Analysis and Design of Full-Order Flux Observers for Sensorless Induction Motors

This paper deals with the flux estimation for sensorless induction motor drives. The linearized model of the speedadaptive full-order flux observer is applied to help choosing the observer gain and the speed-adaptation gains. It is shown that the linearized model reveals potential instability problems that are difficult to find by other means. An observer gain and a method to vary the speed-adaptation gains in the field-weakening region are proposed. Experimental results show stable operation in a very wide speed range.Peer reviewe

Braking Scheme for Vector-Controlled Induction Motor Drives Equipped With Diode Rectifier Without Braking Resistor

This paper deals with sensorless vector control of PWM-inverter-fed induction motor drives equipped with a three-phase diode rectifier. An electronically controlled braking resistor across the dc link is not used. Instead, the power regenerated during braking is dissipated in the motor while a dc-link overvoltage controller limits the braking torque. Losses in the motor are increased by an optimum flux-braking controller, maximizing either the stator voltage or the stator current depending on the speed. Below the rated speed, the braking times are comparable to those achieved using a braking resistor. The proposed braking scheme is very simple and causes no additional torque ripple. Experimental results obtained using a 2.2-kW induction motor drive show that the proposed scheme works well.Peer reviewe

Adaptive Full-Order Observer With High-Frequency Signal Injection for Synchronous Reluctance Motor Drives

A back electromotive force-based position observer for motion-sensorless synchronous reluctance motor (SyRM) drives is augmented with high-frequency signal-injection method for improved low-speed operation. Previously proposed observer structure is further improved to account for the cross saturation in the motor. The combined observer is experimentally evaluated using a 6.7-kW SyRM drive in low-speed operation and under various load conditions. The resulting position error at low speeds and standstill is small.Peer reviewe

Modified integrator for voltage model flux estimation of induction motors

This paper deals with voltage model flux estimators for sensorless induction motor drives. In order to eliminate the drift problems, the pure integrator of the voltage model is replaced with a first-order low-pass filter, and the error due to this replacement is compensated in a very simple way.Peer reviewe

State observer for sensorless control of a grid-connected converter equipped with an LCL filter: Direct discrete-time design

Synchronization with the power system is an essential part of control of grid-connected converters. This paper proposes a grid-voltage sensorless synchronization and control scheme for a grid-connected converter equipped with an LCL filter, measuring only the converter currents and the DC voltage. A discrete-time pole-placement design method is used to formulate an adaptive full-order observer for estimation of the angle and magnitude of the grid voltage. The analytically derived design is experimentally validated, and the results demonstrate rapid convergence of the estimated angle and magnitude.Peer reviewe

Digital implementation of full-order flux observers for induction motors

This paper deals with flux estimation for induction motor drives by using a full-order flux observer. A problem of full-order flux observers is their need for computationally demanding discretization methods in order to work stably and accurately at high speeds. An implementation of the full-order flux observer using the stator and rotor fluxes as state variables in the stator reference frame and in the rotor reference frame, respectively, was recently proposed. This paper describes how an observer gain can be included in this structure. It is shown that discretization errors of the proposed implementation are small and that there is more freedom to choose an observer gain, even if the simple forward Euler discretization is used

Induction Motor Drives Equipped With Diode Rectifier and Small DC-Link Capacitance

This paper deals with sensorless vector-controlled induction motor drives that are fed by a frequency converter equipped with a diode front-end rectifier. A small DC-link capacitance is used, which makes it possible to replace the electrolytic capacitors with film capacitors. The natural frequency of the DC link is chosen considerably higher than six times the mains frequency but lower than the switching frequency. A recently proposed sensorless controller can be exploited; only minor modifications for small capacitances are needed. Simulation and experimental results of a 2.2-kW drive equipped with a capacitance of only 24 µF demonstrate operation in a wide speed range.Peer reviewe

State Observer for Grid-Voltage Sensorless Control of a Converter Equipped With an LCL Filter: Direct Discrete-Time Design

Synchronization with the power system is an essential part of control of grid-connected converters. This paper proposes a grid-voltage sensorless synchronization and control scheme for a converter equipped with an LCL filter, measuring only the converter currents and the DC voltage. A discrete-time pole-placement design method is used to formulate an adaptive full-order observer for estimation of the frequency, angle, and magnitude of the grid voltage. The proposed discrete-time design method enables straightforward implementation and suits low sampling rates better than its continuous-time counterpart. The analytically derived design is experimentally validated, and the results demonstrate rapid convergence of the estimated quantities. Moreover, the experimental tests show that grid-voltage sensorless operation is possible under balanced and unbalanced grid disturbances and distorted grid conditions.Peer reviewe

Observer-Based State-Space Current Control for a Three-Phase Grid-Connected Converter Equipped With an LCL Filter

This paper presents a state-space current control method for active damping of the resonance frequency of the LCL filter and setting the dominant dynamics of the converter current through direct pole placement. A state observer is used, whereupon additional sensors are not needed in comparison with the conventional L filter design. The relationship between the system delay and instability caused by the resonance phenomenon is considered. Nyquist diagrams are used to examine the parameter sensitivity of the proposed method. The method is validated with simulations and experiments.Peer reviewe

Signal-Injection-Assisted Full-Order Observer With Parameter Adaptation for Synchronous Reluctance Motor Drives

A back-EMF-based position observer for motion-sensorless synchronous reluctance motor (SyRM) drives is augmented with parameter-adaptation laws for improved operation at all speeds, including standstill. The augmented observer is theoretically analyzed under various operation conditions. The analysis indicates that the stator-resistance adaptation should be enabled only at low speeds, the d-axis inductance adaptation should be enabled only at medium and high speeds near no load, and the q-axis inductance adaptation should be enabled only at high speeds under high load. The augmented observer is experimentally evaluated using a 6.7-kW SyRM drive.Peer reviewe