Doubly fed induction motor

Abstract

Electric motor, in particular induction motor, comprising a stator, a rotor and a control device which is arranged at the rotor. The three rotor windings are connected to a Rotor Control device with inverter and controller unit mounted on the rotor. A capacitor is placed in the DC link. The capacitor is supplied from the EMF induced in the rotor. The current in the rotor windings is advanced in order to achieve a 90 degree phase shift between rotor current and stator MMF vector. To achieve this the frequency and amplitude of the rotor current as well as the phase shift can be varied. Wherein the frequency of the rotor inverter is matching the slip frequency.

Claims

1. Doubly fed electric motor, comprising a stator (20), a rotor (40), and a control device (60) which is arranged at the rotor (40), wherein the stator (20) is adapted to induce electromotive force (EMF) into the rotor (40) during operation, and wherein the rotor (40) has no less than three rotor windings (42), into which EMF is induceable, and which are electrically connected to the control device (60), and wherein the stator (20) has no less than three stator windings (43), and the control device (60) is adapted to vary the power factor based on and using the induced EMF in the rotor, and to vary or adjust at least one of electric currents in the rotor windings (42), and the angle of rotor magnetic flux vector, characterized in that the control device (60) is adapted to vary the phase, frequency, and/or the magnitude/amplitude of the rotor current using EMF induced to the rotor in such a way to achieve an optimum angle of 90 between the rotor magnetic flux vector and the stator magnetomotive force (MMF) vector.

2. Doubly fed electric motor according to claim 1, wherein the control device (60) is adapted to vary the frequency and/or the magnitude/amplitude of the rotor current to adapt the slip.

3. Doubly fed electric motor according to claim 1, wherein the rotor control device (60) comprises a rotor controller unit (61), a rotor inverter unit (66), an energy storage unit (74), a communication unit (84), and/or an electric potential measuring unit.

4. Doubly fed electric motor according to claim 3, wherein the rotor controller unit (61) is an active control device comprising at least one PI control loop (62).

5. Doubly fed electric motor according to any of the claim 3, wherein the energy storage unit (74) is connected to a switchable resistor.

6. Doubly fed electric motor according to claim 1, wherein the rotor control device (60) comprises an accelerometer sensor capable of measuring tangential and/or radial acceleration.

7. Doubly fed electric motor according to claim 1, wherein a switching unit (68) comprises a plurality of transistors and diodes.

8. Doubly fed electric motor according to claim 1, wherein the control device (60) comprises an power supply unit (72) adapted to power the rotor controller unit (61) with DC.

9. Doubly fed electric motor according to claim 1, comprising a rotor switching unit (68) adapted to electrically connect the rotor windings (42) to the rotor inverter unit (66) in variety of alternatively selectable configurations.

10. Doubly fed electric motor according to claim 9, wherein the rotor switching unit (68) is adapted to change a magnetic pole number of the rotor (40).

11. Doubly fed electric motor according to claim 1, comprising at least one position angle encoder, wherein the at least one position angle encoder is a rotor position angle encoder (78).

12. Doubly fed electric motor according to claim 1, comprising at least one current sensor (80), wherein preferably at least two current sensors (80) are positioned in the windings of the rotor (40).

13. Doubly fed electric motor according to claim 1, comprising a stator control device (82) which is electrically connected to the stator (20), and wherein the stator control device (82) is preferably connected to the rotor control device (60) via a communication unit (84).

14. Doubly fed electric motor according to claim 13, wherein the stator control device (82) is adapted to vary frequency, phase and/or the magnitude/amplitude of the current or voltage of the stator (40).

15. Method to operate a doubly fed electric motor, having a stator (20) with no less than three stator windings (43), a rotor (40) comprising no less than three rotor windings (42), into which EMF is induceable, and a control device (60) which is arranged at the rotor (40), wherein the rotor windings (42) are electrically connected to the control device (60), wherein the control device (60) varies the power factor based on and using the induced EMF in the rotor, and varies or adjusts at least one of: electric currents in the rotor windings (42), and and/or the angle of rotor magnetic flux vector, characterized in that the control device (60) varies the phase, frequency and/or the magnitude/amplitude of the rotor current using EMF induced to the rotor in such a way to achieve an optimum angle of 90 between the rotor magnetic flux vector and the stator magnetomotive force (MMF) vector; wherein the method comprises the steps of: using EMF induced in the rotor windings (42) to store electric energy in the energy storage unit (74, 76); using energy stored in energy storage unit (74, 72) and demanded torque or/and RPM to modify, adjust electric current in rotor windings (42); using information sent from the rotor control device via the communication unit (84) and demanded torque or/and RPM to adjust electric currents in stator windings; using information from sensors such as current sensors on the rotor windings, electric potential sensors on the energy storage unit, temperature sensors, shaft position encoders, or accelerometer sensors, and demanded torque or/and RPM to select and switch to adequate magnetic pole number on rotor and stator; and generating EMF in rotor windings by pulsating, nonrotating stator magnetomotive force (MMF) vector.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

(1) Additional advantages and features of the current invention are shown in the following description of preferred embodiments of the current invention with reference to the attached drawings. Single features or characteristics of respective embodiments are explicitly allowed to be combined within the scope of the current invention.

