Method for determining the position of a freely rotating rotor in a permanent magnet motor and control circuit and system thereof

Abstract

The present disclosure relates to a method for determining the position of a freely rotating rotor in a permanent magnet motor and a control circuit and system thereof. A control circuit, system and method 200 for determining the position of a rotor in a permanent magnet motor PM1 in a state where the rotor is freely rotating, the motor being connected to a DC voltage link 101 via an inverter circuit 102, wherein the inverter circuit is operable to connect the windings of the stator of the motor to the DC voltage link, the method comprising the following steps: step a) short-circuiting the windings of the stator of the permanent magnet motor 201, step b) measuring 202 the back electromotive force EMF of the short-circuited windings of the stator, and step c) determining 203 the position of the rotor by means of the measured back EMF.

Description

Technical Field

[0001] The present invention relates to a method of determining the position of a rotor in a permanent magnet electric machine. More particularly, the present invention relates to a method of determining the position of a freely rotating rotor in a permanent magnet electric machine. Background Art

[0002] In modern rail vehicles with permanent magnet motors, it is highly desirable to determine the position of the rotor, because in order to control the permanent magnet motor, the position of the rotor must be known, which is not easy to achieve, especially if the rotor is in a free-spinning state. This state may occur, for example, if the rail vehicle is traveling downhill and the stator windings are inactive and the rotor is free-spinning to save energy. When the motor is activated again, the position of the rotor needs to be known precisely in order to provide a magnetic field with the correct phase relative to the rotor. A method of determining the position of a freely spinning rotor in a permanent magnet motor is urgently needed.

[0003] Furthermore, if the rail vehicle is equipped with two or more permanent magnet motors connected to the same DC voltage link, it is important that the method for determining the position of the freely rotating rotor does not affect the voltage of the DC voltage link or cause undesired motor torques which affect passenger comfort and / or cause unnecessary wear on the drive train of the rail vehicle.

[0004] There are several known methods for determining the position of a rotor in a stopped permanent magnet motor (ie, the rotor has no angular velocity). However, these methods cannot be used to determine the position of a freely rotating rotor.

[0005] In this disclosure, a "freely rotating rotor" should be interpreted as a rotor that rotates without being affected by the magnetic field provided by the current flowing in the stator windings. Typically, this means that the inverter circuit holds the leads of the windings in a high impedance state without being connected to a direct current voltage link (DC link). Summary of the invention

[0006] The present invention provides circuits, systems and methods for determining the position of a rotor in a permanent magnet motor when the rotor is in a free-spinning state. During the start-up of the free-spinning rotor, small errors in the determined position of the free-spinning rotor can cause severe disturbances in the motor torque and DC link voltage. Several known solutions are required to stop the free-spinning rotor before the rotor position is determined and the motor can be started.

[0007] According to the present invention, a method for determining the position of a rotor in a permanent magnet motor in a state where the rotor is freely rotating is provided. The motor is connected to a DC voltage link via an inverter circuit, wherein the inverter circuit is operable to connect the winding of the stator of the motor to the DC voltage link, and the method comprises the following steps:

[0008] a) short-circuiting the windings of the stator of the permanent magnet motor;

[0009] b) measuring the back electromotive force (EMF) of the short-circuited winding of the stator; and

[0010] c) The position of the rotor is determined with the aid of the measured back EMF.

[0011] This method is advantageous because it allows the position of a freely rotating rotor to be determined without stopping the rotor. Furthermore, it allows the position of a freely rotating rotor to be determined without disturbing the DC voltage link or inducing motor torque, which may be advantageous if several permanent magnet motors are connected to the same DC voltage link. Additionally, this method may also be useful if other vehicles are connected to the same DC link, for example in a DC voltage feed system.

[0012] In one embodiment, step a) also includes: determining a desired maximum back EMF current; determining the speed of the freely rotating rotor; calculating a predetermined short circuit interval based on the desired maximum back EMF current, the speed of the freely rotating rotor and motor parameters; and short-circuiting the winding by means of the inverter circuit during the predetermined short circuit interval.

