Sliding Mode Direct Torque Control of Three Phase Induction Machine Applicable in Electric Vehicles
CHAPTER ONE
Objectives of the Study
Two control methodologies for the IM will be investigated: the constant volts/hertz (v/f) and the direct torque control (DTC) methods. The objective is to design a SM – based DTC, first, the design and implementation of v/f will be discussed and its model results presented and then compared with that of the DTC. The performances of these schemes will be evaluated, compared and conclusions will be drawn from the results.
Thus, the main objective of this study, which is the contribution of this thesis, is to develop encoder less operation of a Sliding Mode Direct Torque Controlled IM drive suitable for application in electric vehicles. In order to achieve this, the main objective is broken down to the following sub-objectives:
- Analysis and modelling of the electric
- Mathematical analysis of the Induction
- Design and implementation of the v/f control strategy for the Induction
- Development of sliding mode direct torque control (SM-DTC) strategy and algorithm for the Induction
- Performance comparisons and inference between the v/f and
CHAPTER TWO
Preliminary Concepts
Modelling the EV
Computers are very useful tools for modelling real world systems. Mathematical models will be developed for the dynamics of the EV. During simulation, effects of varying the model parameters can be seen as it affects acceleration, top speed, motor sizing, vehicle range, etc. Data produced from the model equations will be used to predict vehicle behaviour.
- Tractive Effort
Consider a vehicle of mass m, moving at a velocity v up a slope of angle αg as shown in Figure 2.1. The force propelling the vehicle forward has to accomplish the following:
- Overcome rolling resistance,
- Overcome aero dynamic drag,
- Provide the force needed to overcome the vehicle’s weight acting down the slope,
- Accelerate the vehicle if the velocity is not constant
CHAPTER THREE
The Induction Motor: Mathematical Analysis
Induction Motor Analysis using Space Vectors
Usually referred to as the workhorse of industry, the induction machine has many applications. In this case, it is applied in traction for EV propulsion, and as the sole unit of propulsion, deep understanding of its principles of operation, its behaviour under different conditions and mathematical analysis of its operation is fundamental. In the following analysis, it is assumed that the magnetic material in the stator and the rotor: 1) is operated in its linear region, and 2) has an infinite permeability, i.e. there is no magnetic saturation.
Space Vectors
At any instant of time, each phase winding produces a sinusoidal flux-density distribution (mmf) in the air gap. This is one of the most fundamental principles of the IM, the creation of a rotating and sinusoidally distributed magnetic field in the air gap. Figure 3.1a shows an idealized three-phase, two-pole IM where each phase winding in the stator and rotor is represented by a concentrated coil. This three-phase windings (either in wye or delta form) are distributed sinusoidally in space and embedded in slots [16, 17]. Neglecting the effect of slots and space harmonics due to winding distribution, it can be shown that a sinusoidal three-phase balanced power supply in the three-phase stator windings creates a synchronously rotating magnetic field.
CHAPTER FOUR
Scalar Based Control Schemes
Scalar Control
Induction motors have different ways they can be controlled. The simplest methods are based on changing the structure of stator winding such as the wye-delta switching to change or reduce the starting current, pole changing, i.e. changing the number of magnetic poles on the stator etc. However in modern adjustable speed drives (ASDs), it is the stator voltage and current that are subject to control and in the steady state, are defined by the magnitude and frequency; and if these are the parameters that are adjusted, the control technique belongs in the class of scalar control [17]. Kazmierkowski [57] further explains that the control scheme is based on steady-state characteristics, which allows stabilization of the stator flux magnitude λs for different speed and torque values.
CHAPTER FIVE
Variable Structure Direct Torque Control of the Induction Motor
In high-performance drive systems, in which control variables include the torque developed in the motor, vector control methods are necessary [17]. In chapter two the DTC control technique was introduced and the principle of encoder-less operation was enumerated. Also the sliding mode theory was introduced and conditions for existence and stability was presented. The analysis of the IM carried out in chapter 3 establishes the mathematical requirement for the implementation of the sliding mode DTC control scheme. Two different sliding surfaces for flux and torque will be designed so that the error between reference and actual values are driven to respective sliding surfaces where the error is enforced to zero.
