R.B. Inderka

Control of switched reluctance drives for electric vehicle applications

Dynamic controllers of switched reluctance drives adjust at least three variables, i.e., current amplitude, turn-on, and turn-off angles. In electric vehicle (EV) applications high efficiency of the drive over a wide speed range, wide torque bandwidth, and low torque ripple under varying DC-bus voltage conditions are important design goals. Hence, controllers of switched reluctance drives for EVs usually have a complex structure. In this paper, the demands on control accuracy of switched reluctance machine traction drives and the traction controller sampling frequency, which are necessary to take advantage of the switched reluctance machine dynamic capabilities, are discussed. To integrate the traction drive, the control commands need to be actualized with a sampling frequency of at least 100 Hz to meet the high-dynamic requirements of modern vehicle control systems, e.g., active cruise control, antislip control, and active damping of mechanical drivetrain oscillations. It is found that the switching angles have to be adjusted within one-tenth of a mechanical degree. This study shows that switched reluctance drives can fulfill all requirements needed for electric propulsion using standard microcontrollers or digital signal processors.

DITC-direct instantaneous torque control of switched reluctance drives

This paper presents an on-line instantaneous torque control technique. The method comprises two novel aspects. Torque is estimated as a function of terminal quantities, i.e. flux linkage and phase current. Hence, the control algorithm is independent of the rotor position. Moreover, high bandwidth drive performance is obtained by implementing a digital torque hysteresis-controller. Thus, the method works without torque distribution functions or auxiliary phase commutating strategies. Therefore, the control algorithm offers a wide drive operating range without the use of a high-resolution shaft position sensor or sensitive position estimation techniques. Experimental and simulation results are presented in this paper.

High-dynamic four-quadrant switched reluctance drive based on DITC

This paper presents the development of a four-quadrant switched reluctance machine (SRM) drive for high dynamic applications. Comprehensive fundamentals and analysis for operating switched reluctance machines in four quadrants are presented. The drive is designed based on a high dynamic control strategy called Direct Instantaneous Torque Control (DITC). The functionality of DITC is discussed in detail for both motoring and generating operation. A methodology to generate switching functions directly by the hysteresis torque controllers for SRMs is proposed. The proposed controller was prototyped and tested on a digital signal processor/field-programmable gate array development platform. High dynamic operation in both motoring and generating mode and the transition between these modes are validated by experimental results presented at the end of this paper.

High-dynamic direct average torque control for switched reluctance drives

In switched reluctance machines, no proportionality between torque and phase current exists as in rotating field machines. Therefore, a new technique was developed to estimate online average electromagnetic torque of an SRM drive. This novel average torque and energy-ratio estimation technique can be used for closed-loop torque control. Different control architectures for closed-loop average torque control are analyzed and evaluated in this paper because multiple methods exist to track the average torque by adjusting the control variables (/spl theta//sub on/, /spl theta//sub off/, i/sub ref/). The switched reluctance drive controller used in this study was developed for a 55 kW electric-vehicle traction drive. Hence, the system is controlled by a torque command set by the driver. Experimental results obtained from a 100 kW test bed for traction drives are presented.

Optimizing performance in switched-reluctance drives

This article discusses the requirements of electric vehicle traction drives and the consequences to the control strategies of a switched reluctance motor drive. The selection of these control strategies for different operating regions in the torque-speed diagram is also discussed. To test different control strategies, a specialized simulation program was constructed. The simulation results and the implementation of the optimization strategies are discussed. Finally, the measurement methods and the test results are presented.

On-line estimation of instantaneous torque in switched reluctance machine control

An inextensive and simple method to estimate the instantaneous torque of a switched reluctance machine online is developed and discussed in this paper. The presented technique is characterized by its simplicity and robustness in two ways. Only phase current and voltage are required as input variables. Therefore, shaft position-sensors can be avoided. Furthermore, the estimation algorithm can be realized in hardware, i.e. no processor is needed, the digital unit consists only of a simple memory device such as an EPROM. The estimation technique requires the machine characteristics. The method presented in this paper estimates torque as a function of flux linkage and phase current. With this real-time estimation and feedback of the instantaneous torque it is possible to control the torque with a hysteresis controller to obtain high performance drives. Furthermore, it is shown that the performance of the drive regarding torque-ripple can be significantly enhanced. The system is developed for the control of high performance drives as they are used in traction application. Experimental and simulation results are presented in this paper.

Eddy currents in medium power switched reluctance machines

Switched reluctance machines are characterized by high local flux densities in the core, high current gradients in the windings and relatively large winding areas for the nondistributed windings. In particular, in medium-, high-power and high-speed machines these characteristics may lead to thermal problems. Hot spots in the windings are a result of losses, which originated from high-frequency alternating magnetic fields. It is identified that proximity effects situated at the pole tips and across the entire winding area are causing significant current displacements and in consequence for a considerable amount of copper losses. This paper reports the phenomenon of eddy currents in the windings of switched reluctance machines. Experimental verification and finite-element-simulation are presented for a 4-phase 55 kW switched reluctance machine.

Simulation model of a switched reluctance drive in 42 V application

In modeling switched reluctance drives for low-voltage applications, an accurate converter model is mandatory. The voltage drop across the semiconductor devices is significant in comparison to the available dc-bus voltage and must be taken into account. Furthermore, in 42 V applications, in which batteries are utilized as the energy source, the dc-bus voltage extremely fluctuates due to the load-dependent behavior of batteries. Therefore, the battery behavior should be also regarded in the drive system model. This paper presents the modeling of a switched reluctance drive for 42 V application regarding behavior of the converter as well as the energy source. The entire switched reluctance drive system is modeled in a single simulation platform MATLAB/Simulink. The model accuracy is verified by the experimental results presented at the end of the paper.

Critical states in generating mode of switched reluctance machines

When switched reluctance drives operate in generator mode, overcurrents can occur due to high back EMF voltages. These overcurrents produce large torque pulsations. Moreover, they can become excessive, creating a thermal overload for the power converter. These phenomena have not been discussed in detail before and are analyzed in this paper. Two overcurrent limits have to be distinguished. A so-called critical factor is defined to assess the risk of overcurrents leading to torque pulsations. In addition, an absolute maximum overcurrent is calculated to specify converter device ratings, preventing converter overload or failure. To protect the drive against uncontrollable overcurrents and the resulting torque pulsations, an adaptation of the drive control to avoid such failure states is suggested.

Optimizing performance in switched reluctance drives

It is demonstrated how the overall performance of a switched reluctance drive can be improved by using different control strategies for different regions of the torque-speed diagram. For a 4-phase 30 kW machine these strategies are fitted by connecting different optimizing routines with a simulation program. The importance of a correct tuning of the control parameters like turn-on angle, turn-off angle, reference current and size of the hysteresis band is demonstrated for a current controlled drive. Furthermore, the test bench and the measurement procedure for the experimental verification and comparison to the simulation are presented.