GB2526757A

GB2526757A - Method for controlling a three-phase alternating-current machine

Info

Publication number: GB2526757A
Application number: GB1517763.7A
Authority: GB
Inventors: Ciprian Antaloae
Original assignee: AVL Powertrain UK Ltd
Current assignee: AVL Powertrain UK Ltd
Priority date: 2013-03-19
Filing date: 2014-03-19
Publication date: 2015-12-02
Anticipated expiration: 2034-03-19
Also published as: AT514176A1; WO2014147111A1; GB201517763D0; AT514176B1; GB2526757B

Abstract

The invention relates to a method for controlling a three-phase alternating-current machine, in particular for an electrically driven vehicle or a vehicle roller test stand. Optimal use of the alternating-current machine can be achieved if a group of several different control strategies is provided, preferably stored in the motor controller, at least one operating parameter of the alternating-current machine and/or of the electric vehicle and/or external specifications, preferably regarding power and/or duration and/or range, are detected, a control strategy is selected from the group of control strategies in dependence on at least one detected operating parameter of the alternating-current machine and/or of the electric vehicle and/or external specifications, and the alternating-current machine is operated on the basis of said selected control strategy.

Description

Method for controlling a three-phase alternating-current machine The invention relates to a method for controlling a three-phase alternating current machine, in particular for an electrically driven vehicle -hybrid vehicle or electric vehicle -or a vehicle roller test stand (dynamometer).

The operation of battery-operated electric vehicles is limited by the low energy density of the high-voltage batteries that are carried. Whilst considerable efforts have been made to improve the battery technology, increased consideration must also be given to all improvements of the efficiency of auxiliary systems of the drive such as power electronics, motor design and motor control.

Various control strategies are known for the operation of a three-phase alternating current motor.

US 5,498,945 A describes a method for the maximum torque per ampere for an induction motor, where suitable combinations of the currents and magnetic fluxes of the quadrature axes or direct axes are selected.

US 6,630,809 B2 discloses a system and a method for controlling an induction motor, where the control is accomplished on the basis of the maximum torque per ampere strategy and constant common flux linkages.

The publication "Control strategies and Parameter Compensation for Permanent Magnet Synchronous Motor Drives", Ramin Monajemy, PhD Thesis, Blacksburg, Virginia Polytechnic Institute, 12 October 2000, describes in Chapter 5, pages 55 to 94, various control strategies for permanent magnet synchronous motors, including the maximum efficiency strategy, the maximum torque per ampere strategy, the zero d-axis current strategy, the unit power factor strategy and the constant mutual flux linkage strategy.

However, each of these control strategies can only be used for a specific operating mode of the alternating current machines, whilst disadvantages must be accepted in other types of operation. In known controls of electric motors, usually only a single predefined one of the known control strategies is used.

However, each control strategy merely constitutes a compromise for various operating modes. Consequently, serious disadvantages must usually be accepted in operation depending on the specific control used.

It is the object of the invention to avoid these disadvantages and develop a method which enables optimal operation of three-phase alternating current machines.

According to the invention, this is achieved whereby a group of several different control strategies is provided, preferably stored in the motor controller, at least one operating parameter of the alternating current machine and/or of the electric vehicle and/or external specifications, preferably with regard to power and/or duration and/or range is detected and from the group of control strategies, one strategy is selected as a function of at least one detected operating parameter of the alternating current machine and/or of the electric vehicle and/or external specification, and the alternating current machine is operated on the basis of this selected control strategy.

The control strategies are preferably stored in the motor controller and are selected depending on the particular requirements. Only the control strategy selected in each case is activated and the alternating current machine is operated on the basis of this selected control strategy.

The group of control strategies includes at least two -preferably all -of the following strategies known per se: zero d-axis current strategy, maximum torque per ampere strategy, maximum efficiency strategy, unit power factor strategy, constant mutual flux linkage strategy.

Each of the known operating strategies has good and less good areas of use.

Each of the control strategies is evaluated for defined selection criteria, preferably various operating parameters of the alternating current machine and/or the electric vehicle and/or load requirements, where the evaluation is taken as the basis for the selection of the control strategies. At least one selection criterion can be selected from the group of relationships torque/speed, torque/current, current/speed, power/speed, counterelectromotive force (or countervoltage)/speed, power factor/speed, air gap magnetic flux linkage (or air gap linkage), and the execution complexity (or complexity).

The control strategy can be selected by means of an algorithm -preferably using artificial intelligence, fuzzy logic and/or neural networks.

