Method for operating a permanent magnet synchronous machine and arrangement with a permanent magnet synchronous machine

Abstract

The invention relates to a method for operating an electrical machine (1) having a stator with at least two phase windings and a rotor, wherein the rotor is connected in a torque-transmitting manner to a rotatably mounted load (20) having an imbalance. After the electrical machine (1) has been switched off, the at least two phase windings of the electrical machine (1) are short-circuited to one another. The invention further relates to an arrangement having an electrical machine (1) and a computing unit (3).

Description

The present invention relates to a method for operating a permanent magnet-excited synchronous machine and to a computing unit, to a computer program for carrying out the method and to an arrangement having a permanent magnet-excited synchronous machine.BACKGROUND OF THE INVENTIONIn the case of a mixing drum (for example concrete mixer), the material located in the drum can also exert a restoring force on the drum drive when stationary, which restoring force is usually designed hydraulically with a transmission. The restoring force is obtained in that the center of gravity of the material in the drum during stopping is not located centrally and therefore not centrally below the axis of rotation of the drum, but rather is at a horizontal distance from the axis of rotation. When the torque is turned off in such a state in a drum which is at a standstill, the drum is set in motion due to this restoring force and oscillates back and forth for some time until the potential energy is dissipated and the center of gravity is at the lowest position. This behavior is undesirable because it is uncontrolled and adversely affects the stability of the structure. If a hydraulically driven drum is stopped up to zero speed and the hydraulic valves are closed, the trapped oil column between the hydraulic motor and the valve ensures a holding effect which holds the drum in its position or which slowly passively discharges it into a position with lower potential energy, since leakage slowly reduces the pressure of the oil column.Disclosure of the InventionAccording to the invention, a method for operating a permanent magnet-excited synchronous machine and a computing unit, a computer program for carrying out the method and an arrangement with a permanent magnet-excited synchronous machine having the features of the independent patent claims are proposed. Advantageous embodiments are the subject matter of the dependent claims and of the following description.The invention is based on an electric machine, namely a permanent magnet excited synchronous machine (PSM), having a stator with at least two phase windings and a rotor. The rotor is connected in a torque-transmitting manner to a rotatably mounted load which has an imbalance and thus a restoring force when stationary. The rotatably mounted load is, for example, a drum, in particular a mobile or stationary mixing drum, in particular a mixer, which has an imbalance due to the material located in the drum. Furthermore, the load can be, for example, a winch, in particular a cable winch, or a drive of a vehicle which has a restoring force, for example on account of a slope. Generally, the load may be a load that applies torque to the electric machine rotor in a zero speed electric machine that results in uncontrolled motion when the actively controlled electric machine drive torque is shut down.By means of the restoring force, the electrically driven and rotatably mounted load moves or oscillates in an uncontrolled manner if it is not braked after the electric machine has been switched off, since an electric machine in the switched-off state, in contrast to a hydraulic drive, does not apply any decelerating torque.In the method, the at least two phase windings of the stator of the electric machine are short-circuited after the latter has been switched off.By an active short circuit of the motor phases of the permanent magnet excited synchronous machine and the movement of the rotor, a current is induced in the electric machine, by means of which current energy is dissipated. The movement of the mass can be damped by the characteristic delaying torque occurring due to the induced current, so that the mass slowly enters an energy-favorable state in which the center of gravity of the load (e.g. drum includes. The material) in the lowest position is transferred with the center of gravity of the material being right below the rotational axis without being spaced from the rotational axis in the horizontal direction. As a result, uncontrolled oscillation and long relaxation of a load that is not held by a mechanical holding brake are damped or prevented.By using a permanent magnet excited synchronous machine with its characteristic speed-dependent braking torque, which occurs when the rotor is set in motion via an external force while the motor phases are short-circuited, the motion can be damped quickly at a very low speed, so that no long-lasting oscillation occurs.In one embodiment, the at least two phase windings of the electric machine are short-circuited for a predefined period of time, wherein the predefined period of time is in particular at least 10 s or at least 5 s or at least 1 s and at most 90 s or at most 300 s or at most 600 s. The predetermined period of time is selected in particular such that the horizontal distance of the imbalance from the axis of rotation or the restoring force is completely compensated by movement of the load.In one embodiment, the at least two phase windings are short-circuited until a current through the phase windings of the electric machine or a rotational speed or a torque of the electric machine falls below a threshold value or a predefined angle has been covered by the rotor of the electric machine. This can ensure that uncontrolled oscillations of the mass no longer occur.A computing unit according to the invention, e.g. a control unit of an electric machine, is configured, in particular by programming, to carry out a method according to the invention.The implementation of a method according to the invention in the form of a computer program or computer program product with program code for carrying out all method steps is also advantageous since this causes particularly low costs, in particular if an executing control device is also used for further tasks and is therefore present in any case. Suitable data carriers for providing the computer program are, in particular, magnetic, optical and electrical memories, such as hard disks, flash memories, EEPROMs, DVDs, among others. Download of a program via computer networks (Internet, intranet, etc.) is also possible.The invention further relates to an arrangement having an electric machine with a stator having at least two phase windings and a rotor, wherein the rotor is connected in a torque-transmitting manner to a rotatably mounted load, in particular via a transmission, and a computing unit according to the invention.The arrangement is, for example, a drive with which a load such as a drum, a winch or a vehicle is driven.A truck mixer, i.e. a truckable concrete mixer, with such an arrangement also constitutes an aspect of the invention.Further advantages and embodiments of the invention will become apparent from the description and the accompanying drawing.It is understood that the features mentioned above and those still to be explained below can be used not only in the respectively specified combination, but also in other combinations or alone, without departing from the scope of the present invention.The invention is schematically illustrated in the drawings on the basis of exemplary embodiments and is described in detail below with reference to the drawings.DESCRIPTION OF THE FIGURESFIG. 1 shows an arrangement with an electric machine according to an embodiment of the invention, which is connected to a rotatably mounted load in a torque-transmitting manner; FIG. 2 shows a cross section of a mixing drum with unequally distributed material; and FIG. 3 shows an exemplary characteristic speed-dependent profile of the torque and of the short-circuit current of a permanent-magnet-excited electric synchronous machine, in which at least two phase windings are short-circuited to one another.Detailed Description of the DrawingsFIG. 1 shows an arrangement 10 with an electric machine 1 according to an embodiment of the invention, which is connected to a rotatably mounted load 20 in a torque-transmitting manner. The arrangement 10 can be, in particular, a drive for a drum, a winch or a vehicle, for example a (mixing) drum drive of a stationary mixer or mobile mixer (truck mixer).The electric machine 1 has a stator with at least two, for example three, phase windings and a rotor via which the electric machine 1 is connected to the load 20 in a torque-transmitting manner. The load 20 is, for example, a drum, such as a mobile or stationary mixing drum, a winch, such as a cable winch, or a drive of a vehicle which is placed on a slope and therefore has a restoring force.The arrangement 10 furthermore has, for example, a transmission 2, via which the electric machine 1 is connected to the rotatably mounted load 20. Thereby, the rotational speed and the torque applied to the load can be adjusted.In addition, the arrangement 10 has a computing unit 3. The electric machine 1 is configured for this purpose and is controlled by the computing unit 3 in such a way that a short circuit is produced between the at least two (or all) phase windings of the electric machine 1 when the electric machine 1 is switched off. The short circuit is established in particular independently of the circumstances under which the electric machine 1 is switched off. In this case, the short circuit can be produced if the electric machine 1 is normally switched off or is switched off by a fault.Due to the short circuit of the at least two phase windings of the stator of the electric machine 1, a current is induced in the phase windings of the electric machine 1 by the movement of the load 20, which generates a torque opposite to the movement of the load 20. As a result, the load is slowly transferred into an energy-more favorable state without the load 20 oscillating in an uncontrolled manner. Depending on the load and the short-circuit torque characteristic, the system can nevertheless easily oscillate, but this oscillation is damped and therefore controlled.Figure 2 shows the cross-section of a mixing drum, which is an example of a load 20, with unequally distributed material, for example concrete. The unequally distributed material, the center of gravity S of which is not directly vertically below the axis of rotation D of the drum, but offset by a distance A from the axis of rotation D, ensures imbalance of the mixing drum. As a result of the imbalance, after the electric machine 1 is switched off, the mixing drum would oscillate or move in an uncontrolled manner until the potential energy of the material has dissipated and the center of gravity S of the material is located vertically centrally below the axis of rotation D, i.e. the distance A assumes a value of 0. By short-circuiting the at least two phase windings of the electric machine 1, such a vibration is prevented or at least damped, since the rotational movement is braked by the current induced in the electric machine 1.FIG. 3 shows an exemplary characteristic speed-dependent profile of the torque and of the short-circuit current of a permanent-magnet-excited electric synchronous machine, in which at least two phase windings are short-circuited to one another. The rotational speed n of the synchronous machine is plotted on the x-axis and the curves show the profile of the torque M (solid line) and of the associated current I (dashed line) which is established in the electric machine 1. If a permanent magnet excited synchronous machine with short-circuited motor phases is set in motion by an external force (here the non-uniformly distributed material in the mixing drum and the restoring force occurring as a result), a rotational speed-dependent torque opposite to the motion is generated by the electric machine 1, since a current flows through the phase windings. The current flow dissipates energy, as a result of which the movement of the load is decelerated.