(2) FIG. 1 shows a perspective schematic view of an electric motor comprising a rotor control device attached to the rotor;

(3) FIG. 2a shows a design scheme of one embodiment of the electric motor with rotor and stator switching units connecting rotor and stator windings in double star configuration;

(4) FIG. 2b shows a design scheme of one embodiment of the electric motor with rotor and stator switching units connecting rotor and stator windings in delta configuration;

(5) FIG. 3 shows a chart which visualizes different methods to operate the electric motor;

(6) FIG. 4 shows a method to estimate magnetic flux angle from values of two known currents from three-phase current.

DETAILED DESCRIPTION OF THE INVENTION

(7) FIG. 1 shows a schematic, perspective view of a motor with a rotor 40 comprising a shaft 90. A control device 60 is attached to the rotor 40 or the shaft 90, respectively. The rotor 40 comprises a rotor winding 42 comprising three windings 42 which are mounted at 120 to each other and which can be connected in star or in delta connection. This way, three-phase AC can be induced into the three windings 42 due to a rotating magnetic field of a stator (not shown). The control device 60 is adapted to correct power factor and slip by varying the phase and/or magnitude/amplitude of rotor three phase current.

(8) FIGS. 2a, b show a design scheme of an embodiment of an electric motor according to the invention motor with rotor and stator switching units connecting rotor and stator windings in double star configuration (FIG. 2a) and in delta configuration m (FIG. 2b). A rotor 40 comprises a rotor winding 42 comprising/forming three phases. The rotor 40 is combined with a control device 60. The control device 60 comprises a controller unit 61 which is supported by a power supply unit 72 and an energy storage unit 74 comprising at least one capacitor 76. The control device 60 comprises a rotor inverter unit 66 comprising a plurality of transistors 68 and diodes 70. The rotor inverter unit 66 is electrically connected to the rotor winding 42 or the appropriate phases, respectively. At least two phases of the rotor 40 are provided with a current sensor 80. In addition, the rotor 40 comprises a rotor angle encoder 78. The controller unit 61 or the control device 60, respectively, are connected to a rotor control device 82 by a communication unit 84. The rotor control device 82 is electrically connected to a stator 20 which comprises also three phases wherein at least two phases comprise a current sensor 80. The rotor control device 82 is connected to a three phase AC system.

(9) FIG. 3 shows a chart which visualizes different methods to operate an electric motor. A control loop 62, preferably a PI control loop 62, and a rotor inverter unit 64 are connected to a rotor 40. They process {right arrow over (I)}.sub.rotor, {right arrow over (I)}.sub.stator and . Two control loops 62, preferably two PI control loops 62, are connected to a stator 20. They process {right arrow over (I)}.sub.rotor, {right arrow over (I)}.sub.stator, .sub.stator, .sub.rotor, .sub.shaft, and commanded torque or commanded shaft RPM, respectively. With regard to FIG. 3, different operation modes are described.

(10) Field Oriented Control OperationUsing Current Sensors

(11) First step: measuring of stator and rotor currents in two phases each using current sensors 80.

(12) Second step: estimating magnetic flux angles of rotor and stator ({right arrow over (I)}.sub.rotor, {right arrow over (I)}.sub.stator) (knowing two measured rotor phase currents and two stator phase currents). Rotor flux angle is determined by adding angle of rotor physical position and rotor flux angle. Rotor physical instantaneous position is determined from rotor position angle encoder 78. Also sensorless methods to estimate the angle ({right arrow over (I)}.sub.rotor, {right arrow over (I)}.sub.stator) are known from prior art. It is known that the angle between rotor and stator flux vectors affects stator current phase shift (back EMF). The rotor flux vector on its turn also induces EMF in the stator 20. This back EMF creates an additional current vector in the stator affecting the overall phase shift in the stator. By measuring this influence, methods exist to estimate values for the angle between rotor 40 and stator 20 flux from measured corresponding stator electrical current values, their phase shift and angular velocity. In the present invention the rotor current vector {right arrow over (I)}.sub.rotor is known exactly from measurements taken on the rotor 40 itself, whereas in prior art it is estimated from the effects caused in the stator which is a lot slower and inaccurate method.

