Patent Application
20190052212
This invention relates to a doubly fed electric motor and to a method to operate a doubly fed electric motor.
Although synchronous electric motors in general are more efficient than induction electric motors, induction motors are still widely used in many applications. The reasons are low cost, less maintenance and simple design compared to synchronous motors. One of the reasons for lower efficiency of induction motors is the fact that the rotor magnetic flux field is very hard to control in terms of strength and direction. In steady state conditions (constant RPM and torque) the angle between stator MMF (magneto motive force) vector and rotor magnetic flux vector is not at optimal 90° (electrical). As a result from that greater electrical current has to be maintained in stator and rotor leading to ohmic and air gap losses which cuts efficiency and can cause overheating problems. Also it takes considerable amount of time to increase the rotor field when instant torque is required from the motor. The process of additionally exciting the rotor can take up to seconds in which time the motor may stall, leading to variety of problems. A more efficient alternative to the simple induction motors are so-called doubly fed electric motors, where the rotor field is more precisely controlled externally via slip rings or additional electromagnetic exciters.
The document US 2015/145466 A1 shows a doubly fed electric motor with a single phase grid power supply or a direct current power supply to the stator. The further documents U.S. Pat. No. 4,982,147 A and US 2015/349687 A1 also show doubly fed electric machines, however, in order to control the rotor vector field these documents propose to use an additional external power supply connected to a rotor inverter through conventional slip rings.
However, slip rings are subject to wear and need frequent maintenance.
There are also so-called brushless doubly fed electric machines known from prior art, such as U.S. Pat. No. 6,278,211 B1. These machines use additional stator and rotor windings dedicated only to transfer energy to and from the rotor through electromagnetic induction.
Therefore, it is an object of the current invention to provide a doubly fed electric motor without the use of slip rings or electromagnetic exciters, with better ability for rotor field vector control, both in terms of direction and strength than prior art induction and doubly fed electric motors.
This object is achieved by means of an electric motor according to claim 1 and by a method to operate an electric motor according to claim 15. Additional advantages and features of preferred embodiments of the current invention are defined in the dependent claims.
According to the invention, a doubly fed electric motor comprises a stator, a rotor and a control device, in particular a rotor control device, which is arranged at the rotor, wherein the stator is adapted to induce EMF (electromotive force) into the rotor during operation, and wherein the rotor has no less than three rotor windings, into which EMF is induceable, and which are electrically connected to the control device, and wherein the stator has no less than three stator windings, and wherein the control device is adapted to vary the power factor based on and using the induced EMF in the rotor, and to vary at least one of the electric currents in the rotor windings, and/or the angle of rotor magnetic flux vector, and wherein the control device 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.
The rotor control device is adapted to draw electric energy from rotor windings at appropriate moment of time and to store a part of that energy in the form of electric charge in a rotor energy storage unit, namely a bank of capacitors. Consequently electric potential builds up between the terminals of the rotor energy storage unit. The energy stored in the rotor energy storage unit is then used to vary the rotor magnetic flux field strength and angle, relative to the rotor itself and/or the slip of the motor by applying electric potential to the rotor windings to modify the current in rotor windings at appropriate moment of time, using the electric potential already built up in the rotor energy storage unit.
Advantageously, the rotor control device is adapted to vary the phase and/or the magnitude/amplitude of the rotor current to adapt the angle and magnitude of the rotor magnetic flux vector, in particular the rotor magnetic flux field. Thus, the current can be shifted forwards or backwards in time for synchronization with the induced EMF in the rotor. In addition, the level, magnitude or amplitude of the current can be increased or decreased which means that the current in the windings can be varied/modulated.
Further, the rotor control device is preferably adapted to vary the frequency and/or the magnitude/amplitude of the rotor current to adapt the slip. The slip frequency is defined as the difference between stator MMF vector rotation (synchronous) frequency and actual shaft rotation frequency. The slip is defined as the ratio of slip frequency over synchronous frequency. Slip varies from zero at synchronous speed and one when the rotor is at rest.
According to one embodiment the rotor comprises a plurality of rotor windings, respectively, electrically connected to the rotor control device. More precisely, electrically connected to a rotor inverter unit as part of the rotor control device. According to a preferred embodiment, three rotor windings are wound at 120° pitch to each other and are connected in star or delta connection. This way, three-phase AC will be induced into the three rotor windings due to rotating stator magnetic field around rotor windings.