The proposed schematic, of the drive system is presented in Figure 5.1 comprising of the power circuit, the co-ordinate transforms from dq to abc coordinates, the estimator block and the sliding mode controller. The estimator and sliding mode controller block is explained in the next sections.
CHAPTER SIX
Results and Discussion
V/f Experimental Results
Signal generated in the laboratory for the v/f control are presented in this section. Initially, the SPWM was designed using analog and logic circuits, the circuit and signals obtained have been discussed in SPWM section of Chapter 4. The six-step wave was generated and applied in the eventual design and implementation of the v/f drive discussed in Chapter 4. Results for the step-wave operation include the phase and line voltages obtained at the output of the three-phase inverter and the v/f graphical output from the control logic circuit.
Six-step Output Voltage
Figure 6.1 shows the output voltage waveforms for the six-step operation of the voltage- fed three-phase inverter. Figure 6.1a shows the line voltage output while (b) shows the phase voltage waveform. This waveforms match the MATLAB® simulated output, hence the results are satisfactory.
The logic circuit controls the ratio of voltage to frequency. The circuit was tested at different step voltages (2 units/step) to check the resulting change in frequency. The graph of voltage per frequency is plotted in Figure 6.2. The solid line shows the measured voltage- to-frequency obtained from the laboratory implementation while the dashed line shows the desired linear voltage-to-frequency ratio plot.
CHAPTER SEVEN
Conclusion and Recommendations
Conclusion
The two techniques employed in this research work are the constant volts/hertz controland the direct torque control using the sliding mode approach. Depending on the purpose and application of the drive, both techniques are able to achieve speed control.
The V/f control is one of the most popular control techniques and possessing a simple algorithm with no dependence on sensors and no requirement of speed measurement (as it can operate in open-loop). It is a much simpler control strategy than the DTC control and does not require high performance digital processing. The V/f drive response to torque is poor compared to the DTC drive but can be employed efficiently where precise speed control is not critical.
Speed and torque control of induction motor is usually attained by application of speed or position sensors, and its implementation is straightforward. However, the use of these encoders require the additional mounting space, reduction in reliability of the system in harsh environments [79] (such as vibrations from the EV) and also increase in the cost of motor drive. Furthermore, the encoders used for position and speed measurement may lead to problems. Faults such as loss of output information, offset, disturbances, measurement deviation [80], and so on, may occur. On the other hand, encoderless DTC control of induction motor drives estimates position using an observer and eliminates the need for the speed sensor. It reduces hardware complexity, size, maintenance and ultimately cost. It also eliminates direct sensor wiring and has been shown to have better noise immunity [81] and increased reliability.
The objective of the DTC is to maintain the motor torque and stator flux within a defined band of tolerance, hence it requires an accurate knowledge of the magnitude and angular position of the controlled flux. But its greatest drawback is the sensitivity to uncertainties in the motor parameters. The speed estimation is affected by parameter variations especially the stator resistance due to temperature rises, particularly at low speeds which may cause significant performance degradation and even instability of the system.
The sliding mode approach to the drive design is to ensure the stability and high performance levels in spite of the disturbances and mismatches. The sliding mode controller is designed to drive, force and confine the system state to lie within a very small neighbourhood of the switching function despite the disturbances and perturbations in harsh EV environments, and variations in IM parameters. The ease of implementation and relatively short computation required in the implementation of SMC in microcontrollers is an advantage of this technique.
The result of this study is an SM-DTC controller design which eliminates some limitations of the two individual controls and retains their merits. The performances of SM-DTC IM drive even in the presence of parameters uncertainties and mismatching disturbances has been presented. The drive delivers high performance and quicker response and can be applied in EV with satisfactory performance.
Recommendations
Modifications and improvement can be made in the control scheme such as an improved filter and observer design of the sliding mode controller also the design of an adaptive algorithm for increased robustness and reduced sensitivity to disturbances. The SM-DTC concept discussed so far remains theoretical until it is implemented, thus implementation is recommended and is possible with the use of microprocessors for computations.
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