Method for starting a synchronous motor

Abstract

A method for starting a synchronous motor is provided. The synchronous motor includes a rotor for creating a first magnetic field and a stator with stator windings connected to an electrical energy converter for converting a supply voltage into a stator voltage to be applied to the stator windings to create a rotating second magnetic field interacting with the first magnetic field. The method includes applying reference stator voltages to the stator windings, where the reference stator voltages are determined from a reference current vector and a reference rotor speed, measuring stator currents, calculating an estimated rotor speed and rotor position from the applied stator voltages and the measured stator currents, calculating a speed error by subtracting the estimated rotor speed from the reference rotor speed, determining a reference torque producing current component from the speed error, and modifying the reference current vector with the reference torque producing current component.

Claims

1. A method for starting a synchronous motor, the synchronous motor comprising a rotor for creating a first magnetic field and a stator with stator windings connected to an electrical energy converter for converting a supply voltage into a stator voltage to be applied to the stator windings to create a rotating second magnetic field interacting with the first magnetic field, the method comprising: applying reference stator voltages to the stator windings, wherein the reference stator voltages are determined from a reference current vector and a reference rotor speed; measuring stator currents in the stator windings; calculating an estimated rotor speed and an estimated rotor position of the rotor from the applied reference stator voltages and the measured stator currents; calculating a speed error by subtracting the estimated rotor speed from the reference rotor speed; determining a reference torque producing current component from the speed error; modifying the reference current vector with the reference torque producing current component; calculating a position error by subtracting the estimated rotor position from a reference rotor position, wherein the reference rotor position is determined from the reference rotor speed and a reference rotor speed correction, wherein the reference rotor speed correction increases and decreases with the position error, wherein the reference rotor speed correction is subtracted from the reference rotor speed to determine a corrected reference rotor speed, and wherein the reference rotor position is determined by integrating the corrected reference rotor speed; correcting the reference current vector by transforming the reference current vector into a corrected reference current vector by the position error, wherein a rotating coordinate system of the corrected reference current vector is aligned with the estimated rotor position; determining switching signals for the electrical energy converter from the reference stator voltages; and applying the switching signals to the electrical energy converter.

2. The method of claim 1, further comprising: determining from the position error whether the estimated rotor position is accepted as correct or not, when the estimated rotor position is accepted as correct: using the estimated rotor position as the reference rotor position and changing a magnitude of the reference current vector to an initial value for normal operation of an electric drive system comprising the synchronous motor; and/or when the estimated rotor position is not accepted as correct: changing the magnitude of the reference current vector according to a predefined magnitude profile and/or changing the reference rotor speed according to a predefined rotor speed profile.

3. The method of claim 2, wherein an absolute value of the position error is compared to a predefined threshold value, and wherein the estimated rotor position is accepted as correct when the absolute value of the position error stays below the predefined threshold value for a predefined time period.

4. The method of claim 3, further comprising: determining a reference magnetizing current component from the reference torque producing current component and a reference magnitude of the reference current vector; and modifying the reference current vector with the reference magnetizing current component.

5. The method of claim 4, wherein the reference magnetizing current component is calculated with:
i.sub.sd**=√{square root over (I.sub.s.sup.2*−i.sub.sq.sup.2**)}

6. The method of claim 5, wherein the reference rotor speed correction is determined as a function of the position error.

7. The method of claim 6, wherein the reference torque producing current component is determined dependent on the reference magnitude of the reference current vector.

8. The method of claim 7, further comprising: determining a reference voltage vector from the corrected reference current vector and the measured stator currents; transforming the reference voltage vector into a stationary three-phase coordinate system; and determining the switching signals by pulse-width modulating phase voltages of the transformed reference voltage vector.

9. The method of claim 1, further comprising: determining a reference magnetizing current component from the reference torque producing current component and a reference magnitude of the reference current vector; and modifying the reference current vector with the reference magnetizing current component.

10. The method of claim 9, wherein the reference magnetizing current component is calculated with:
i.sub.sd**=√{square root over (I.sub.s.sup.2*−i.sub.sq.sup.2** )}

11. The method of claim 10, wherein the reference rotor speed correction is determined as a function of the position error.

12. The method of claim 1, wherein the reference rotor speed correction is determined as a function of the position error.

13. The method of claim 1, wherein the reference torque producing current component is determined dependent on a reference magnitude of the reference current vector.