Preferably during operation of the alternating current machine it is checked continuously or discontinuously whether this is operated with the optimal control strategy for the particular situation. If the test based on a selection criterion from the group of relationships torque/speed, torque/current, current/speed, power/speed, counterelectromotive force/speed, power factor/speed, air gap magnetic flux linkage, and the execution complexity (or complexity) reveals that a different control strategy is more suitable, the control strategy is then changed and -depending on at least one operating parameter of the alternating current machine and/or the electric vehicle and/or external specifications -a suitable control strategy is selected and the alternating current machine is further operated with this selected control strategy. As a result, the control strategy can be optimally adapted to the respective situation and the respective requirements.

For example, in normal operation, in particular in hybrid vehicles, the unit power factor strategy or the constant mutual flux linkage strategy is selected for three-phase alternating-current machines. The high base speed made possible by the lower air gap magnetic flux linkage improves the driving properties of the vehicle at high speeds of the internal combustion engine on account of the permanent operation of the electric machine(s) at constant torque. In addition, the lifetime of the alternating current machine can be maximized with the unit power factor strategy or with the constant mutual flux linkage strategy since these strategies enable low current fluctuations in electric driving operation and a low switch-on current. As soon as the speed of the internal combustion engine is reduced (for example, engaging a high gear at high vehicle speed), the speed of the motor shaft is thus reduced and either the maximum torque per ampere strategy or the maximum efficiency strategy is selected for further operation in order to enable energy-efficient operation of the electric high-voltage system when the vehicle is operated in high gear.

For high power requirements the maximum torque per ampere strategy or the maximum efficiency strategy can be selected.

For maximum power and improved driving dynamics, for example, the driver can at the press of a button select the maximum torque per ampere strategy in order to achieve a maximum torque/speed ratio.

On the other hand, when there is an impending energy shortage and/or a predefined distance over a critical length, it is advantageous if the maximum efficiency strategy is selected. If, for example, a destination is preset by the driver, possibly using a navigation unit of the vehicle, which is close to the limit of the maximum range defined by the charging state of the battery, by selecting the maximum efficiency strategy, energy can be saved and the range therefore optimized.

Alternatively to the application in electric vehicles, the method according to the invention can also be used in stationary applications, for example, in vehicle roller test beds (dynamometers) in order to simulate defined driving scenarios.

An adaptive control strategy for the electric motor of the dynamometer can in this case adaptively investigate the planned driving scenario and select and apply the optimal control strategy for the particular driving scenario in each case. For example, in a section of the driving scenario with simulated ascent, the maximum torque per ampere control strategy can be selected. In simulated city driving scenarios on the other hand, the maximum efficiency strategy or the unit power factor strategy or the constant mutual flux linkage strategy can be selected to save energy or increase the lifetime of components of the dynamometer.

The method according to the invention is particularly suitable for three-phase asynchronous and synchronous machines, both for torque-controlled and for speed-controlled systems. In torque control the motor speed is obtained by the load applied to the motor shaft. In speed control the torque is obtained by the load applied to the motor shaft.

The method, in particular the detection of the operating parameters, the examination of the current control strategy, the selection of the optimal operating strategy, the finding of an optimal switchover time for the control strategy etc. can be carried out fully automatically by means of a control and monitoring program. In addition, it is possible to override the automatic control and monitoring program of the motor control by manual intervention, for example, if a high power is spontaneously required by the driver.

If a change of the control strategy is required during a known driving route, the optimal time for a change can be planned and executed automatically, for example, on the basis of a predefined topography or external information such as traffic situation etc. The invention is explained in detail hereinafter with reference to the figures. In the figures: Fig. 1 shows normalized power characteristics of the zero d-axis current control strategy; Fig. 2 shows normalized power characteristics of the unit power factor control strategy; Fig. 3 shows normalized power characteristics of the constant mutual flux linkage control strategy; Fig. 4 shows normalized power characteristics of the maximum torque per ampere control strategy; and Fig. 5 shows the method according to the invention in a block diagram.

The motor controller controls the intensity of the current flowing in the electrical machine so that the torque of the electrical machine follows a certain desired value or reference value. The regulating algorithm for controlling the current has been developed on the basis of a mathematical model of the electrical machine.

The most commonly used mathematical model of the electrical machine is composed of the following equations (1) to (4) and is also designated as "d-q system machine model". The equations (1) to (5) are specific for an inner-running permanent magnet synchronous machine. Other three-phase alternating current machines have a representation in the d-q system which differs slightly from this system of equations. dl

V =Iq R5 +Par (9PM dH1q (1) Vd=Jd*RsPCO,.*Lq*Jq+Ld (2) 9q=Lqlq (3) T =1.5.P.Jq +1 5P(L.J -,)Td.J,, (4) dxo =1(7 BOr (5) This mathematical system of equations has the advantage that the representation of the alternating current machine is simplified and that the control is easier to detect and to apply. In fact the "d-q system machine model" is a direct-current equivalent representation of the AC voltages and currents.

