Matlab/Simulink models have been ported to PSCAD to compare the actual control capabilities, cost, complexity, and robustness of asymmetric half-bridge (AHB), n+ 1, and C-dump SRM converter topologies. The torque split function switching strategy for low-ripple torque control is presented and simulated with hysteresis current control for 8/6 SRM fed by a four-phase AHB converter.
CHAPTER 4
Contour function of four-phase SRM ..4.9 TSF torque controller with ideal current control ..4.12 PSCAD TSF torque controller with ideal current control.
CHAPTER 5
Performance Comparison of Four Inverter Topologies ..3.30 Total Locked Rotor Torque High with Ideal Current Control.
LIST OF ABBREVIATIONS AND SYMBOLS
Vfd Forward diode voltage drop Qll Gate signal for upper switch of phase 1 Q12 Gate signal for lower switch of phase 1. Tall Torque derived from lookup table Ta Torque reference for phase 1 Tb Torque reference for phase 2 Te Torque reference for phase 3 Td Torque reference for phase 4 Ia Current reference for phase 1 Ib Current reference for phase 2 Ie Current reference for phase 3 Id Current reference for phase 4.
CHAPTER ONE INTRODUCTION
- General
- Thesis Project Background and Objectives
- Thesis Layout
- Page 1.6
- Contributions
- Research Publications
- Summary
All of the above aspects of SRM drives are covered in more detail in the main body of this thesis. A Matlab/Simulink model of the prototype 8/6 SRM is presented and validated using locked rotor and free rotor alignment tests.
CHAPTER TWO
SWITCHED RELUCTANCE MOTOR MEASUREMENTS AND SIMULATION MODELS
Page2.8
- Phase Inductance and Torque Measurements
In the two analytical methods, the first is that the SRM torque characteristics can be derived from the nonlinear characteristics of the phase inductance. WI = W =-L(O)l 2 Then the instantaneous torque reduces to. 2.25), torque characteristics can be derived from the derivative of the phase flux with respect to the rotor position.
Page2.14
- Comparison of Torque from Calculation and from Torque Transducer Measurement
- Comparison of Different Connection of Each Stator Phase
- Simulation and Measured Results
- Electrical Simulation Model and Locked Rotor Test
- Four Phase Simulation Model and Simple Torque Control Strategy for 8/6 SRM
The next section develops the Matlab and Simulink models from the measured phase inductances and torque characteristics obtained from measurements of the inline phase connection SRM torque converter. The torque characteristic is entered in the form of a 2-D lookup table with current and position indices.
CHAPTER THREE
POWER CONVERTER STRATEGIES FOR SWITCHED RELUCTANCE MOTOR
Introduction
Generic Properties of SRM Power Converters
PSCAD [3.4] is a powerful and flexible graphical user interface for the well-established EMTDC (Electromagnetic Transients including DC) simulation engine. This implementation is described in detail in Section 3.9, where simulation results are also shown to agree closely with measured results.
The Asymmetrical Half Bridge Converter
- AHB Converter Introduction and Principle of Operation
Hard switching on and off for the S I-S 1 ' phase is achieved by feeding the signal Q 1 shown in Fig. Comparison of the current response immediately after O.1, however, shows that the switching current conduction error is smaller for the strong cutoff.
- N1S Converter Introdnction and Principle of Operation
Hard start mode for phase Sl-Sl' b) Idle mode for phase Sl-Sl' .. c) Hard shutdown mode for phase Sl-Sl'. The advantage of this converter is that it has fewer switches and thus a slightly lower cost.
- SRM Converter Topology Selection
The capacitor supplies the phase demagnetization energy to the next phase, which turns on instead. This significantly reduces the voltage values for the switching devices (as shown in Figure 3.22(c)) and thus the cost of the converter. A further advantage of this approach is that one phase can be operated in hard-off mode while the next stage is implemented in hard-on mode, as shown in Fig. 3.22(d).
The capacitor current and voltage in the turn-on and hard-off phases of MCD and CECD are compared in Fig. In the locked rotor test, the rotor is fixed at an angle of 5° (ie the same condition presented for the Matlab/Simulink model in Chapter 2).
- Summary
Vdc015.dat is the measured Vdc, i015.dat is the measured phase current, th015.dat is the measured rotor angle, and Vp015 is the measured phase voltage. This chapter explained the principle of operation and used PSCAD models and simulations to compare the performance of four different SRM power converter topologies under different operating conditions. However, it is suggested that future work should aim to minimize the control complexity and exploit the potential advantages of faster independent phase commutation and tighter current control offered by the cost-effective c-dump topology.
This chapter has also presented and validated PSCAD SRM and ARB models using locked and free rotor alignment tests. The next chapter makes use of these models to develop and test a PSCAD torque control strategy based on optimal simultaneous phase commutation and current control.
