Research On Speed Sensorless Induction Motor Drive At Low Speed Range

In applications speed sensor is expensive and unreliable in hostile environments.It is necessary to use speed sensorless drives for instead.In general the advantages of speed sensorless drives include low cost,reduced hardware complexity and size,improved reliability and less maintenance requirements.The main disadvantage of speed sensorless drives is the poor performance when induction motor runs at low speed range or at zero speed point.The reasons mainly include:(1)there is the unstable range when the motor operates at low speed regenerating mode;(2)the estimated flux and rotor speed accuracy is sensitive to the motor parameter deviation;(3)the inverter non-linear error has large influence on the flux and speed estimation at low speed range;(4)the rotor speed can not be estimated when the current frequency is zero;(5)the response of torque is slow at low speed range because the bandwidth of current loop is limited.In this paper,all above five problems at low speed range will be solved.Many approaches of speed sensorless induction motor drives have been proposed in past years.There are mainly two basic approaches in sensorless control system.One approach is based on fundamental model of induction motor.Fundamental model has poor performance when motor runs at low speed range in motoring mode or in regenerating mode.The other approach is signal-injection-based method for speed estimation,which is based on rotor slot harmonics,main inductance saturation,artificial saliency or rotor slot leakage.Wit h signal injection,the motor can run at zero speed.In this paper,the speed sensorless induction motor drive is researched for the low speed operation:The performance of adaptive full order observer for sensorless induction motor drives can be improved further though it has been researched for many years.In this paper,two different design methods for improving the speed estimation stability and robustness are proposed.1)In classical speed estimation algorithm used in adaptive full order observer,the estimated rotor flux error is neglected.A robust design method of feedback gains with classical speed estimation algorithm is proposed.Compared with existed three main design methods of feedback gains,the stabilit y of speed estimation and estimated speed error are improved.2)A robust speed estimation algorithm with estimated rotor flux error and the corresponding feedback gains are proposed in this paper.Through Lyapunov's theorem,the stability of proposed speed estimation algorithm is proved.The stability of flux estimation and estimated speed error can be improved much more.Finally the feasibility of two proposed methods is verified by experiments.The performance of speed sensorless induction motor drive is poor at low speed range.That is because the inverter non-linear error,such as inaccurate motor parameters,deadtime,turn on/off delay and on-state voltage drop of power switches,has strong influence on the speed estimation at low speed range.In this paper,the feedback gains of adaptive full order observer(AFO)are designed to guarantee the stability of speed estimation and to reduce the influence of inverter non-linear error on the speed estimation.In addition,the estimated current error caused by inverter non-linear error is identified off-line.It is stored in the look-up-table to compensate the influence of inverter non-linear error on the speed estimation.The feasibility o f proposed method is verified by experiments.It is well known that the operation of speed sensorless induction motor dr ive at zero stator current frequency is unstable theoretically if signal injection is not used.In this paper,the unstable reason is analyzed.The error between stator input voltage of adaptive full order observer and motor is introduced for the operation at zero stator current frequency without any signal injection.When the stator current frequency is reduced to zero,the traditional speed estimation fails to estimate the rotor speed but the rotor speed can be estimated by the proposed stator input volta ge error.The stability of the proposed method is analyzed.Though the unstable problem at zero stator current frequency can be solved by the proposed method,there is a disadvantage that an error between estimated and actual rotor speed is caused.Finally the feasibility of proposed method is verified by experiments.A novel robust predictive control scheme,with indirect vector control system,for the induction motor is proposed in this paper.In order to improve the current loop bandwidth in control system,the restrictions to current-loop bandwidth improvement is analyzed.The predictive control scheme based on mathematical model of induction motor in synchronous frame,is proposed to improve the current-loop bandwidth and the performance of dynamic response.Meanwhile reasons of destabilization in system are analyzed and two robust factors are added to predictive current controller to upgrade the robustness of system and reduce the sensitivity to motor parameters.Experiment results show that the propose d robust predictive current control scheme improves the robustness of control system,and raise system response time.