Frequency of the EMF in the rotor is equal to the slip frequency. The rotor control device is adapted to phase shift electrical current in all three rotor windings so that the current in any winding is preferably in phase with the induced EMF in it. However the induced EMF in the rotor is always delayed by electric 90° relative to the rotating stator MMF vector (according to Faraday's law of electromagnetic induction). Also the rotor magnetic flux vector is directly proportional to the rotor current vector. This way if rotor AC current is kept in phase with the induced EMF the rotor magnetic flux vector will also be delayed by exactly 90° electrical in relation to the stator MMF vector. This condition is ideal, as maximum torque per unit current is achieved exactly when rotor flux vector and stator MMF vector are at 90° electrical. Clearly, this is not true for standard induction motors, where there is an additional delay in rotor current and hence of the rotor flux vector, namely 90°+ϕ relative to the stator MMF vector and always the offset angle ϕ>0. That is because the prior art rotor consists of a number of purely RL (inductive resistive) electric loops, causing a delay of current in every loop of the rotor with respect to the induced EMF in the loop. In other words, the power factor of the rotor loops in prior art is less than 1, which offsets the rotor flux vector not to be at 90° with respect to stator MMF vector.
According to one embodiment the rotor control device comprises a rotor controller unit a rotor inverter unit, an energy storage unit, a communication unit, and /or an electric potential measuring unit. Preferably, the rotor control device comprises an accelerometer sensor capable of measuring tangential and/or radial acceleration.
As the rotor is adapted to operate at a variety of slip frequencies it is not appropriate to use passive elements (like simple capacitors) or simple RL circuits to correct the phase of rotor current. Therefore, the rotor control device contains a controller unit which is preferably an active programmable device comprising at least one control loop, for example a PI (Proportional Integral) control loop, and/or at least one rotor inverter unit. According to one embodiment, every PI control loop comprises a feedback or a loop controller.
The feedback is provided by current sensors on rotor windings, electric potential sensor on the energy storage unit and other sensors like temperature sensor, shaft position encoder, accelerometer or other. In particular, the rotor controller unit is adapted to operate the PI control loops and/or the rotor inverter unit(s). The whole system is expediently arranged at the rotor and therefore spins or rotates with the rotor.
Preferably, the rotor control device having a controller unit, energy storage device and a rotor inverter unit, which comprises for example a plurality of transistors and diodes. The windings of the rotor are electrically connected to the rotor inverter unit.
Furthermore, the control device comprises a power supply unit adapted to power the rotor controller unit with appropriate DC power, in particular needed for its operation. The windings of the rotor are also electrically connected to the power supply unit.
According to a preferred embodiment, the energy storage unit comprises at least one capacitor. These components are preferably also connected to the windings of the rotor. Electric potential measuring sensor is attached to both terminals of the energy storage unit and passes information to rotor controller unit. Also a switchable resistor is connected to the energy storage unit. In case too much energy is accumulated in the energy storage unit part of that energy can be dissipated as heat via the switchable resistor
According to one embodiment, the energy storage unit is a capacity bank in which electric energy is stored in the form of electric charge. This energy is then used to appropriately modulate frequency and/or magnitude/amplitude of rotor current vector so that it is kept as close as possible to 90° with respect to the stator MMF vector. In steady condition, rotor inverter module outputs sinusoidal PWM duty cycle, managed by the rotor controller unit which brings rotor current to come earlier and by doing so eliminates phase lag of rotor current in its otherwise purely RL circuits. Frequency generated by the rotor inverter is kept equal to instantaneous slip frequency. When the rotor inverter increases its three-phase frequency, the slip frequency also equally increases. More slip brings greater induced EMF in the rotor and the current in the rotor windings also increases which increases rotor flux. If the rotor inverter decreases its three-phase frequency, the slip frequency also equally decreases. Less slip brings drop in induced EMF in the rotor and the current in the rotor windings also drops which decreases rotor flux. Also the rotor flux can be varied by adjusting the rotor inverter module duty cycle without having to adjust the slip. In this manner, the rotor flux can be advantageously varied depending on demanded torque.
