Research On Key Technologies For Sensorless Control Of Switched Reluctance Motor Drives

The switched reluctance motor(SRM)has received considerable attention in connection with variable speed motor drive in many applications.Owing to their rugged structure,simple geometry,and high fault-tolerant capability,SRM drives have made a successful entrance into various sectors of industry such as aerospace,automotive,and home appliances.The rotor position of the SRM must be properly synchronized with the electromagnetic orientation of the machine to efficiently operate the motor.Conventionally,mechanical position sensors,such as optical encoders,resolvers,or Hall-effect sensors are installed on the motor frame to provide the precise rotor position information for SRM drives.To avoid additional cost,size,and unreliability associated with the external position sensors,developing a reliable,high grade and low cost sensorless control for SRM is quite necessary.This thesis mainly studies the SRM control strategies and sensorless algorithms from the viewpoint of engineering applications.In this thesis,a systematic and in-depth theoretical analysis,simulation and experimental study on several key technologies such as rotor position identification methods at high speed operation,initial rotor position estimation and continuous position estimation at low speed range,wide speed range sensorless schemes and fault-tolerant control of position detection technology for SRM drives.Aiming at the medium-high speed regions,the sensorless control methods based on the detection of the particular positions for each phase cycle are researched.The rotor position can be estimated by detection of the special positions for each phase cycle.The conventional current gradient sensorless method and phase inductance gradient sensorless method are improved,and a novel sensorless control for SRM based on gradient of phase inductance is proposed.In this method,current chopped control is adopted during the un-energized period.This proposed method uses the change of the gradient of the phase inductance to detect the un-aligned position.At the same time,an improved method based on EMF intersection is presented.The sensorless method uses the cross point position of transformer electromotive force(EMF)and motional back electromotive force(BEMF)to estimate the rotor position.The crossing point position can be derivate from nonlinear inductance model,and it is regarded as a reference point to estimate rotor position.The position sensorless operation of the full speed range is a key issue in the SRM sensorless technology.This chapter describes a sensorless position estimation technique for SRM over a wide-speed-range using pulse width modulation(PWM)voltage control.To achieve sensorless wide speedrange operation,a current-waveform-based method is presented.The proposed position estimation scheme can be implemented based on an improved current-slope-difference strategy and a current-gradient method over a wide speed range.At zero and low speeds,an improved current-slope-difference based algorithm is used to estimate the rotor position.For medium and higher speeds,a current-gradient method is used and combined to work with the low speed algorithm.The sensorless control of a SRM from startup at standstill to high speed operation can be easily implemented by the proposed methods.The performance of the sensorless SRM drive is fully investigated.A new sensorless control method based on full-cycle phase inductance vectors of SRM is presented.This method can realize sensorless control of SRM from starting up at standstill to high-speed operation and is quite easy to implement.For high speed,current chopped control is adopted during the un-energized period.For chopping at currents near zero,continuous monitoring of phase inductance is possible.At zero and low speeds,the full-cycle phase inductance can be estimated from current-slope-difference.The low-and high-speed methods are integrated together to provide a wide-speed-range rotor position estimator for SRM.To eliminate the position sensor,which is the aim of sensorless control,the rotor position has to be obtained through special sensorless control algorithms,which use only measured electrical quantities(such as motor currents,inverter dc link voltage,and voltage at motor terminals).Even if the position sensor is not completely removed(such as in automotive and aerospace applications),the sensorless control is used to provide a backup rotor position information in case of position sensor failures.Due to the harsh operational environments such as high temperature,humidity,dusty and wet conditions,position sensors are prone to failure.Through the analysis of the fault position signals of SRM,a new fault diagnosis method of position signals based on the predictions of the edges is adopted.To increase switched reluctance motor drive system reliability regarding position sensor failures,a maximum-likelihood voting(MLV)algorithm is proposed.Thus,a dual-backup operation of the SRM drive,wherein the driving mode is switch smoothly between the sensor and the sensorless driving according to the position signals of SRM,can realize position sensor fault-tolerant control.