14. The method of claim 1, further comprising: determining a reference voltage vector from the corrected reference current vector and the measured stator currents; transforming the reference voltage vector into a stationary three-phase coordinate system; and determining the switching signals by pulse-width modulating phase voltages of the transformed reference voltage vector.

15. A computer program for starting a synchronous motor, the synchronous motor comprising a rotor for creating a first magnetic field and a stator with stator windings connected to an electrical energy converter for converting a supply voltage into a stator voltage to be applied to the stator windings to create a rotating second magnetic field interacting with the first magnetic field, which, when executed on a processor, is adapted to start the synchronous motor comprising by directing the processor to: apply reference stator voltages to the stator windings, wherein the reference stator voltages are determined from a reference current vector and a reference rotor speed; measure stator currents in the stator windings; calculate an estimated rotor speed and an estimated rotor position of the rotor from the applied refernece stator voltages and the measured stator currents; calculate a speed error by subtracting the estimated rotor speed from the reference rotor speed; determine a reference torque producing current component from the speed error; modify the reference current vector with the reference torque producing current component; calculate a position error by subtracting the estimated rotor position from a reference rotor position, wherein the reference rotor position is determined from the reference rotor speed and a reference rotor speed correction, wherein the reference rotor speed correction increases and decreases with the position error, wherein the reference rotor speed correction is subtracted from the reference rotor speed to determine a corrected reference rotor speed, and wherein the reference rotor position is determined by integrating the corrected reference rotor speed; correct the reference current vector by transforming the reference current vector into a corrected reference current vector by the position error, wherein a rotating coordinate system of the corrected reference current vector is aligned with the estimated rotor position; determine switching signals for the electrical energy converter from the reference stator voltages; and apply the switching signals to the electrical energy converter.

16. A non-transitory computer-readable medium in which the computer program according to claim 15 is stored.

17. A controller for an electrical energy converter, wherein the controller is adapted to start a synchronous motor, the synchronous motor comprising a rotor for creating a first magnetic field and a stator with stator windings connected to the electrical energy converter for converting a supply voltage into a stator voltage to be applied to the stator windings to create a rotating second magnetic field interacting with the first magnetic field, wherein the controller, to start the synchronous motor, is configured to: apply reference stator voltages to the stator windings, wherein the reference stator voltages are determined from a reference current vector and a reference rotor speed; measure stator currents in the stator windings; calculate an estimated rotor speed and an estimated rotor position of the rotor from the applied reference stator voltages and the measured stator currents; calculate a speed error by subtracting the estimated rotor speed from the reference rotor speed; determine a reference torque producing current component from the speed error; modify the reference current vector with the reference torque producing current component; calculate a position error by subtracting the estimated rotor position from a reference rotor position, wherein the reference rotor position is determined from the reference rotor speed and a reference rotor speed correction, wherein the reference rotor speed correction increases and decreases with the position error, wherein the reference rotor speed correction is subtracted from the reference rotor speed to determine a corrected reference rotor speed, and wherein the reference rotor position is determined by integrating the corrected reference rotor speed; correct the reference current vector by transforming the reference current vector into a corrected reference current vector by the position error, wherein a rotating coordinate system of the corrected reference current vector is aligned with the estimated rotor position; determine switching signals for the electrical energy converter from the reference stator voltages; and apply the switching signals to the electrical energy converter.

18. An electric drive system, comprising: the controller of claim 17; the synchronous motor comprising the rotor for creating the first magnetic field and the stator with the stator windings; and the electrical energy converter connected to the stator windings and adapted to convert the supply voltage into the stator voltage to be applied to the stator windings to create the rotating second magnetic field interacting with the first magnetic field.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The subject matter of the invention will be explained in more detail in the following text with reference to exemplary embodiments which are illustrated in the attached drawings.

(2) FIG. 1 schematically shows an electric drive system according to an embodiment of the invention.

(3) FIG. 2 schematically shows the transformation component from FIG. 1 in more detail.

(4) FIG. 3 shows a diagram of a predefined magnitude profile which may be used with the electric drive system from FIG. 1.

(5) FIG. 4 shows a diagram of a predefined rotor speed profile which may be used with the electric drive system from FIG. 1.

(6) FIG. 5 schematically shows the current controller from FIG. 1 in more detail.

(7) FIG. 6 shows a flow diagram of a method for starting a synchronous motor according to an embodiment of the invention.