The stator control device is adapted to provide AC to the stator. According to one embodiment the stator control device comprises also stator controller unit with at least one control loop built in it, for example a PI (Proportional Integral) control loop, and/or at least one inverter unit.
According to one embodiment, the stator control device module is adapted to vary frequency, phase and/or the magnitude/amplitude of the current or voltage of the stator.
In a preferred embodiment the rotor control device and rotor controller unit in particular can be programmed to adjust the slip and other rotor parameters based on built in logic (PI control loops). In another embodiment the rotor control device can receive external commands via wireless communication unit and adjust the slip and other rotor parameters accordingly. In such embodiment the PI control loops are run outside the rotor. Preferably the stator control device and its integrated programmable stator controller unit run PI loops outside the rotor. Also the rotor control device sends information regarding rotor status and condition to the outside world or/and to the stator control device via the same wireless communication unit. Such information is used as part of the feedback to the PI loops running in stator controller unit. Another part of the feedback is data from stator current sensors (80).
Preferably, the rotor control device is powered entirely by the induced EMF in the rotor by the stator rotating field and does not need any additional power transfer modules like slip rings or electromagnetic exciters.
According to one embodiment, the doubly fed electric motor comprises a rotor switching unit adapted to electrically connect the rotor windings to the rotor inverter unit in variety of alternatively selectable configurations. Preferably, the rotor switching unit is adapted to change a magnetic pole number of the rotor.
Preferably, the electric motor can be operated as motor and/or as generator.
According to one embodiment, the rotor and/or the stator can also be provided with pole number switching units to advantageously change the pole numbers of the doubly fed electric motor during operation to optimize efficiency with respect to momentary RPM and torque requirements. A rotor switching unit is preferably attached to the rotor which means that it also rotates/spins with the rotor. In such embodiment rotor switching unit is part of the rotor control device and is commanded by the rotor controller unit. According to a preferred embodiment, the rotor switching unit does also not need any slip rings or electromagnetic exciters. In such embodiment rotor and stator contain preferably 6 windings each. Windings are distributed at 60° pitch on rotor and stator. Rotor and stator switching units can simultaneously electrically connect rotor and stator windings to respectively rotor and stator inverter units in delta configuration to operate the doubly fed electric motor in four magnetic poles configuration. Also rotor and stator switching units can be alternatively switched to electrically connect rotor and stator windings respectively to rotor and stator inverter units in double star configuration to operate the doubly fed electric motor in two magnetic poles configuration. Other embodiments with different number of alternatively switchable rotor and stator magnetic pole number sets are also possible and preferred. Magnetic pole number switching takes place in three steps performed in quick succession:
Likewise switching from four to two magnetic poles takes place in a similar fashion when RPM exceeds a pre-set point, however the duty cycle of rotor and stator inverters resume at half their initial frequencies each. Frequency adjustment is important to synchronize rotation velocity of stator MMF vector and rotor field vector with the actual shaft rotation velocity, which remains almost unchanged during the magnetic pole number switching process. However the physical rotation velocity of both vectors would change because of magnetic pole number change and needs adjustment to match again with the shaft rotation velocity. The process of magnetic pole number switching is synchronized over the communication unit to take place simultaneously on the rotor and the stator.
According to one embodiment, the doubly fed electric motor comprises at least one position angle encoder, wherein the at least one position angle encoder is a shaft position angle encoder. Thus the rotor current vector is known exactly from measurements taken on the rotor itself and adding the angle of the shaft known from position angle encoder. In another embodiment a sensorless method is used to determine the angle of the rotor flux vector.
According to one embodiment, the doubly fed electric motor comprises at least one current sensor, wherein preferably at least two current sensors are positioned in the windings of the rotor.
According to one embodiment the doubly fed electric motor comprises a stator control device which is electrically connected to the stator windings, wherein the inverter module operated by stator controller unit which is a programmable device. Preferably stator controller unit can exchange information with rotor controller unit via the communication unit.