[0013] This allows a very small back EMF current to be selected to determine position, thereby reducing unnecessary heating of the permanent magnet motor and inverter circuits. If the expected maximum back EMF current is small enough, the regenerated power and motor torque can be very small.

[0014] In one embodiment, step a) further comprises: determining a desired maximum back EMF current; short-circuiting the windings of the stator and measuring the back EMF current, and terminating the short circuit when the measured back EMF current is equal to or greater than the desired maximum back EMF current.

[0015] This allows the use of the standard current protection circuitry of the permanent magnet motor to set the maximum back EMF current. For example, during determination of the position of the rotor, the overcurrent protection setting of the permanent magnet motor is reduced to the desired maximum back EMF current.

[0016] In a preferred embodiment, steps a) to c) are performed in a synchronous reference frame phase locked loop.

[0017] This allows for a fast and efficient determination of the rotor position of a permanent magnet motor while the rotor is freely spinning, which is easy to implement in both software and hardware. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 is a block diagram of a control circuit according to an embodiment of the present invention;

[0019] Figure 2 is a schematic block diagram illustrating a method according to an embodiment of the present invention;

[0020] Figure 3 is a schematic block diagram of a phase-locked loop according to an embodiment of the present invention;

[0021] Figure 4 is a diagram illustrating an experiment performed according to an embodiment of the present invention;

[0022] Figure 5 is a diagram illustrating an experiment performed according to an embodiment of the present invention; and

[0023] Figure 6 is a schematic diagram of a rail vehicle according to an embodiment of the present invention. DETAILED DESCRIPTION

[0024] As will be explained in detail below, the inventors have devised a way to determine the position of the rotor in a permanent magnet motor while the rotor is spinning freely. This is achieved by short-circuiting the stator windings and measuring the induced back electromotive force (EMF) current and using a phase locked loop to determine the position.

[0025] The rotating rotor of a permanent magnet motor generates a three-phase phase EMF with a u-phase according to the following formula:

[0026] e(t)=ω el ·Ψ m sin(α)

[0027] α=ω el ·t

[0028] ω el =p / 2·ω

[0029] Among them Ψ m is the flux linkage generated by the permanent magnets, α is the position of the rotor, ω is the speed of the rotor, and p is the number of poles in the motor.

[0030] Short circuit time (t ON ) is much smaller than 1 / (2π·ω el ) of the short-circuited permanent magnet motor. ph can be deduced to be equal to:

[0031] i ph (t k )=ω el ·Ψ m ·cos(ω el ·t k )·t ON / L q [Equation 0]

[0032] where tON is the short circuit time, ω el is the speed relative to the motor poles, Ψ m is the flux generated by the permanent magnet, L q is the motor inductance in the q direction, and i ph is the back EMF current.

[0033] Back EMF current i ph Allows the rotor position to be determined without stopping the rotor.

[0034] First embodiment

[0035] Figure 1 1 is a schematic block diagram of a control circuit, generally denoted by CC, for determining the position of a rotor in a permanent magnet motor PM1 in a state where the rotor is free to rotate. The motor is connected to a DC voltage link 101 via an inverter circuit 102. The DC voltage link has a capacitor C1 connected between a positive line indicated by "+" and a negative line indicated by "-". In addition, the DC voltage link has an inductor L1 connected in series to a DC voltage source V1. The voltage source may be an AC source and a rectifier network providing a DC voltage to the DC voltage link. The inverter circuit 102 has a control line connected to the control circuit. The inverter circuit includes a plurality of switching elements S1, S2, S3 controlled by respective control lines. After the switching element is activated by means of the respective control lines, the output of the switching element is connected to the positive line of the DC voltage link or the negative line of the DC voltage link, depending on the control signal applied to the control line. A common implementation of the switching element is two transistors connected in series between the positive line and the negative line of the DC voltage link. The output of the switching element is connected to a wire connecting the transistors to each other. A common transistor type used in switching elements is the IGBT (Insulated Gate Bipolar Transistor), but power MOSFETs can also be used.

