FACULTY OF ELECTRICAL ENGINEERING

Eng. Nicolae Ștefan PREDA

PhD THESIS

- Abstract -

OPTIMIZATION AND IMPLEMENTATION OF THE DUAL FIELD ORIENTED VECTOR

CONTROL FOR THE CAGE INDUCTION MACHINE

PhD Supervisor, Prof. Dr. -eng. Maria IMECS

Evaluation committee PRESIDENT: Prof.Dr.Ing Radu Ciupa – Dean, Faculty of Electrical Engineering,

Technical University of Cluj-Napoca; MEMBERS: Prof.Dr.Ing. Maria Imecs – PhD Supervisor, Technical University of Cluj-Napoca; Prof.Dr.Ing. Răzvan Măgureanu – Reviewer,

„Politehnica” University of Bucharest; Prof.Dr.Ing. Alexandru Bitoleanu – Reviewer,

University of Craiova; Prof.Dr.Ing. IulianTudor Mircea Birou – Reviewer,

Technical University of Cluj-Napoca.

Chapter 1 represents the introductive part of the thesis, where the general context, the objectives and the structure of the thesis are presented. Starting from the main objective, which is the implementation of the dual field oriented vector control, and considering the general structure of an electrical drive, the drives’ components (motor, converter, modulation technique, vector control structure) will be modeled and simulated. For implementation and experimental tests, the models of the motor and converter will be removed and replaced with the correspondent physical ones. A second objective was aimed by this thesis, which is the energetical optimization of the dual field oriented vector control structure for large power drives.

Therefore, the thesis is divided into 7 chapters out of which the first one is introductory and the seventh one contains conclusions, contributions and perspectives. The thesis also contains a reference list, a list of published papers and annexes.

The 2nd chapter deals with induction machine modelling and simulation. Starting from the general equations of the asynchronous motor, a set of expressions are derived which are used for modeling in Simulink® the squirrel-cage induction motor, using stator-fixed (natural) coordinates.

Fig 1. Structure used for simulation of the induction motor.

Validation of the model is done through simulation, by supplying the motor from a three-phase sinusoidal balanced voltage reference. The simulation is performed using the nameplate data and parameters of a real asynchronous motor from Siemens.

Chapter 3 presents the power converter and the modulation method used to control the semiconductor devices.

Fig. 2. Diode bridge rectifier topology. Fig. 3. Voltage source inverter topology.

The solution chosen for supplying the motor is an indirect static frequency converter, composed of a three-phase bridge diode rectifier and a three-phase voltage source inverter. These two form the most commonly used solution for a.c. motor supply and can be found in compact form along with integrated circuits for protection and control.

The space-vector (or space-phasor) modulation – SVM – was used for controlling the switching of the inverter. Considering the topology of the chosen inverter, only 8 basic voltage vectors can be produced at the inverter’s output (out of which two are zero).

Fig. 4. Basic voltage vectors produced by the VSI.

Using space-vector modulation, a given reference voltage vector can be synthesized using two non-zero basic voltage vectors and the zero ones:

6,1,11 =⋅+⋅= ++ kuuu kkkkref δδ .

The on-time of the active voltage vectors can be easily computed:

−⋅= γπδ3

sinˆ3 ref

dck u

V γδ sinˆ

31 ⋅=+

ref

dck u

V

while the on-time of the zero vectors )(1 1870 ++−=+= kk δδδδδ (also called free-wheeling

duration), can be distributed regarding different optimization criteria. According to this distribution, several modulation methods can be obtained. In the thesis, five SVM techniques were treated:

- the classical (simple suboptimal) SVM :

20

87

δδδ ==

- and four discontinuous (flat-top) methods:

For Simulink modeling of the SVM methods, a series of computation steps were taken in consideration, with which the duty cycle for each inverter leg is obtained. An original is made to the reference voltage vector position determination. The method used divides the

voltage hexagon into sextants and sectors and uses simple comparisons to determine the vectors position, without using any trigonometric functions or look-up tables.

After modeling, the five SVM methods and the converter were validated through simulation. Also an harmonic analysis of the modulation techniques was performed, which was confirmed experimentally through implementation on a digital signal processor – DSP.

Chapter 4 deals with the dual-field oriented vector control of the squirrel-cage induction motor. The dual-field orientation principle came from analyzing the advantages and disadvantages of the two classical field-oriented methods (the rotor field and the stator field one):

- rotor-field oriented control benefits from decoupled control of flux and torque through stator-current components and linear static mechanical characteristics, but the stator voltage computation is highly complex so usually closed-loop current PWM inverters are needed;

- stator-field oriented control benefits from a simple stator voltage computation, making it applicable to open-loop voltage inverters, but decoupled control is only obtained with compensation of the stator current components and the static mechanical characteristics are nonlinear.

The dual-field orientation claims the advantages of both methods and eliminates the disadvantages. Therefore, the flux control and generation of the stator current components is done in the rotor-field oriented frame:

m

rsd L

ir

Ψ=λ

r

e

mp

rsq

m

Lz

Li

r Ψ=

3

2λ

The stator voltage components are computed in the stator-field oriented frame:

dt

diRu s

sdssd ss

Ψ+= λλ ssqssq sss

iRu Ψ+= λλλ ω

Two dual-field oriented vector control structures were taken in consideration, one with voltage compensation loop and one simplified, without voltage compensation.

Fig. 5. Block diagram of the simulation structure for the dual-field oriented vector control for the squirrel-cage induction motor fed by a VSI.

For identification of the orientation fields, three field computation methods were compared. Each method was modeled and simulated, and the method yielding the best results was chosen for field identification in the dual-field oriented vector control structure.

