rotation at the time the first coil is already excited. Such a method is known from DE 102 29 443 A1. The content of this patent application is hereby incorporated in its entirety into the disclosure of the present invention, also for the purpose of incorporating features of this patent application into the claims of the present invention. This measure counteracts the risk of uneven rotor running and, even, of it stopping.

[0004] In view of the prior art described above, a technical problem of the invention is to provide a method of the type in question by means of which improved control of a reluctance motor running at a very high speed can be achieved even at very low speeds.

[0005] This problem is initially and essentially solved by the subject matter of claim 1, wherein the excitation of the first coil is carried out in accordance with a predetermined, but changeable, time window and within the time window the following coil is excited in the sense of a phase overlap in accordance with the position detection of the rotor, wherein the phase overlap is maintained until the expiry of the time window, and wherein, in order to avoid a single tone, an essentially fixed rotational frequency of the rotor is composed of rotor steps of different movement intensities, wherein the rotor steps recurring after a rotation of 360° also each differ in their movement intensities.The time window preferably extends beyond the so-called aligned position, which represents the dead center position of the rotor segment relative to the coil following in the direction of rotor rotation. This means that, unlike when energized with just one phase, the rotor no longer tends towards unpleasant oscillating movements but rather is gently braked as a result of the generator operation that begins after the aligned position. The rotor reaches a position that is stable in the figurative sense. The phase overlap is maintained until the phase energization time has elapsed. Sensors then evaluate the rotor position reached. The control system monitors, among other things, the time it takes for the rotor to reach the position responsible for switching on the next phase.By switching on the second phase, a quasi-random energization time results for the time range in which one phase and possibly both phases are switched on. This also significantly improves the acoustic behavior, as individual tones can no longer be identified. This acoustic optimization is achieved primarily by dividing a rotor step into two ranges by the position responsible for switching on the next phase: a range before the position in which only one phase is energized, and a range after this position in which two phases are energized. The time until the decisive position is reached is fundamentally random, i.e. not constant, which means that no individual tone is acoustically audible. Rather, each step has a different frequency, resulting in a more pleasant sound overall.In addition to this advantageous acoustic impression, the method according to the invention also results in a visually modified rotor rotation behavior. Particularly at very low speeds of less than 100 revolutions per minute, a slightly hesitant running can be observed, which results from the alternating acceleration in motor mode and braking in generator mode, each within a phase current supply. This rotor rotation characteristic can be particularly advantageous when the method is used with a reluctance motor provided in a kitchen appliance for preparing food or the like, since the slightly hesitant running achieves continuous movement of the material to be stirred, but does so while avoiding undesired damage to the material, in particular its comminution.

[0006] In a preferred embodiment, the position for switching on the second coil is the position approximately 7.5° before the apex between the rotor segment and the first coil in the direction of rotor rotation. The apex position between the rotor segment and the first coil assigned in the direction of rotor rotation is the aligned position already mentioned. Accordingly, according to the invention, the switch-on point for the second coil following in the direction of rotor rotation is the position -7.5° before the aligned position. It is also advantageous if the current profile is changed depending on the time required by the rotor to reach the position responsible for switching on the further phase. The time window can also be changed depending on the load. At high loads, it may be more advantageous if the switch-on position for the second coil is reached earlier.In a further embodiment, the applied load can be responded to in an advantageous manner. For example, depending on a possibly repeated failure to reach the position decisive for triggering the energization of the further phase, a reversal of the direction of rotation of the rotor can be effected. The rotor is then moved in the opposite direction in a targeted manner. This can prevent jamming of material or the like in the vessel when the reluctance motor to be controlled by the method according to the invention is arranged, for example, in a mixing vessel intended for food preparation. A sufficiently severe jamming could destroy and possibly unintentionally crush the material. This provides a sensitively reacting method for controlling a reluctance motor. The reversal of direction is preferably only maintained over a predetermined angle of rotation, for example around 90°.Alternatively, the reversal can also be time-limited. It is also intended that the current used to excite the first phase is not reduced below a minimum current. Setting a minimum current for motor operation in a stage of the reluctance motor at very low speed results in a characteristic rotational behavior. The motor does not run smoothly, but rather in steps. This is supported by a corresponding operating noise, which provides a clear visual and acoustic distinction from normal motor operation.

[0007] According to the invention, in order to avoid a single tone, a rotor rotation frequency that is essentially fixed (for the observer) is composed of rotor steps of different movement intensities that cannot be determined by the observer. The rotor steps that recur after a rotation of 360° also differ in their movement intensities. A rotor step comprises an acceleration phase and a deceleration phase. Preferably, the motor current is controlled by evaluating the rotor position reached within such a rotor step. Position detection can be performed using sensors that, in an exemplary embodiment of the motor as a four- or six-reluctance motor, i.e., a motor with four rotor segments and six stator poles, can detect the rotor within a rotation angle of approximately 15°.With regard to the rotor position detection as well as the arrangement of the sensors and the evaluation of the sensor signals generated by the rotor rotation, reference is made to DE 102 29 443 A1 mentioned above.