This system of equations is therefore preferably used for the implementation of the alternating current motor control.

Within the nonlinear d-q system, various combinations of d-and q-axis currents d and q can lead to the same motor speed w and the same torque Te. The simplest combination of d-and q-axis currents d and q consists in equating d to zero, with the result that the torque is a linear function of the q-axis current q.

This method is the usual control method used in motor controllers since it is easy to implement and is generally designated as "zero d-axis current control strategy ZDAC".

However, this is only one of many possible combinations of the d-and q-axis currents d and q which can be used to obtain the same level of torque Te. Thus, other combinations of d-and q-axis currents d and q can be used for the same torque Te in order to optimize specific aspects in regard to the operation of the alternating current machine.

One variable which is desirable to maximize is the extent of the torque Te generated by the alternating current machine in relation to the magnitude of the current used by the energy storage unit for the supply of the alternating current machine. This method, also known as the "maximum/optimal torque per ampere control strategy MTA" is based on the nonlinear relationship which is described by the above equation (4) in order to obtain the best combinations of the d-and q-axis currents d and q with regard to the torque Te, i.e. the largest ratio between Te and the vector sum of the currents,J +1. These pairs of d-and q-axis currents d and I with respect to the torque Te can either be determined analytically by solving the equation (4) or empirically by carrying out suitable test runs of the alternating current machine.

Another variable which is desirable to optimize is the energy efficiency of the alternating current machine. In order to achieve this, it is necessary_to find the pairs having the lowest value of the expressionsi +J and.jV +V, whereby both the torque Te, and also the speed w of the alternating current machine are maximized. As a result, the maximum mechanical power (product of torque Te and speed w), can be achieved at the lowest electrical power (product of current and voltage). This method which is known as "maximum efficiency control strategy ME" can be based on the analytical calculation of the best d-and q-axis currents d and I or carried out experimentally on the alternating current machine.

Based on a similar procedure as in the maximum/optimal torque per ampere MTA control strategy or the maximum efficiency ME control strategy, other aspects of the operation of the alternating current machine and/or the power electronics unit (DC/AC Inverter) can also be maximized. Thus, known under the designation "unit power factor UPF control strategy" is a method which is based on pairs of d-and q-axis currents d and I which result in in-phase alternating voltages and alternating currents in the power electronics unit. The method "constant mutual flux linkage control strategy (CMFL)" aims to find pairs of d-and q-axis currents d and I for which the total magnetic flux,,Jp +ço remains constant. These two methods can also be solved analytically or experimentally.

Thus, the following known control strategies are obtained, which are taken into consideration here: ii Zero d-axis current control strategy (ZDAC) Fig. 1 shows normalized power characteristics of the zero d-axis current control strategy (ZDAC).

Properties: * The power factor deteriorates with increasing rotor speed and increasing stator current I. * The lower power factor necessitates a higher VA rating of the inverter.

* The zero d-axis current control strategy (ZDAC) is suitable for applications which require high torque.

* A simpler execution is possible due to the linear current-torque relationship.

2i Unit power factor control strategy (UPF) Fig. 2 shows normalized power characteristics for the unit power factor control strategy (UPE).

Properties: * This strategy is not optimal for producing high torque.

* A low reactive VA rating is required * A low supply voltage is required.

* On account of the available reserve voltage, the unit power factor control strategy (UPE) is suitable for applications for extending the range of vehicles.

3.) Constant mutual flux linkage control strategy (CMFL) Fig. 3 shows normalized power characteristics of the constant mutual flux linkage control strategy (CM FL).

Properties: * Unlike other methods, this strategy can be used above the base speed * In this strategy a saturation of the motor core is avoided, thus the electrical drive torque is improved and high current transitions are avoided * The power factor is located close to one.

* The maximum speed at which a weakening of the flux is required is raised.

4.) Maximum/optimum torque per ampere control strategy (MTA) Fig. 4 shows normalized power characteristics of the maximum torque per ampere control strategy (MTA).

Properties: * This strategy is suitable for high salient machines (Lq>>Ld) * An approximately li.% increase in torque can be achieved compared with a zero d-axis current control strategy * Poor inverter utilization (i.e. a high VA rating is required).