[0036] The control circuit has control lines connected to the inverter circuit and thence to control signal outputs of the corresponding switching elements. The control signal outputs are operable to control the power supply to the windings of the stator of the motor by means of the inverter circuit during intervals. If the switching elements simultaneously connect the windings to the positive or negative lines, then these control signal outputs can be used to short-circuit all windings. This means that the inverter circuit for driving the permanent magnet motor can be used to short-circuit the windings.

[0037] The control circuit also has back electromotive force (EMF) current sensors 103, 104, which are provided for measuring the back EMF current of the windings of the stator of the motor during a short circuit of the windings.

[0038] The control circuit also includes an output α that provides a determined position of the rotor, wherein the control circuit is configured to short-circuit the windings of the stator with the aid of the inverter circuit during an interval, and is configured to measure a back EMF current induced in the windings, wherein the control circuit is configured to calculate the position of the rotor using the measured back EMF current.

[0039] To understand how the determination of the rotor position is performed, now refer to Figure 2 , Figure 2 is a flow chart illustrating the method, which is generally indicated at 200 .

[0040] The method involves:

[0041] a) Short-circuiting the winding of the stator of the permanent magnet motor (201). Since the rotor is in a rotating state, if the winding of the stator is connected to a load or is short-circuited, the permanent magnet of the rotor will induce a back EMF current in the stator.

[0042] b) measuring (202) the back electromotive force (EMF) of the short-circuited winding of the stator; and

[0043] c) Determining (203) the position of the rotor with the aid of the measured back EMF.

[0044] Second embodiment

[0045] Preferably, the short circuit interval is as short as possible because the short circuit may cause stress in the motor and inverter circuit. In one embodiment, the method may include:

[0046] Determine the expected maximum back EMF current for the permanent magnet motor, inverter circuit, and DC voltage link.

[0047] The speed of the freely spinning rotor is determined, for example if the permanent magnet motor is used on a rail vehicle this can be done via its speedometer or GPS speed.Other embodiments may utilize some rotation sensor.

[0048] The predetermined short circuit interval is calculated based on the expected maximum back EMF current, the speed of the freely spinning rotor and the motor parameters. This can be done using the following equation:

[0049] t ON =ω el ·Ψ m / (L q ·i ph,max ) [Equation 1]

[0050] This is obtained using the above equation and definition, where t ON is the short circuit time, ω el is the rotation speed relative to the pole, Ψ mis the flux generated by the permanent magnet, L q is the motor inductance in the q direction, and i ph,max is the maximum expected back EMF current.

[0051] Third embodiment

[0052] In a third embodiment, this third embodiment may not involve calculation of the short circuit interval. The method involves determining a desired maximum back EMF current. The method also involves short-circuiting the stator and measuring the back EMF current, and when the measured back EMF current is equal to or greater than the desired maximum back EMF current, the short circuit is terminated. This third embodiment may use the internal overcurrent protection of the permanent magnet motor to set the desired maximum back EMF current.

[0053] Phase-locked loop

[0054] The methods and embodiments discussed above are preferably implemented in a phase-locked loop, wherein the stator current is used as the input of the phase-locked loop. Figure 3 It is shown in the form of a block diagram, and is generally represented as 300.

[0055] The phase locked loop comprises an input terminal 301 for receiving the measured back EMF current, the three-phase quantities of the received back EMF current at an angle of 120 degrees to each other are transformed in a block 302 to an orthogonal fixed reference frame, where Iα (along the α axis) and Iβ (along the β axis) are perpendicular to each other but in the same plane as the three-phase reference frame, and the transformed signals are input to a first multiplier 303 and a second multiplier 304. The difference between the outputs of the first multiplier and the second multiplier is calculated in a difference block 305. This difference is used as an input to a first gain stage 306 and an input to a second gain stage 310. The amplified difference from the first gain stage 306 is input to a first integrator 307. The output from the first integrator 307 is input to a summing block 308. The output from the second gain stage 310 is also input to the summing block 308. The output from the summation block 308 is integrated in a second integrator 309, wherein the output from the second integrator is the determined position of the rotating rotor available at the output terminal 311. However, the determined position is used as a feedback signal to the sine block 312 and the cosine block 313. The output from the sine block is used as an input to the first multiplier 303. The output from the cosine block 313 is used as an input to the second multiplier 304. In this way, the determined position of the rotor is calculated.