The complete vector control structure was modeled using SimulThe PI controller gains were tuned using SISO Design tool, for the 4 control loops (two current loops, one rotor flux loop and one speed loop), taking in consideration a few design requirements regarding bandwidth and overshoot.. For tunintransfer functions from each loop were determined and the gains were determined graphically in SISO tool.

The two control structures were simulated, different test being run in order to compare the two structures and analyze their steady

Chapter 5 describes the experimental setup built for implementation and testing of the dual-field vector control.

The experimental setup can be considered of being composed of two parts: a computational and control part (consisting of a computer, a dSPACE development system and software) and a high-power part (consisting of induction converters, transducers etc.)

Fig. 6. Block diagram of the experimental setup

Fig. 7. Block diagram of the experimental setup

The complete vector control structure was modeled using SimulThe PI controller gains were tuned using SISO Design tool, for the 4 control loops (two current loops, one rotor flux loop and one speed loop), taking in consideration a few design requirements regarding bandwidth and overshoot.. For tuning, each loop was described, the transfer functions from each loop were determined and the gains were determined graphically

The two control structures were simulated, different test being run in order to compare e their steady-state and dynamic behavior.

describes the experimental setup built for implementation and testing of the

The experimental setup can be considered of being composed of two parts: a rol part (consisting of a computer, a dSPACE development system and power part (consisting of induction motor, load motor, power

Fig. 6. Block diagram of the experimental setup – computational

. Block diagram of the experimental setup – power part.

The complete vector control structure was modeled using Simulink® and PLECS®. The PI controller gains were tuned using SISO Design tool, for the 4 control loops (two current loops, one rotor flux loop and one speed loop), taking in consideration a few design

g, each loop was described, the transfer functions from each loop were determined and the gains were determined graphically

The two control structures were simulated, different test being run in order to compare

describes the experimental setup built for implementation and testing of the

The experimental setup can be considered of being composed of two parts: a rol part (consisting of a computer, a dSPACE development system and

motor, load motor, power

computational part.

part.

Fig. 8. View of the whole experimental setup.

The load motor used is a surfacefor torque generation a simple vector control structure was chosen ( the constant torque ancontrol strategy).

The dual-field oriented control for the induction machine was implemented on the experimental setup through a realprogram was structured into 5 modules and synchronizsed with the PWM generation through a software interrupt. The program was controlled through an interface developed in ControlDesk.

Fig. 9. Triggering of the implemented real

Several test were carried on both fieldand steady-state. A very good steadystructures, over the whole speed range.

Chapter 6 deals with an optimization problem, regarding theoriented control for large power drives. The objective was to reduce the power losses in the switching devices and two solutions were proposed.

The first solution is the implementation of discontinuous modulation methods (presented and analyzed in chapter 3) in the dualreduce the number of commutations by 1/3 over the fundamental period. The discontinuous SVM was successfully implemented in the control and it was tested on the experimental rInfluence on speed and flux control was analyzed during tests.

Fig. 8. View of the whole experimental setup.

The load motor used is a surface-mounted permanent magnet synchronous motor, and for torque generation a simple vector control structure was chosen ( the constant torque an

field oriented control for the induction machine was implemented on the experimental setup through a real-time program, created from the Simulink model. The program was structured into 5 modules and synchronizsed with the PWM generation through a software interrupt. The program was controlled through an interface developed in

Fig. 9. Triggering of the implemented real-time program and its modular

ried on both field-oriented control structures, for both dynamic A very good steady-state precision in speed control was noticed for both

structures, over the whole speed range.

deals with an optimization problem, regarding the usage of dualoriented control for large power drives. The objective was to reduce the power losses in the

and two solutions were proposed.

is the implementation of discontinuous modulation methods d analyzed in chapter 3) in the dual-field oriented vector control structure

reduce the number of commutations by 1/3 over the fundamental period. The discontinuous SVM was successfully implemented in the control and it was tested on the experimental rInfluence on speed and flux control was analyzed during tests.

synchronous motor, and for torque generation a simple vector control structure was chosen ( the constant torque angle

field oriented control for the induction machine was implemented on the e program, created from the Simulink model. The

program was structured into 5 modules and synchronizsed with the PWM generation through a software interrupt. The program was controlled through an interface developed in

modular structure.

oriented control structures, for both dynamic state precision in speed control was noticed for both

usage of dual-field oriented control for large power drives. The objective was to reduce the power losses in the

is the implementation of discontinuous modulation methods field oriented vector control structure, to

reduce the number of commutations by 1/3 over the fundamental period. The discontinuous SVM was successfully implemented in the control and it was tested on the experimental rig.

The other solution was to use a single complex current regulator (instead of two PI regulator) for the current control loops, that allows to decrease the switching frequency of the power converter.

Fig. 10. The complex current control loop.

This regulator compensates the delays introduced in the control loop by the motor, the PWM controlled power converter and the digital processor.

After implementation of the complex current regulator, it was tested showing that it allows decreasing the switching frequency. Test were made for reference current steps applied on the d and q oriented axes, and also for the whole control.

Chapter 7 presents the general conclusions of the thesis. The components and the vector control structures were modeled and simulated. The simulation results were validated through experimental tests. Also an optimization was performed on the vector control.

Some contributions were also presented: the proposal and usage of a new implementation method for the five SVM methods, a new dual-field oriented vector control structured was proposed, an experimental setup was built, two optimization methods were implemented and tested for the dual-field oriented control.

In perspective, implementation of the control on a stand-alone processing system (DSP, MCU) is desired and also its programming directly in C/C++ or assembler language. Also, further optimization regarding the decrease of switching frequency can be considered.