[0008] A reluctance motor is preferably used, which enables speeds of up to 12,000 revolutions per minute and more. The method can therefore also be used with an 8 / 6-SR motor equipped with sensors, for example with two forked light barriers. Such a sensor system is known from DE 102 29 443 A1 mentioned above. This delivers 24 pulses per revolution of the rotor and simultaneously specifies a fixed relationship between the rotor position and stator phase. The sensor system mechanically assigns the range from -22.5° to -7.5° in front of the aligned position of a phase. With the method according to the invention, when the -7.5° position is reached, the next phase is switched on simultaneously, meaning that the rotor, from a purely static perspective, can reach a position of up to approximately +10° behind the aligned position of the last switched-on phase.In contrast to single-phase power supply, this avoids the unpleasant oscillating motion of the rotor. Instead, the rotor is gently decelerated and reaches a quasi-stable position. The phase overlap is maintained until the phase power supply time has elapsed. The sensors can then evaluate the position reached. There are three possible scenarios: a) the rotor does not reach the -7.5° position with respect to the aligned position, b) the rotor is in the range -7.5° to + 7.5° with respect to the aligned position, c) the rotor is in the range >+7.5° with respect to the aligned position.

[0009] The motor current is adjusted depending on the position reached.

[0010] In the attached diagram, the currents of two phases PH arranged one after the other in the direction of rotor rotation are shown. 1 and PH 2which are carried out according to the recorded time window t. The latter can be changed depending on the detected rotor position after the expiration of the time window, which accordingly also affects the current supply to the phases PH 1 and PH 2 and thus also influences the phase overlap. For example, the switch-off time of the first phase in the direction of rotor rotation PH 1 Depending on the position detection, the current supply to phase PH preferably extends 1 but beyond this point up to 7.5° behind the aligned position.

[0011] The time window t can also be chosen (as shown in the dashed line) so that the subsequent phase PH 2It is only switched on in the angular range between -7.5° and the aligned position, and the first phase is switched off in the angular range between the aligned position and +7.5°. Accordingly, the phase overlap is selected to be smaller.

[0012] The method reacts very tolerantly to load changes. Overshoot tendencies are very low. Furthermore, load changes are compensated for very quickly. The rotor speed can be adjusted using the method according to the invention in the range of 0 to 500 revolutions per minute, preferably 0 to 200 revolutions per minute.

[0013] As a result of the method according to the invention, a motor that is nominally designed for much higher speeds can also be operated at very low speeds. The use of an appropriately controlled reluctance motor in a cooking / stirring appliance used for food preparation has the advantageous effect of reducing damage to the food being cooked, compared to conventional control. It is detected whether the food is jammed in the container, which results in a reaction in the form of the rotor rotating backwards. Phase overlap reduces torque ripple, and the rotor's tendency to oscillate is reduced by utilizing the braking effect when the motor exceeds the aligned position. The method according to the invention proves to be particularly advantageous in that it allows a gearless reluctance motor to be operated at speeds of 0 to 12,000 revolutions per minute and more.In principle, the method according to the invention can be used for starting SR motors, since an automatic adaptation to the load conditions can be achieved.

Claims

[1] Method for controlling a reluctance motor with a rotor and a stator, wherein the rotor has a plurality of rotor segments and the stator has a number of coils dependent thereon, for example eight coils in the case of six rotor segments, with a control device which applies a voltage to a coil of a respective phase of the stator, wherein at a time when a first coil is already excited, namely when a rotor segment moves towards the coil, the second coil following in the direction of rotation of the rotor can already be additionally excited and wherein a position detection of the rotor is carried out by means of sensors, characterized bythat the excitation of the first coil is carried out in accordance with a predetermined, but changeable time window and within the time window the following coil is excited in the sense of a phase overlap in accordance with the position detection of the rotor, wherein the phase overlap is maintained until the expiry of the time window, and wherein in order to avoid a single tone a fixed rotational frequency of the rotor is composed of rotor steps of different movement intensities, wherein the rotor steps recurring after a rotation of 360° also each differ in their movement intensities. [2] Method according to claim 1, characterized by that the position for switching on the second coil is the position in the direction of rotor rotation approximately 7.5° before the apex between the rotor segment and the first coil. [3] Method according to claim 1 or 2, characterized bythat the current profile is changed depending on the time required for the rotor to reach the position responsible for switching on the next phase. [4] Method according to one of the preceding claims, characterized by that the time window is changed depending on the load. [5] Method according to one of the preceding claims, characterized by that, depending on a possibly repeated failure to reach the position required to trigger the energization of the further phase, a reversal of the direction of the rotor rotation is effected. [6] Method according to claim 5, characterized by that the reversal of direction is only maintained over a given angle of rotation. [7] Method according to one of the preceding claims, characterized by that the current with which the first phase is excited is not reduced below a minimum current. [8] Method according to one of the preceding claims, characterized by that a rotor step comprises an acceleration phase and a deceleration phase. [9] Method according to one of the preceding claims, characterized by that the motor current is controlled by evaluating the rotor position reached within a rotor step.

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