[0056] Experiment 1

[0057] exist Figure 4 , a data graph from a first experiment is disclosed. This experiment was conducted on a permanent magnet motor with a target of a desired maximum back EMF current of 10A. Figure 3Calculate the switching frequency f at low speed. sw , so that the duty cycle (f sw *t ON ) is equal to or less than 0.5. The maximum switching frequency is selected as 2kHz. Using the motor parameters and equation [Equation 1], we get t ON =27us.

[0058] Figure 4 The first graph in shows the measured back EMF current as a function of time as a triangular pulse. A peak hold circuit is used to measure the maximum back EMF current during each interval and is shown in the graph as a square signal.

[0059] Figure 4 The second figure in shows the position difference over time between the true position of the freely rotating rotor and the determined position of the freely rotating rotor. From this figure it can be clearly seen that the error in the estimated position is close to zero after 10ms. This clearly shows the beneficial features of the second embodiment.

[0060] Experiment 2

[0061] exist Figure 5 , a data graph from a second experiment is disclosed. This experiment was conducted on a permanent magnet motor with a target of a desired maximum back EMF current of 10A. Figure 3 The maximum switching frequency is chosen to be 2kHz. Figure 5 The first graph in Figure 1 shows the measured back EMF current as a function of time as a trapezoidal waveform. A peak hold circuit is used to measure the maximum back EMF current during each interval and is shown as a square signal in the graph. From this graph, the maximum back EMF current of 10A can be easily seen in the trapezoidal motor current.

[0062] Figure 5 The second figure shows the position difference over time between the true position of the freely rotating rotor and the determined position of the freely rotating rotor. It can be clearly seen from this figure that the error in the estimated position is close to zero after 25ms. This clearly shows the beneficial features of the third embodiment.

[0063] Fourth embodiment

[0064] exist Figure 6In the figure, a rail vehicle generally indicated as 600 is schematically illustrated. The rail vehicle 600 has a control circuit CC according to any one of the embodiments disclosed above. The rail vehicle has a first permanent magnet motor PM1 and a DC voltage link 101. The DC voltage link is connected to the catenary 601 via a pantograph 602 and a rectifier circuit RC. Of course, other ways of connecting the DC voltage link to the power supply are also possible. The rail vehicle also has an inverter circuit 102 connected to the DC voltage link and the first permanent magnet motor, wherein the inverter circuit includes control lines for controlling the power supply to the windings of the stator of the permanent magnet motor. The rail vehicle may also have a second permanent magnet motor PM2 connected to the DC voltage link 101 via a second inverter circuit IC2. Such a rail vehicle will now be discussed in the case where only the second permanent magnet motor is used and the first permanent magnet motor is free to rotate. When the load on the rail vehicle increases, the first permanent magnet motor is needed, and since the rail vehicle is in motion, the first permanent magnet motor cannot be stopped to determine the position of the rotor therein. The rotor position may be used to provide a smooth start of the first permanent magnet motor. Thus, the embodiments disclosed above may be useful for determining the position of the rotor in the first permanent magnet motor and thus provide a way to start the motor without any disturbances on the DC voltage link. Because, starting a permanent magnet motor with a known rotor position is easy to perform by a person skilled in the art.

[0065] Fifth embodiment

[0066] According to a fifth embodiment, a non-transitory computer readable medium has stored therein a data storage program that causes a computer to execute the method disclosed above and control an inverter circuit and receive information about the measured back EMF current. The computer may be a field programmable gate array (FPGA) device and / or a digital signal processor (DSP).

[0067] Modifications and Changes

[0068] Many modifications and variations of the embodiments described herein may be made.