Claims

Method for operating a permanent magnet-excited synchronous machine (1) having a stator with at least two phase windings and a rotor, wherein the rotor is connected in a torque-transmitting manner to a rotatably mounted load (20) which has an imbalance, the method comprising: short-circuiting the at least two phase windings of the permanent magnet-excited synchronous machine (1) after the electric machine (1) is switched off.Method according to Claim 1, wherein the at least two phase windings of the permanent-magnet-excited synchronous machine (1) are short-circuited for a predefined period of time.The method of claim 2, wherein the predetermined time period is at least 10 s or at least 5 s or at least 1 s and at most 90 s or at most 300 s or at most 600 s.Method according to Claim 1, wherein the at least two phase windings of the permanent-magnet-excited synchronous machine (1) are short-circuited until a current which flows through the at least two phase windings of the permanent-magnet-excited synchronous machine (1) or a rotational speed or a torque of the permanent-magnet-excited synchronous machine (1) has fallen below a threshold value or a predefined angle has been covered by the rotor of the permanent-magnet-excited synchronous machine (1).Method according to one of the preceding claims, wherein the rotatably mounted load (20) is a drum, in particular without a mechanical holding brake, in particular a mobile or stationary mixing drum, a winch, in particular a cable winch, or a drive of a vehicle.A computing unit (3) comprising a processor configured to perform the method of any preceding claim.A computer program comprising instructions which, when the program is executed by a computer, cause the computer to carry out the method of claims 1 to 5.Computer-readable data medium on which the computer program according to Claim 7 is stored.Arrangement (10) having a permanent magnet-excited synchronous machine (1), having a stator having at least two phase windings and a rotor, wherein the rotor is connected in a torque-transmitting manner to a rotatably mounted load (20) which has an imbalance, and having a computing unit according to Claim 6.Arrangement (10) according to Claim 9, wherein the rotor of the permanent-magnet-excited synchronous machine (1) is connected to the rotatably mounted load (20) via a transmission (3).

Images