Patent Application
20190356254
This application claims priority to German Patent Application No. DE 10 2017 112 388.1, filed Jun. 6, 2017, which is incorporated by reference herein in its entirety.
The present invention relates to a method for operating a synchronous machine having a permanent-magnet rotor. The present invention furthermore relates to a corresponding synchronous machine, to a corresponding computer program and to a corresponding storage medium.
In power electronics and electrical engineering, control concepts in which sinusoidal alternating variables—or alternating variables which are assumed to be largely sinusoidal—such as voltages or currents cannot be controlled directly in respect of their instantaneous time value but rather in respect of an instantaneous value which is corrected by the phase angle within the periods, in summary called field-oriented or vector control. To this end, the detected alternating variables are each transferred into a coordinate system which rotates at the frequency of the alternating variables. Identical variables, to which all customary methods of control engineering can be related, are then produced from the alternating variables within this rotating coordinate system.
For practical reasons, a Cartesian coordinate system comprising two axes d and q which are perpendicular to one another is generally selected, in particular for field-oriented control of synchronous machines. Therefore, the actuation signals for a three-phase four-quadrant actuator are formed by way of space vector modulation (space vector pulse width modulation, SVPWM) on the basis of suitable reference values for the corresponding current components.
In order to achieve a required torque, different combinations of the two phase currents id and iq generally come into consideration. In this case, that combination with the lowest phase current and therefore maximum torque per ampere (MTPA) is often selected in practice. In this way, the copper losses of the machine which occur primarily in the stator should be largely minimized.
CN104319969, which is incorporated by reference herein, and US2013207589, which is incorporated by reference herein, disclose control methods for an electrical machine, in particular a permanent-magnet synchronous machine for electric and hybrid vehicles, in which methods the currents id and iq are analyzed and eddy current and hysteresis losses are minimized.
DE102010050344, which is incorporated by reference herein, discloses a control method for an electrical machine in which iron losses are generally calculated.
US2005073280, which is incorporated by reference herein, US2013006593, which is incorporated by reference herein, US2013119900, which is incorporated by reference herein, WO04073156, which is incorporated by reference herein, US2016028343, which is incorporated by reference herein, and U.S. Pat. No. 6,605,912, which is incorporated by reference herein, disclose further control methods in which magnetic noise is partially reduced and the torque ripple is compensated for.
The invention provides a method for operating a synchronous machine having a permanent-magnet rotor, a corresponding synchronous machine, a corresponding computer program and a corresponding storage medium as claimed in the independent claims.
In this case, the method according to aspects of the invention is based on the finding that the rotor often becomes too hot for constant powers in very powerful machines. In permanent-magnet synchronous machines, a rotor of this kind comprises sheet metal (iron) and magnets; therefore, the rotor losses are made up substantially of iron losses and magnet losses.
Therefore, a combination of the two phase currents id and iq, which does not necessarily minimize the copper losses, but rather the hysteresis losses, in order to relieve the rotor of loading is selected for the purpose of achieving a required torque.
It is therefore proposed, in accordance with the—two- or three-dimensional—finite element method (FEM), to calculate the magnetic and iron losses depending on id, iq and the rotation speed. Since the rotor constitutes the critical component in respect of heat dissipation and therefore continuous power of the synchronous machine, a combination of id and iq which minimizes losses can be found. Overshooting of the minimum phase current in the sense of the MTPA approach has to be accepted in the process.
One advantage of the proposed solution is therefore the improved constant power of a machine which is driven according to aspects of the invention. Further advantageous refinements of the invention are specified in the dependent claims.
An exemplary embodiment of the invention is illustrated in the drawings and will be described in greater detail below.
Using the two-phase system model (process 11) used as part of field-oriented control as a basis, the iron and magnetic losses which occur in the rotor can be numerically approximated (process 12) as a function of the first phase current, of the second phase current and of a specified rotation speed of the synchronous machine on the basis of the results of the simulation. The function obtained in this way can serve as an evaluation function as part of an optimization problem which—for the specified rotation speed and a required torque—seeks to minimize the rotor losses by selecting an optimum combination of the two phase currents id and iq. The combination which is found by solving the optimization problem can now be used in the conventional manner as a reference for controlling (process 13), for example, the electric motor (20) illustrated in
This method (10) can be implemented, for example, using software or hardware or in a mixed form comprising software and hardware, for example in a motor controller.