(13) Third step: If the angle between rotor and stator flux vectors ({right arrow over (I)}.sub.rotor, {right arrow over (I)}.sub.stator) is less than 90, then at least one of the following steps is performed: the rotor controller unit 61 commands rotor inverter unit 64 to reduce the angular velocity .sub.rotor of rotor three phase current. Such action will decrease slip, increase shaft RPM (.sub.shaft) and will bring the angle between rotor and stator flux vectors ({right arrow over (I)}.sub.rotor, {right arrow over (I)}.sub.stator) closer to 90. Such action is not possible with prior art induction motors; commanding the rotor control device 82 to increase stator angular velocity .sub.rotor. Such action will also increase shaft RPM or .sub.shaft; commanding the rotor control device 82 to decrease stator duty cycle. Such action will keep RPM and decrease torque.

(14) If the angle between rotor and stator flux vectors ({right arrow over (I)}.sub.rotor, {right arrow over (I)}.sub.stator) is greater than 90, then at least one of the following steps is performed: the rotor controller unit 61 commands rotor inverter unit 66 to increase frequency of rotor three phase current (.sub.stator) to advance rotor current vector. Such action will increase slip, decrease shaft RPM (.sub.shaft) and will bring the angle between rotor and stator flux vectors ({right arrow over (I)}.sub.rotor, {right arrow over (I)}.sub.stator) closer to 90; such action is not possible with prior art induction motors; commanding the rotor control device 82 to reduce stator frequency (.sub.stator); such action will also decrease shaft RPM or .sub.shaft; commanding the rotor control device 82 to increase stator duty cycle; such action will increase torque; If necessary, repeating at least one of the preceding steps.

(15) Field Oriented Control OperationUsing PI Control Loops

(16) Motor control can be done using Proportional Integral (PI) control loops: In a first control loop 62, angle ({right arrow over (I)}.sub.rotor, {right arrow over (I)}.sub.stator) is compared to 90. If ({right arrow over (I)}.sub.rotor, {right arrow over (I)}.sub.stator)<90 an error message is fed to a PI controller 62 which adequately decreases .sub.rotor. Otherwise PI controller 62 adequately increases .sub.rotor. In a second control loop 62, instantaneous torque is estimated from the values of rotor and stator currents and the assumption that their vectors are at 90. Instantaneous torque ({right arrow over (I)}.sub.rotor, {right arrow over (I)}.sub.stator) is compared to commanded torque and error message is fed to a PI controller 62. If more torque is needed than rotor control device 82 is commended to adequately increase its duty cycle, which eventually will increase stator and rotor currents, reflecting in higher torque. In a third control loop 62, commanded RPM is compared to .sub.stator.sub.rotor, which is actual shaft angular velocity.sub.shaft. If measured .sub.shaft is less than commanded, stator control device 82 is commanded to adequately increase stator frequency. Otherwise the opposite command is given.

(17) FIG. 4 shows a method to estimate magnetic flux angle from values of two known currents from three-phase current.

(18) $I_0\sin \left(t\right)=A_t$$I_0\sin \left(t+\frac{2}{3}\right)=B_t,$

(19) Wherein A.sub.t is instantaneous current value in phase A and B.sub.t is instantaneous current value in phase B.

(20) If sin(t)=0, A.sub.t=0 and current angle t=0.

(21) If sin(t)0, the current angle can be calculated as follows:

(22) $I_0\sin \left(t\right)=A_t.Math.I_0=\frac{A_t}{\sin \left(t\right)}$$I_0\sin \left(t+\frac{2}{3}\right)=B_t.Math.I_0\sin \left(t\right)\cos \left(\frac{2}{3}\right)+I_0\cos \left(t\right)\sin \left(\frac{2}{3}\right)=B_t$$\mathrm{Substituting}$$\sin \left(\frac{2}{3}\right)=\frac{\sqrt{3}}{2};$$\cos \left(\frac{2}{3}\right)=-\frac{1}{2};$$I_0=\frac{A_t}{\sin \left(t\right)}$$A_t\frac{\sqrt{3}\cos \left(t\right)}{2\sin \left(t\right)}-\frac{A_t}{2}=B_t.Math.\cot \left(t\right)=\frac{2B_t+A_t}{A_t\sqrt{3}}$$\mathrm{momentary}\mathrm{current}\mathrm{vector}\mathrm{angle}t={\cot }^{-1}\left[\frac{2B_t+A_t}{A_t\sqrt{3}}\right]$

REFERENCE NUMERALS

(23) 20 stator 40 rotor 42 rotor winding 43 stator winding 60 rotor control device 61 rotor controller unit 62 (PI) control loop, (PI) controller 66 rotor inverter unit 68 rotor switching unit 70 electric potential measuring sensor 71 switchable resistor 72 power supply unit 74 energy storage unit 76 capacitor 78 shaft angle encoder 80 current sensor 82 stator control device 83 stator controller unit 85 stator inverter unit 86 external interface 87 stator switching unit 84 communication unit 85 stator inverter unit 90 shaft P power I current EMF angular velocity t current vector angle A.sub.t instantaneous current value in phase A B.sub.t instantaneous current value in phase B