(8) FIG. 7 shows a diagram of different drive signals with disabled reference rotor speed correction according to an embodiment of the invention.

(9) FIG. 8 shows a diagram of different drive signals with enabled reference rotor speed correction according to an embodiment of the invention.

(10) The reference symbols used in the drawings, and their meanings, are listed in summary form in the list of reference symbols. In principle, identical parts are provided with the same reference symbols in the figures.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

(11) FIG. 1 shows an electric drive system 100 with a synchronous motor 102, an electrical energy converter 104 and a controller 106. The synchronous motor 102, which may be a permanent-magnet synchronous machine (PMSM), comprises a stator 108 with stator windings 110 which are each connected to outputs of the electrical energy converter 104. A rotor 112 is configured to rotate within the stator 108. In order to create a first magnetic field, the rotor 112 comprises one or more permanent magnets 114 which may be mounted on and/or buried within the rotor 112. Additionally or alternatively, the rotor 112 may comprise a number of electrical windings to create the first magnetic field. The stator windings 110 are arranged around the rotor 112. The electrical energy converter 104 is connected to an electrical grid 116 providing an AC supply voltage V.sub.cc. The electrical energy converter 104 is configured to convert the supply voltage V.sub.cc into a three-phase AC voltage in the form of three stator voltages V.sub.sx, v.sub.sy, v.sub.sz based on switching signals s.sub.1, s.sub.2, s.sub.3, e. g., pulse-width modulation or space vector modulation signals, generated by the controller 106. The stator voltages V.sub.sx, V.sub.sy, v.sub.sz are applied to respective terminals of the stator windings 110. An electrical current through the stator windings 110 sets up a rotating second magnetic field within an air gap between the rotor 112 and the stator 108. The interaction between the two magnetic fields causes the rotor 112 to rotate, producing torque. The speed and torque of the synchronous motor 102 may be controlled by controlling the current through the stator windings 110.

(12) The synchronous motor 102 may be controlled using field-oriented control (FOC) techniques without any sensors or encoders. In this case, the flux and torque components of the stator currents are controlled independently by the controller 106 based on a reference rotor speed ω.sub.r**, which may be an external speed reference signal, and an estimated rotor position {tilde over (θ)}.sub.r, estimated based on a back electromagnetic force (back-EMF) calculated from quantities of the stator windings 110. This implies that the synchronous motor 102 must be rotating at a minimum speed for a sufficient amount of back-EMF to be detected by the controller 106 to accurately calculate the estimated rotor position {tilde over (θ)}.sub.r. Therefore, the controller 106 is configured to perform a startup procedure in order to start the synchronous motor 102 from zero speed.

(13) FIG. 1 depicts a block diagram of an algorithm implemented in the controller 106 for starting the synchronous motor 102 from standstill. During startup, a reference current generator 118 generates a reference magnitude I.sub.s* for a reference current vector with a reference magnetizing component i.sub.s** and a reference torque producing component i.sub.sq**. For example, the reference current generator 118 may output the reference magnitude I.sub.s* according to a desired profile as shown in FIG. 3, which may be a ramping function ramping the reference magnitude I.sub.s* up to a specified value and keeping it constant until the estimated rotor position {tilde over (θ)}.sub.r is accepted as correct. The reference current generator 118 may then change the reference magnitude I.sub.s* to a value defined as an initial value for a torque control mode.

(14) A reference speed generator 122 generates a reference rotor speed ω.sub.r** for the rotor 112. The reference rotor speed ω.sub.r** may also be generated according to a desired profile, e. g., ramped up from a specified minimum value to a specified maximum value, as shown in FIG. 4.

(15) Flux estimation is active from the very beginning of the startup procedure. An estimator block 124 comprises a rotor flux estimator 126 for calculating an estimated rotor flux {tilde over (Ψ)}.sub.mfdq and a rotor position estimator 128 for calculating the estimated rotor position {tilde over (θ)}.sub.r and an estimated rotor speed {tilde over (ω)}.sub.r based on the estimated rotor flux {tilde over (Ψ)}.sub.mfdq. The estimated rotor position {tilde over (θ)}.sub.r and/or the estimated rotor speed {tilde over (ω)}.sub.r may be calculated in a phase-locked loop (PLL).