In one embodiment the communication unit can be a wire- or wireless-based communication unit. In another embodiment rotor and stator control devices are equipped with encoding /decoding units which impose additional low power modulated high frequency signal to rotor and stator windings. Messages are FM encoded in this variable high frequency, which passes through the air gap by inducing small EMF with the same FM frequency in the opposite windings. The signal is filtered out on the other end out of the opposite windings, decoded by the corresponding encoding/decoding unit. This way rotor and stator control devices can exchange information without the need of additional antennas or wires.
The invention refers also to a method to operate a doubly fed electric motor according to claim 15.
According to one embodiment the method comprises the steps:
A starting procedure of the electric motor may be as follows. Initially, there is no current in the rotor and the stator. Then, the stator control device module generates a pulsating (not rotating) MMF vector in the stator by modulating constant low duty cycle on the stator inverter unit. This can be done by simply by switching “on” and “off” only one of the six transistors of the stator inverter unit. As this so generated MMF vector is not rotating there is no torque acting in the shaft, only small radial forces acting on the rotor will be generated. Relatively small EMF is induced in the rotor windings due to the stator pulsating MMF vector. As the MMF vector is not rotating there will be no torque on the shaft. Diodes of the rotor inverter unit act as rectifiers and the energy storage unit (capacitor(s) receive(s) electric charge leading to build up of electric potential between its terminals. The power supply device is activated by the electric potential from the capacitor(s) and outputs precisely conditioned DC power to the rotor controller unit. Rotor controller unit boots up and sends “ready” signal to stator controller unit via the communication unit. After this state is achieved the electric motor still generates no torque, but it is in ‘stand by’ mode and ready to operate. When rotating stator MMF vector is initiated in the stator with rotating frequency above some minimal slip frequency there will be torque on the shaft and eventual rotation. Commanded torque and/or RPM are received via the external interface, connected to stator control device.
In the following, different operation modes are described using the subsequent expressions:
First step: measuring of stator and rotor currents in two phases each using current sensors.
Second step: estimating magnetic flux angles of rotor and stator Θ({right arrow over (I)}rotor, {right arrow over (I)}stator) (knowing two measured rotor phase currents and two stator phase currents). Rotor flux angle is determined by adding the angle of rotor physical position and rotor flux angle with respect to the rotor itself. Rotor physical instantaneous position is determined from rotor position angle encoder. Rotor flux angle with respect to the rotor itself is estimated on the basis of two measured currents in two rotor windings. Also sensorless methods to estimate the angle Θ({right arrow over (I)}rotor, {right arrow over (I)}stator) are known from the 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. 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 and stator flux from measured corresponding stator electrical current values, their phase shift and angular velocity.
Preferably, the rotor current vector {right arrow over (I)}rotor is known exactly from measurements taken on the rotor itself, whereas in prior art it is estimated from the effects caused in the stator which is a lot slower and which is an inaccurate method.
Third step: If the angle between rotor and stator flux vectors Θ({right arrow over (I)}rotor, {right arrow over (I)}stator) is less than 90°, then at least one of the following steps is performed:
If the angle between rotor and stator flux vectors Θ({right arrow over (I)}rotor, {right arrow over (I)}stator) is greater than 90°, then at least one of the following steps is performed:
Motor control can be done using Proportional Integral (PI) control loops:
First step: measuring of stator and rotor currents in two phases each using current sensors 80.
Second step: estimating magnetic flux angles of rotor and stator Θ({right arrow over (I)}rotor, {right arrow over (I)}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 Θ({right arrow over (I)}rotor, {right arrow over (I)}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)}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.
Third step: If the angle between rotor and stator flux vectors Θ({right arrow over (I)}rotor, {right arrow over (I)}stator) is less than 90°, then at least one of the following steps is performed:
If the angle between rotor and stator flux vectors Θ({right arrow over (I)}rotor, {right arrow over (I)}stator) is greater than 90°, then at least one of the following steps is performed:
Motor control can be done using Proportional Integral (PI) control loops:
Wherein At is instantaneous current value in phase A and Bt is instantaneous current value in phase B.
If sin(ωt)=0, At=0 and current angle ωt=0.
If sin(ωt)≠0, the current angle can be calculated as follows:
| Number | Date | Country | Kind |
|---|---|---|---|
| 16156807.6 | Feb 2016 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2017/051904 | 1/30/2017 | WO | 00 |
