Sensorless Operation Research Articles

Integrated design of high‐speed permanent‐magnet machines considering sensorless operation

Sensorless operation can significantly improve the reliability of the high‐speed permanent‐magnet (HSPM) machines. Thus, from the point of view of the whole system, it is necessary to consider sensorless control at the design stage for the HSPM brushless DC machines. In this paper, an integrated design is presented for sensorless control using the third‐harmonic back‐Electromagnetive force (EMF). Finite element (FE) method is used to integrally design the machine considering electromagnetism, temperature, and rotor structure strength, and the optimum rotor and stator structures are determined to obtain sufficient third‐harmonic back‐EMF for sensorless control. Simultaneously, the temperature distribution and rotor structure strength are predicted by the FE method. Finally, a prototype is fabricated and the experimental system is built to verify the machine structure, design method, and sensorless control strategy. © 2018 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Enhanced Low-Speed Operations for Sensorless Anisotropic PM Synchronous Motor Drives by a Modified Back-EMF Observer

The high-frequency signals normally used for sensorless operations in anisotropic permanent-magnet synchronous motors cause unwanted torque ripple, noise, and vibrations. The target of the present research is to extend downwards the speed range in which the sensorless drive can work without any signal injection. Specifically, this paper proposes a new back-electromotive force observer that eliminates the instability that may arise in conventional observers when the motor drive operates in generating mode. Two different versions of the observer are proposed and discussed, to include motors with high harmonic content in the back-electromotive force. The detailed mathematical analysis of the proposed observer is supported by simulations and the practical feasibility is proven through experimental results.

Post-Fault Speed Sensor-Less Operation Based on Kalman Filter for a Six-Phase Induction Motor

Post-Fault Speed Sensor-Less Operation Based on Kalman Filter for a Six-Phase Induction Motor

Modified SVPWM Technique for a Sensorless Controlled Induction Motor Drive using Neural Network Observer and Predictive Controller

The use of multi-level inverter in a sensorless control scheme increases reliability in state parameter estimation. In this paper, sensorless control is presented using a neural network observer that uses the direct and quadrature current and voltage components for speed estimation. Distortion in current and voltage will result in deviations in speed estimation. In order to address the problem, this paper presents a modified space vector modulation scheme for sensorless control of induction motor drive fed by a multi-level inverter. The modulation scheme uses lesser switching states and is employed on a cascaded H-bridge inverter configuration. This results in reliable speed estimation by reducing distortion in current and voltage measurement. Moreover the paper uses predictive controller for speed control. Simulation is carried out in MATLAB and results show improved performance of sensorless operation.

A Novel Technique for Controlling Speed and Position of Bearingless Switched Reluctance Motor Employing Sensorless Sliding Mode Observer

High-speed motors working in harsh environments such as high temperatures, radiation, and poisonous substances are limited because of motor breakdown due to vibrations of mechanical bearings. The concept of magnetic bearings is an alternative to the issues of slide or ball bearings because of its advantages of friction-free, negligible thermal problems, and lubrication free. Rotor vibration control during start-up, acceleration and deceleration phases are one of the key problems besides stable levitation, in high-speed applications of bearingless switched reluctance motor (BSRM). The use of linear control strategies alone is not effective in suppressing vibration due to residual unbalance and external disturbances. In this paper, a global sliding mode controller (GSMC) is proposed to control the speed and position of BSRM and also sensorless operation with sliding mode observer. In this method, rotor displacement tracking error functions are used in the sliding mode switching functions and the new sliding mode displacement control and speed tracking equations are obtained with an extra exponential fast decaying nonlinear function along with conventional linear sliding mode switching surface. Simulation is carried on 12/14 BSRM with the proposed controller and observer and the results of rotor displacements and speed are presented. In addition to the improvement of performance characteristics when compared to sliding mode controller, GSMC cancels the reaching mode, reduce the chatting, and overcome the disturbance and the time delay.

Sensorless Control Strategy of a 315 kW High-Speed BLDC Motor Based on a Speed-Independent Flux Linkage Function

A sensorless control strategy of a high-speed 315 kW brushless dc (BLDC) motor based on a speed-independent flux linkage function is proposed in this paper. It addresses the two key challenges on the sensorless control of high-speed BLDC motor: proposing a speed-independent flux linkage function to ensure reliable operation in the low-speed range, and presenting correction strategy to provide accurate commutation points in the high-speed range. First, the sensorless detection circuit model of a 315 kW BLDC motor is discussed. The analysis shows that the speed-independent flux linkage function is really suitable for BLDC motor sensorless control strategy in the low-speed range. Then, an improved closed-loop system based on the change of current in the full speed range is proposed. The experimental results show that the proposed control strategy can provide a highly accurate and robust sensorless operation from near zero to 20 000 r/min.

A Comparative Study of Methods for Estimating Virtual Flux at the Point of Common Coupling in Grid-Connected Voltage Source Converters With LCL Filter

Grid synchronization based on virtual flux (VF) estimation allows for control of grid-connected power converter without depending on ac-voltage measurements. This is useful in voltage-sensor-less applications for reducing cost and complexity of the control hardware, and can be utilized in case of limited reliability or availability of voltage measurements at the intended point of synchronization to the grid. However, for voltage source converters (VSC) with LCL filters, the influence of the capacitor current must be taken into account to ensure accurate VF estimation at the point of common coupling (PCC) with the grid. This paper presents a comparative evaluation of three VF-based methods for grid synchronization of VSCs with LCL filters, with three different ways of obtaining the capacitor current. The VF estimation in the first method is based only on the measured converter currents. The second method includes capacitor voltage measurements used for estimating the capacitor currents, while the capacitor currents are measured in the third approach. Comparative results from time-domain simulations are presented, demonstrating good performance of the estimation and accurate control of the active and reactive power at the PCC with all three methods, as long as sufficiently accurate filter parameters and current measurements are available. However, the approach based on capacitor current measurements is sensitive to noise due to the high ripple current compared with the fundamental frequency current in the capacitors. The operation of a converter with VF-based grid synchronization including estimation of the capacitor current is demonstrated by experimental results, verifying the voltage sensor-less operation with LCL filter.