(16) The estimated rotor flux {tilde over (Ψ)}.sub.mfdq is calculated based on the applied stator voltages V.sub.sx, V.sub.sy, V.sub.sz and stator currents i.sub.sx, i.sub.sy measured in the stator windings 110. In the example illustrated in FIG. 1, only two phases i.sub.sx, i.sub.sy of the stator current are measured, since a third phase may be calculated by the controller 106 based on the measurements of the other two phases i.sub.sx, i.sub.sy.

(17) The measured stator currents i.sub.sx, i.sub.sy as well as the applied stator voltages V.sub.sx, V.sub.sy, V.sub.s, may be provided in a stationary three-phase xyz coordinate system. Prior to calculating the estimated rotor flux {tilde over (Ψ)}.sub.mfdq, the measured stator currents i.sub.sx, i.sub.sy and the applied stator voltages V.sub.sx, V.sub.sy, V.sub.s, may be transformed into a stationary orthogonal αβ coordinate system by a transformation component 130, as illustrated in FIG. 2.

(18) For damping of rotor speed oscillations, the estimated rotor speed W.sub.r is compared with the reference rotor speed ω.sub.r** in a speed comparator 132 which subtracts the estimated rotor speed {tilde over (ω)}.sub.r from the reference rotor speed ω.sub.r** to generate a speed error ω.sub.e as an error signal. The speed error ω.sub.e is amplified by a torque controller 134, e. g., a PI controller, which generates the reference torque producing current component i.sub.sq**. A limit controller 136 limits the reference torque producing current component i.sub.sq** according to a given value of the reference magnitude I.sub.s*.

(19) Furthermore, a reference rotor position θ.sub.r* is calculated by a position reference generator 138, which may be an integrator for integrating the reference rotor speed ω.sub.r*. A position comparator 140 subtracts the estimated rotor position {tilde over (θ)}.sub.r from the reference rotor position θ.sub.r* to generate a position error θ.sub.e as an error signal.

(20) The position error θ.sub.e is input to a reference position corrector 142 which amplifies the position error θ.sub.e with an appropriate gain factor and outputs a reference rotor speed correction ω.sub.c. The reference position corrector 142 is configured to increase and decrease the reference rotor speed correction ω.sub.c in the same proportion as the position error θ.sub.e. Alternatively, the reference rotor speed correction ω.sub.c and the position error θ.sub.e may be modified in different proportions.

(21) The reference rotor speed correction ω.sub.c is used by a reference speed corrector 144 to calculate a corrected reference rotor speed ω.sub.r*, for example, by subtracting the reference rotor speed correction ω.sub.c from the reference rotor speed ω.sub.r** as output by the reference speed generator 122. The corrected reference rotor speed ω.sub.r* is then input to the position reference generator 138 for calculating the reference rotor position θ.sub.r*.

(22) The reference rotor speed correction ω.sub.c is to be understood as an additional rotation of the reference rotor position θ.sub.r* towards the estimated rotor position {tilde over (θ)}.sub.r. This results in faster convergence of θ.sub.r* and {tilde over (θ)}.sub.r and thus in better oscillation damping and shorter startup time.

(23) To provide the reference current vector during startup, the reference magnetizing current component i.sub.sd** has to be calculated in addition to the reference torque producing current component i.sub.sq**. The reference magnetizing current component i.sub.sd** is calculated by a current magnitude limiter 148 based on the reference magnitude I.sub.s* and the reference torque producing current component i.sub.sq**. For example, the reference magnetizing current component i.sub.sd**, which is orthogonal to component i.sub.sq**, may be calculated as the square complement to the reference magnitude I.sub.s* with i.sub.sd**=√{square root over (I.sub.s.sup.2*−i.sub.sq.sup.2** )}. Prior to entering the current magnitude limiter 148, the reference magnitude I.sub.s* and the reference torque producing current component i.sub.sq** are each squared in a squaring component 149. The current magnitude limiter 148 then subtracts the squared reference torque producing current component i.sub.sq.sup.2** from the squared reference magnitude i.sub.s.sup.2*. A square rooting component 150 calculates the reference magnetizing current component i.sub.sd** from the resulting difference, i. e., the squared reference magnetizing current component i.sub.sd.sup.2**.

(24) The reference magnetizing current component i.sub.sd**, the reference torque producing current component i.sub.sq** and the position error θ.sub.e are each input to a reference transformation component 151 which is configured to transform the reference magnetizing current component i.sub.sd** and the reference torque producing current component i.sub.sq** by the position error θ.sub.e from a reference rotating orthogonal dq* coordinate system into a rotating orthogonal dq coordinate system aligned with the estimated rotor position {tilde over (θ)}.sub.r. The resulting corrected reference current vector has a corrected reference magnetizing current component i.sub.sd* and a corrected reference torque producing component i.sub.sq*.