5.) Maximum efficiency control strategy (ME) The maximum efficiency control strategy (ME) is based on minimizing the loss function: P1 =1.5R *(/; *[(Lq Iq)2 +@Pr'u +L. .J)2] (6) Fig. 5 shows the method according to the invention in a block diagram.

In a step 1, ambient and boundary conditions such as outside temperature, accelerator pedal pressure or -when used on a roller test stand -the type of dynamometer -drive cycle are determined.

In a step 2 it is interrogated whether the currently applied control strategy is actually the most optimal strategy on the basis of the ambient and boundary conditions established in step 1 and also based on the selection criterion from the group of relationships torque/speed, torque/current, current/speed, power/speed, counterelectromotive force /speed, power factor/speed, air gap magnetic flux linkage, and the execution complexity. If this is the case ("yes"), in step 3 there is a return to step 1.

If the interrogation from step 2 is negative ("no"), an adaptive control strategy algorithm is started in step 4 in order to determine the most suitable control strategy for the conditions detected in step 1.

In step 5 the optimum time for changing the control strategy is determined and this time is awaited.

In step 6 a change is made to the control strategy determined in step 4.

-10 -Nomenclature AC Alternating current B Friction coefficient DC Direct current Vg q-axis voltage (in relation to "d-q" alternating current machine model) q-axis current (in relation to d-q" alternating current machine model) L, q-axis inductance (in relation to d-q' alternating current machine model) Vd d-axis voltage (in relation to "d-q" alternating current machine model) d-axis current (in relation to d-q" alternating current machine model) L-d-axis inductance (in relation to d-q" alternating current machine model) Stator current V5 Stator voltage J Rotor inertia Pi Power losses VA Power Rc Core resistance of alternating current machine Stator resistance of alternating current machine (Dr Mechanical speed (PPM Permanent magnetic flux of alternating current machine q-axis generated flux (in relation to d-q' alternating current machine Pg model) d-axis generated flux (in relation to d-q' alternating current machine (Pd model) Torque angle between rotor field position and stator current indicator position 0 Phase angle between sinusoidal voltage and sinusoidal current Torque of alternating current machine Load torque

Claims

-11 -CLAIMS1. Method for controlling a three-phase alternating current machine, in particular for an electrically driven vehicle or a vehicle roller test stand, characterised in that a group of several different control strategies is provided, preferably stored in the motor controller, that at least one operating parameter of the alternating current machine and/or of the electric vehicle and/or external specifications, preferably with regard to power and/or duration and/or range is detected and that from the group of control strategies, one strategy is selected as a function of at least one detected operating parameter of the alternating current machine and/or of the electric vehicle and/or external specification and the alternating current machine is operated on the basis of this selected control strategy.
2. The method according to claim 1, characterised in that the group of control strategies includes two -preferably all -of the following strategies: zero d-axis current (ZDAC) strategy, unit power factor (UPF) strategy, constant mutual flux linkage (CMFL) strategy, maximum torque per ampere (MTA) strategy, maximum efficiency (ME) strategy.
3. The method according to claim 1 or 2, characterised in that each of the control strategies is evaluated for defined selection criteria, preferably various operating parameters of the alternating current machine and/or the electric vehicle and/or load requirements and that the evaluation is taken as the basis for the selection of the control strategies.
4. The method according to claim 3, characterised in that at least one selection criterion is selected from the group torque/speed, torque/current, current/speed, power/speed, countervoltage/speed, air gap linkage, power factor/speed and complexity.
5. The method according to one of claims 1 to 4, characterised in that the selection of the control strategies is made by means of an adaptive algorithm -preferably using artificial intelligence, fuzzy logic and/or neural networks.
6. The method according to one of claims 1 to 5, characterised in that during operation of the alternating current machine it is checked whether this is operated with the optimal control strategy for the particular situation and -if the test result is negative -the control strategy is changed and -depending on at least one operating parameter of the alternating current machine and/or the electric vehicle and/or external specifications -a -12 -suitable control strategy is selected and the alternating current machine is further operated with this selected control strategy.
7. The method according to one of claims 1 to 6, characterised in that in normal operation of the three-phase alternating-current machine, in particular in hybrid vehicles, the unit power factor (UPF) strategy or the constant mutual flux linkage (CMFL) strategy is selected.
8. The method according to one of claims 1 to 7, characterised in that at high load requirements on the three-phase alternating-current machine, the maximum torque per ampere (MTA) strategy or the maximum efficiency (ME) strategy is selected.
9. The method according to one of claims 1 to 8, characterised in that when there is an impending energy shortage and/or a predefined distance over a critical length, the maximum efficiency (ME) strategy is selected.
10. The method according to one of claims 1 to 9, characterised in that the method is carried out automatically.