[0069] For example, the inverter circuit may be connected to a battery instead of a DC voltage link. Thus, the position of a freely spinning rotor may also be determined in a battery-operated vehicle or machine using the method disclosed herein. Thus, the method of the invention requires only information about the rotational speed of the freely spinning rotor and sensors for sensing the back EMF current and some motor parameters.

Claims

1. A method (200) for determining the position of a rotor in a permanent magnet motor (PM1) in a state where the rotor is freely rotating, the motor being connected to a DC voltage link (101) via an inverter circuit (102), wherein the inverter circuit is operable to connect the windings of the stator of the motor to the DC voltage link, the method The following steps are involved: a) short-circuiting (201) the winding of the stator of the permanent magnet motor for a predetermined short-circuit interval; b) measuring (202) the back electromotive force EMF of the short-circuited winding of the stator; and c) determining (203) the position of the rotor with the aid of the measured back EMF, characterised in that steps a) to c) are performed in a synchronous reference frame phase locked loop, Wherein step a) further comprises: - Determine the expected maximum back EMF current; - Determine the speed of the freely rotating rotor; - calculating the predetermined short circuit interval based on the expected maximum back EMF current, the speed of the freely spinning rotor and the motor parameters; - short-circuiting the winding by means of the inverter circuit (102) during said predetermined short-circuit interval.

2. The method of claim 1, wherein step a) further comprises include: - Determine the expected maximum back EMF current; -Short-circuiting the stator and measuring the back EMF current, and when the measured back EMF current is equal to or greater than the expected maximum back EMF current, the short-circuiting is terminated.

3. The method according to any one of claims 1 and 2, wherein steps a) to c) are performed in a loop at a switching frequency of the inverter circuit so that a duty cycle is equal to or less than 0.

5.

4. A control circuit (CC) for determining the position of a rotor in a permanent magnet motor (PM1) in a state where the rotor is freely rotating, the motor being connected to a DC voltage link (101) via an inverter circuit (102), the inverter circuit (102) having a control line connected to the control circuit, the control circuit (CC) include: - a control signal output terminal connected to a control line of the inverter circuit (102), and operable to control the supply of power to the windings of the stator of the motor (PM1) by means of an inverter circuit (102) during certain intervals; - a back electromotive force (EMF) current sensor (103, 104) provided for measuring the back EMF current of a winding of a stator of said electric machine (PM1) during a short circuit of the winding; - an output terminal (α) providing a determined position of the rotor, wherein the control circuit (CC) is configured to short-circuit the windings of the stator by means of the inverter circuit (102) during the short-circuit interval and is configured to measure the back EMF current induced in the windings, wherein the control circuit (CC) is configured to calculate the position of the rotor using the measured back EMF current, characterized in that the control circuit (CC) also includes - a synchronous reference frame phase locked loop circuit (300) connected to the back EMF current sensor (103, 104) and configured to output a rotor position signal; - a maximum current circuit configured to determine a desired maximum back EMF current; and - an input for receiving information about the rotation speed of the rotor; Wherein a control circuit (CC) is configured to determine the short circuit interval using a desired maximum back EMF current, a rotational speed of the rotor and motor parameters.

5. A control circuit (CC) according to claim 4, The control circuit (CC) is configured to short-circuit the windings of the rotor by means of the inverter circuit (102), and measure the back EMF current during the short-circuit, and if the measured back EMF current is equal to or greater than the maximum back EMF current, then the short-circuit is terminated.

6. A control circuit (CC) according to any one of claims 4 to 5, in, Steps a) to c) are configured to be performed at a switching frequency during a predetermined period such that a duty cycle is equal to or greater than 0.

5.

7. A rail vehicle (600), include: - A control circuit (CC) according to any one of claims 4 to 6; - Permanent magnet motor (PM1); - a DC voltage link (101); - an inverter circuit (102), connected to the DC voltage link and to the electric machine, The inverter circuit (102) includes control lines for controlling the supply of power to the windings of the stator of the motor.

8. A non-transitory computer-readable medium containing program instructions for causing a computer to execute the method as claimed in any one of claims 1 to 3.

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