(25) A current controller 152 receives both the corrected reference torque producing current component i.sub.sq*, and the corrected reference magnetizing current component i.sub.sd*. The current controller 152 compares the corrected reference torque producing current component i.sub.sq* to a measured and transformed stator current i.sub.sq, and the corrected reference magnetizing current component i.sub.sd* to a measured and transformed stator current i.sub.sd in order to generate the switching signals s.sub.1, s.sub.2, s.sub.3, as it will be described in more detail in FIG. 5.

(26) For example, the estimated rotor position {tilde over (θ)}.sub.r is accepted as correct by the reference positon corrector 142 when the absolute value of the position error θ.sub.e stays below a given threshold for a certain amount of time. In this case, the reference current generator 118 may ramp the reference magnitude I.sub.s* to a value used as an initial value for a normal operation control scheme. This initial value may be equal to the reference torque producing current component i.sub.sq** generated by the torque controller 134. After that, the startup procedure is considered as successfully finished and the controller 106 switches to a closed-loop torque or speed control mode based on FOC. In this case, the orientation angle of the rotating dq coordinate system changes from the reference rotor position θ.sub.r* to the estimated rotor position {tilde over (θ)}.sub.r. Furthermore, the current references i.sub.sd* and i.sub.sq* are changed to values generated by a torque and flux control loop. In this way, seamless switching from the startup procedure to the normal operation of the synchronous motor 102 may be achieved.

(27) The different components of the controller 106 may be realized in hardware and/or in software. The controller 106 may also comprise a processor and a memory for storing instructions which, when being executed by the processor, may perform the method as described above and below.

(28) FIG. 2 schematically shows the transformation component 130 from FIG. 1 in more detail. The transformation component 130 comprises a first transformation unit 200 for transforming the measured stator currents i.sub.sx, i.sub.sy from the stationary three-phase xyz coordinate system to the stationary orthogonal αβ coordinate system by performing a Clarke transformation. The resulting stator currents i.sub.s∝, i.sub.sβ are then transformed to the rotating orthogonal dq coordinate system by a second transformation unit 202 which performs a Park transformation based on the estimated rotor position {tilde over (θ)}.sub.r. As a result, the second transformation unit 202 outputs the measured and transformed stator currents i.sub.sd, i.sub.sq which are used by the current controller 152 to generate the switching signals s.sub.1, s.sub.2, s.sub.3 for controlling the electrical energy converter 104.

(29) The transformation component 130 may be configured to transform the applied stator voltages V.sub.sx, V.sub.sy in an analogous manner. The reference transformation component 151 may have only the transformation component 202.

(30) FIG. 3 shows a diagram of a predefined magnitude profile 300, as it may be stored in the reference current generator 118 of FIG. 1. According to this example, the magnitude profile 300 comprises a first portion 302 in the form of a rising curve. The first portion 302 transitions into a constant second portion 304 in the form of a horizontal line. Alternatively, the reference magnitude profile 300 may only comprise the rising curve. The rising curve may be a linear or non-linear function.

(31) FIG. 4 shows a diagram of a predefined rotor speed profile 400, as it may be stored in the reference speed generator 122 of FIG. 1. According to this example, the rotor speed profile 400 has the form of a rising curve, which may be linear or non-linear function for ramping the reference rotor speed ω.sub.r** from a predefined minimum up to a predefined maximum.

(32) FIG. 5 schematically shows the current controller 152 from FIG. 1 in more detail. The current controller 152 comprises a first current comparator 500 for generating a first error signal 502 by subtracting i.sub.sd, as generated by the transformation component 130, from i.sub.sd*, as generated by the reference transformation component 151. The first error signal 502 is amplified by a first PI controller 504 to generate a reference d-component voltage V.sub.sd*.

