Research On Online Rotor Time Constant Estimation For Indirect Field Oriented Induction Motor Drives

The indirect field-oriented control(IFOC)method is widely used in induction motor(IM)drives due to its simple structure and high stability.The field orientation,determing the performances of the drives,is dependent on the rotor time constant,which undergoes considerable variation in application due to machine saturation,thermal drift,etc.The mismatch between rotor time constant used in the vector control scheme and its actual value would lead to incorrect field orientation,deteriorating both the steady-state and dynamic performances of the IM drives.Therefore,online updating of rotor time constant has been the focus of the researches currently.Model reference adaptive system(MRAS)based rotor time constant updating scheme is attractive because of its clear physical concept,easy implementation,and stability.Reviewing the publications however,the following problems associated with this kind of identification algorithm should be analyzed and solved to improve its estimation perfermance further in the drives with speed sensor:(1)the intrinsic relationship of the adaptive schemes based on different functional candidates are still not clarified,and the optimization of the MRAS scheme lacks the theoretical basis;(2)the proportional-integral(PI)gain in the MRAS is selected by trial and error,relsuting in that the stability and optimizing dynamic performance of the estimation system is not confirmed;(3)the scheme available in literatures is usually designed to update the rotor resistance,while the variation of mutual inductance,which is often experienced in wide flux weakening speed range,is not considered;(4)there are few papers reporting about the coupling between rotor time constant estimation and iron loss,stray loss.In this paper,the above four problems are analyzed and solved in turn,developing the theory of MRAS and rotor time constant estiamtion.A unified mathematical model of rotor time constant identification algorithm based on MRAS is proposed in this paper with the help of the Lyapunov theorem and Popov hyperstability theorem,revealing its intrinsic unification and providing theoretical basis for the optimizing selection of different MRASs.Moreover,it is also shown that stator frequency or q-axis component of stator current must be incorporated into the traditional parameter estimation model to expand its effective range.Besides,with the established closed-loop transfer function,not only the high nonlinearity of the estimation system is exposed,but also ramp response of the estimator could be analyzed quantitatively.Three typical MRASs including reactive power model,torque model and dot product of rotor flux and stator current model are chosen and corresponding small signal model is established to further analyze the stability of the estimation system,supplying foundation for optimizing selections of PI gains.As for the reactive power model,the information of stator frequency is introduced into the adaptive scheme and design strategy of PI gains,by which the system collapse is avoided and optimized convergence process is achieved,is proposed.As for the torque model,it is concluded the effective range is limited with heavy-load operation due to the false equilibrium and the selection range of PI gains from the perspective of stability is also given.As for the dot product of rotor flux and stator current model,the stator resistance in the stator flux estimator is designed to vary according to the operating points of the motor to guarantee the stable operation.A parallel estimation method of rotor time constant and mutual inductance is developed,overcoming the negative effect of detuned mutual inductance on the rotor time constant estimation.The proposed scheme is implemented by estimating rotor time constant with dot product of stator current and rotor flux model and updating mutual inductance with the amplitude and phase error of rotor flux.In order to obtain the rotor flux,an improved stator flux estimator based on dual feedback compensation of DC offset is proposed.When compared with the traditional schemes,the DC offset in the estimated flux is eliminated and better dynamic response speed is achieved with the proposed algorithm.Based on the parallel iron loss model incorporated with a series stray loss model,two novel iron loss and stray loss compensation schemes are given in this paper to deal with the adverse effect of the above two nonideal factors on rotor time constant estimation.The first one is to correct the commanded current and and slip frequency with the known coefficient of iron loss and stray loss in the IFOC system.The second compensation scheme consists of the following two steps: identifying the equivalent rotor time constant including iron loss and stray loss based on reactive power model to achieve field orientation,realizing the output of expected torque and flux through the correction of current references.Besides,the identification results of equivalent rotor time constant with the second scheme can be used to fit the coefficient of hysteresis,eddy current and stray loss,providing necessary information for online compensation scheme without tedious off-line test process.