CHAPTER FOUR SRM TORQUE CONTROL
Introduction
A comparison of phase 1 current and inductance shows that the negative torque is due to the coincidence of non-zero phase current and negative dL/d8 (ie dL/dt < 0 and d8/dt > 0). It is therefore possible to increase or decrease (i.e. roughly control) the average torque by adjusting the cut-off angle. A slight progression of engagement angle from 30° to 27" results in negative torque until the angle reaches 30°.
However, this negative torque is very small due to the low dL/d8 in this range and the relatively slow current response, but this is more than compensated by the resulting increase in the coincidence of high current values and positive dUd8. The trip angle is also increased from 60° to 51° to prevent non-zero current from coinciding with large negative values of dUd8, and thus significant negative torque values.
SRM Torque Control
Page 4.9
- Torque Control PSCAD Model and Results
- TSF Torque Control with Ideal Current Controllers
This TSF is defined by a contour function that describes each phase contribution to the total torque reference, as a function of the instantaneous angle. To obtain the corresponding reference current for each phase, the torque angle current characteristic (T - B - i) is needed. The measured information is then stored as a look-up table (B - i - T) to estimate each phase torque in the simulation model, and as (T - B - i) to obtain the current reference for each phase as a function of the instantaneous torque reference and rotor angle.
Accurate knowledge of the engine's torque characteristics is therefore essential to the success of such an approach. The next section presents a PSCAD implementation of the above six-step TSF torque control method and simulation results under several locked and free rotor operating conditions.
Page 4.15
Torque Control ofSRM
4.15, and Table 4.2 show that the magnitude of the rotor free-rotation ripple (due to errors in the inversion of i - B - T to obtain T - B - i, and the linear interpolation error) increases with the magnitude of the pair reference torque, but that the ripple percentage is lower at higher torque magnitudes. The corresponding torque accumulation is seen to be significantly higher (70.0%) due to the strong nonlinearity of the torque characteristic with respect to the current. This is most likely due to increased back emf, resulting in lower hysteresis current control ripple (due to lower attenuation) and hence lower torque ripple.
The overall effect, however, is a decrease in relative (percentage) torque ripple as torque magnitude increases. IReg.Rippel and IComm.Rippel represent torque ripple due to current regulation and commutation respectively.
Torque Control of SRM
Summary
The corresponding PSCAD torque control model has been presented and tested under several locked and free rotor conditions, assuming ideal current control. A PSCAD model for hysteresis current control with a fixed error sampling rate (and thus upwardly limited switching frequency) has also been combined with the TSF model and tested under various operating conditions. It has been shown that this hysteresis current control method requires a relatively high sampling rate to produce low current (and hence torque) switch ripple at low speeds with higher values of DC link voltage.
It was therefore suggested that future work investigate PWM current control to reduce voltage switching induced current and torque ripple. The next chapter presents a practical implementation of the TSF torque and current control strategy, and measured results to confirm the various predictions obtained from the simulations of this chapter.
CHAPTER FIVE
PRACTICAL IMPLEMENTATION
Introduction
Prototype 8/6 SRM Drive Hardware
- Dynamic Torque and Current Responses With Constant Speed Load
A photo of the corresponding experimental setup is shown in Figure 5.2. Photo of the experimental setup of the prototype 8/6 SRM drive. The PC is used to develop the real-time version of the torque and current control strategy presented in Chapter 4. The main computational burden comes from the T-8-i lookup table used to determine the phase current references as a function of the instantaneous total. torque reference and rotor angle (the i-8-T table is also used in this experimental work to estimate the actual torque produced by each stage and the total torque, for comparison with simulation results).
It has been shown that while the step time (ie the time the reference torque changes from zero to +lNm or -lNm) is arbitrary, the corresponding current rotor angle has a significant effect on the torque and current transient response. The simulated current responses in Figs. iv) 10.0° closely with the measured result during periods when the DSP does not drop samples, e.g. during most of the first current pulses in Fig. 56.5°. The actual cut-off angles differ for negative rotation because the rotor angle is always (iii) 45.0° twisted to lie between 0° and 60° as shown in fig.
The next set of tests examines the speed and current dependence of the current and torque step responses.
Summary
CHAPTER SIX CONCLUSION
- General
- Switched Reluctance Motor Measurements and Simulation Models
- SRM Torque Control
- Practical Implementation
- Suggestions for Further Work
The ARB converter topology was chosen for the experimental work of this dissertation because of its relative simplicity in terms of phase commutation and current control, its ability to allow independent phase commutation and current control to reduce torque ripple, and relatively good phase commutation and current control performance. The control performance of the selected TSF torque and hysteresis current control strategy was tested under different locked rotor, motor and generator conditions. PSCAD simulation results were compared with measured results to validate the PSCAD torque and current strategy models presented in Chapter 4.
All these results show that the selected Torque Sharing Function (TSF) and hysteresis current control strategy perform well under locked rotor and car and generator test conditions at low speed. Possible static and dynamic optimizations of the angles of TSF for the 8/6 four-phase SRM in this thesis;
APPENDIX A