(33) Furthermore, the current controller 152 comprises a second current comparator 506 for generating a second error signal 508 by subtracting i.sub.sq, as generated by the transformation component 130, from i.sub.sq*, as generated by the reference transformation component 151. The second error signal 508 is amplified by a second PI controller 510 to generate a reference q-component voltage V.sub.sq*. A voltage transformation component 512 is configured to transform the reference d-component voltage V.sub.sd* and the reference q-component voltage V.sub.sq* into the stationary three-phase xyz coordinate system by inverse Park and Clarke transformations. The resulting three reference stator voltages V.sub.sx*, V.sub.sy*, V.sub.sz* are to be applied to the stator windings 110. To achieve this, the reference stator voltages V.sub.sx*, V.sub.sy*, V.sub.sz* are input to a modulation component 514 configured to generate the switching signals s.sub.1, s.sub.2, s.sub.3 by pulse-width modulating the stator voltages V.sub.sx, V.sub.sy, V.sub.sz, each of the switching signals s.sub.1, s.sub.2, s.sub.3 corresponding to one modulated voltage. The stator voltages V.sub.sx, V.sub.sy, V.sub.sz may be modulated with a space vector modulation algorithm implemented in the modulation component 514.

(34) FIG. 6 shows a flow diagram of a method 600 for starting the synchronous motor 102 from FIG. 1 without a position and speed sensor.

(35) In a step 610, the stator voltages V.sub.sx, V.sub.sy, V.sub.s, are applied to the stator windings 110.

(36) In a step 620, the resulting stator currents i.sub.sx, i.sub.sy are measured in the stator windings 110.

(37) In a step 630, the measured stator currents i.sub.sx, i.sub.sy as well as the applied stator voltages V.sub.sx, V.sub.sy, V.sub.sz are used to estimate the rotor position {tilde over (θ)}.sub.r and the rotor speed {tilde over (ω)}.sub.r based on back-EMF created by rotation of the rotor 112.

(38) In a step 640, the estimated rotor speed {tilde over (ω)}.sub.r is compared with the reference rotor speed ω.sub.r** to determine the speed error ω.sub.e.

(39) In a step 650, the speed error ω.sub.e is used to determine the reference torque producing current component i.sub.sq**.

(40) In a step 660, the estimated rotor position {tilde over (θ)}.sub.r is subtracted from the reference rotor position θ.sub.r* to determine the position error θ.sub.e. The reference rotor position θ.sub.r* is determined from the corrected reference rotor speed ω.sub.r*, e.g., by integration of the corrected reference rotor speed ω.sub.r*. The corrected reference rotor speed ω.sub.r* is calculated based on the reference rotor speed ω.sub.r** by subtracting the reference rotor speed correction ω.sub.c which is calculated based on the position error θ.sub.e. It may be that an absolute value of the position error θ.sub.e is compared to a predefined threshold value in an optional step 662. When the absolute value of the position error θ.sub.e is smaller than the threshold value for at least a predefined period of time, the reference rotor position θ.sub.r* is set to a value of the estimated rotor position {tilde over (θ)}.sub.r in an optional step 664. This may be achieved by setting the position error θ.sub.e and the reference rotor speed correction ω.sub.c to zero. Also, the magnitude of the reference current vector may be changed to an appropriate initial value for normal operation of the synchronous motor 102 to ensure smooth transition to a closed-loop torque or speed control mode.

(41) On the other hand, when the absolute value of the position error θ.sub.e is equal to or greater than the threshold value, the reference rotor speed correction ω.sub.c may be calculated based on the position error θ.sub.e in an optional step 666 to minimize the position error θ.sub.e in a further correction loop. In other words, the greater the position error θ.sub.e, the greater the reference rotor speed correction ω.sub.c, and vice versa. Briefly summarized, the reference rotor position θ.sub.r* may be corrected in the direction of the estimated rotor position {tilde over (θ)}.sub.r by changing the reference rotor speed ω.sub.r* accordingly.

(42) In a step 670, the reference current vector modified with the reference torque producing current component i.sub.sq** is transformed from the reference dq* coordinate system by the position error θ.sub.e into the dq coordinate system which is aligned with the estimated rotor position {tilde over (θ)}.sub.r. In other words, once the position error θ.sub.e is accepted as correct, e. g., set to zero, the reference rotor position θ.sub.r* is identical to the estimated rotor position {tilde over (θ)}.sub.r.

(43) In a step 680, the switching signals s.sub.1, s.sub.2, s.sub.3 are generated based on the corrected reference current vector components i.sub.sd* and i.sub.sq*.

(44) During startup of the synchronous motor 102, the reference magnitude I.sub.s* may be changed from a specified minimum value up to a specified maximum value and kept constant until the estimated rotor position {tilde over (θ)}.sub.r is accepted as correct. Then, the reference magnitude I.sub.s* may be changed to a value defined as an initial value for a torque controller. Also during startup, the reference rotor speed ω.sub.r** may be changed from a specified minimum value up to a specified maximum value.

(45) FIG. 7 depicts a timing chart to illustrate oscillations of the rotor speed of the synchronous motor 102 from FIG. 1 when started from standstill with disabled reference rotor speed correction. The timing chart shows a curve 700 of a motor current, a curve 702 of an estimated motor speed, a curve 704 of a measured motor speed, a curve 706 of a measured rotor position, and a curve 708 of an estimated rotor position. One can clearly see rotor speed oscillations during starting, which last about 2.3 s.

(46) FIG. 8 depicts a timing chart to illustrate the effectiveness of the oscillation damping when the synchronous motor 102 from FIG. 1 is started from standstill with enabled reference rotor speed correction. The parameters shown are the same as in FIG. 7. Oscillations are almost completely damped. The starting time is reduced to about 1 s. This may be the point where the controller 106 switches to normal operation mode, e. g., closed-loop torque or speed control of the synchronous motor 102.

(47) While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art and practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single processor or controller or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

LIST OF REFERENCE SYMBOLS

(48) 100 electric drive system 102 synchronous motor 104 electrical energy converter 106 controller 108 stator 110 stator winding 112 rotor 114 permanent magnet 116 electrical grid 118 reference current generator 122 reference speed generator 124 estimator block 126 rotor flux estimator 128 rotor position estimator 130 transformation component 132 speed comparator 134 torque controller 136 limit controller 138 position reference generator 140 position comparator 142 reference position corrector 144 reference speed corrector 148 stator current magnitude limiter 149 squaring component 150 square rooting component 151 reference transformation component 152 current controller 200 first transformation unit 202 second transformation unit 300 reference magnitude profile 302 rising portion 304 constant portion 400 reference speed profile 500 first current comparator 502 first error signal 504 first PI controller 506 second current comparator 508 second error signal 510 second PI controller 512 voltage transformation component 514 modulation component 600 method for starting a synchronous motor 610 applying stator voltages 620 measuring stator currents 630 calculating an estimated rotor speed and an estimated rotor position 640 calculating a speed error 650 determining a reference torque producing current component 660 calculating a position error 662 comparing the position error to a threshold value 664 using the estimated rotor position as the reference rotor position 666 changing a magnitude of a reference current vector and/or a reference rotor speed 670 correcting the reference current vector 680 determining switching signals 700 curve of a motor current 702 curve of an estimated motor speed 704 curve of a measured motor speed 706 curve of a measured rotor position 708 curve of an estimated rotor position dq rotating orthogonal coordinate system i.sub.s∝ transformed stator current i.sub.sβ transformed stator current i.sub.sd transformed stator current i.sub.sq transformed stator current i.sub.sx measured stator current i.sub.sy measured stator current i.sub.sd** reference magnetizing current component i.sub.sq** reference torque producing current component [i.sub.sd** i.sub.sq**] reference current vector i.sub.sd* corrected reference magnetizing current component i.sub.sq* corrected reference torque producing current component [i.sub.sd* i.sub.sq*] corrected reference current vector I.sub.s* magnitude of the reference current vector s.sub.1 switching signal s.sub.2 switching signal s.sub.3 switching signal t time V.sub.cc supply voltage V.sub.sd* reference d-component voltage V.sub.sq* reference q-component voltage [V.sub.sd* V.sub.sq*] reference voltage vector [V.sub.sx* V.sub.sy* V.sub.sz*] transformed reference voltage vector V.sub.sx applied stator voltage V.sub.sy applied stator voltage V.sub.sz applied stator voltage V.sub.sx* reference stator voltage, reference phase voltage V.sub.sy* reference stator voltage, reference phase voltage V.sub.sz* reference stator voltage, reference phase voltage xyz stationary three-phase coordinate system αβ stationary orthogonal coordinate system {tilde over (Ψ)}.sub.mfdq estimated rotor flux θ.sub.e position error θ.sub.r* reference rotor position {tilde over (θ)}.sub.r estimated rotor position ω.sub.c reference rotor speed correction ω.sub.e speed error ω.sub.r* corrected reference rotor speed ω.sub.r** reference rotor speed {tilde over (ω)}.sub.r estimated rotor speed