A Global Bivalued-Observer for the Sensorless Induction Motor

Sensorless control of Induction Motors has received a lot of attention both from the applied as well as from the theoretical perspectives. In its more formal approach, a complete observability analysis of this machine, assuming the load torque is known, has been developed and several nonlinear observers have been reported in the literature. Since the system has indistinguishable trajectories, none of the existing observers is able to operate correctly under all operating conditions of the machine. In this paper we present a novel observer that is able to estimate all (non trivial) distinguishable or indistinguishable trajectories exhibited by the induction motor model under sensorless operation. The structure of the estimator comes from a detailed observability analysis under assumption that the load torque is unknown, that shows that the non trivial indistinguishable trajectories always appear in pairs, i.e. for each actual trajectory there exists at most another indistinguishable one. Hence we propose a Bi-valued observer which is always well-behaved and its convergence is guaranteed for any operation regime. The usefulness of the proposed scheme is validated by numerical simulations.

DIRECT POWER CONTROL OF A DFIG BASED WIND TURBINS UNDER UNBALANCED GRID VOLTAGE WITHOUT ROTOR POSITION SENSOR

In this paper, the behavior of a doubly fed induction generator (DFIG) is proposed under unbalanced grid voltage and without using a rotor position sensor. There are two main methods that are been used for the detection of rotor position: using shaft sensor and sensorless algorithm. In this paper the shaft sensor is eliminated and a position sensorless algorithm is used for estimating the rotor position. Sensorless operation is more desirable than using shaft sensor, because the shaft sensor has several disadvantages related to the cost, cabling, robustness and maintenance. Also, during network unbalance, three selectable control targets are identified for the rotor side converter (RSC), i.e., obtaining sinusoidal and symmetrical stator currents, mitigation of active and reactive powers ripples and the cancellation of electromagnetic torque oscillations. The effectiveness of the proposed control strategy is confirmed by the simulation results from a 2-MW DFIG system. It is concluded that the sensorless algorithm is able to produce accurate results similar to the case of that used from shaft sensor and it can be used in the practical applications.

Predictive Torque Control of Induction Motor Sensorless Drive Fed by a 3L-NPC Inverter

A finite-state predictive torque control system for a speed-sensorless induction motor drive supplied from a three-level neutral-point clamped inverter is proposed. For sensorless operation, the controller requires estimated speed and rotor/stator flux. In this study, the rotor speed and the rotor flux are estimated accurately by using an extended Kalman filter. Due to the large number of available voltage vectors, the control algorithm for a multilevel inverter-fed drive is computationally expensive. As a consequence, the controller requires longer execution time that yields worse torque, flux, and speed responses, especially at low-speed. In order to reduce the computational burden, a reduced number of voltage vectors for prediction and optimization in the control algorithm is proposed in this paper. The sign of the stator flux deviation and the position of the stator flux are predicted to lessen the number of voltage vectors tested. Experimental results illustrate that the proposed encoderless strategy can estimate the speed accurately over a wide speed range including field-weakening region while maintaining robustness and excellent torque and flux responses.

Sensorless Estimation of Single-Phase Hybrid SRM using Back-EMF

This paper presents a novel scheme to estimate the rotor position of a single-phase hybrid switched reluctance motor (HSRM). The back-EMF generated by the permanent magnet (PM) field whose performance is motor parameter independent is adopted as an index to achieve the sensorless control. The differential value of back-EMF is calculated by hardware and processed by DSP to capture a fixed rotor position four times for every mechanical cycle. In addition, to accomplish the normal starting of HSRM, the determination method of the turn-off time position at the first electrical cycle is also proposed. In this way, a sensorless operation scheme with adjustable turn on/off angle can be achieved without substantial computation. The experimental verification using a prototype drive system is provided to demonstrate the viability of the proposed position estimation scheme.

Sensorless Control of PMSM by a Four-Switch Inverter with Compensation of Voltage Distortion and Adjustment of Position Estimation Gain

This paper proposes performance improvement schemes for sensorless PMSM control drive using a four-switch three-phase inverter (so-called B4 inverter). In the proposed scheme, the back-EMF estimation-based sensorless control algorithm is used to control the brushless PMSM without position sensors. In order to have stable operation, this paper presents a gain adjustment scheme that compensates the reduction of stable sensorless operation range as long as the rotor speed increases. In B4 topology, the center point of dc-link capacitors is connected to 3-phase load, and it is prone to have the load current distortion. Hence, to mitigate this problem, a distortion compensation scheme by modifying voltage commands using measured dc-link potentials is proposed in this paper. The validity of the proposed method is evaluated by simulations and experiments.

Design Consideration on the Square-Wave Voltage Injection for Sensorless Drive of Interior Permanent-Magnet Machines

Although it is widely known that the saliency-based position-sensorless drive is able to achieve the closed-loop control at zero and low speed, there is little literature addressing the consideration on the selection of injection voltage frequency. This paper evaluates the square-wave injection voltage at different frequencies for the design of an interior permanent-magnet (PM) machine sensorless drive. It is shown that more flux saturation on high-frequency (HF) d -axis inductance occurs than the saturation on q -axis inductance due to the magnetic relaxation. The HF saliency ratio $\rm{(}L_{\rm{qhf}}\rm{/ }L_{\rm{dhf}}\rm{)}$ is increased by increasing the injection frequency. The performance of a saliency-based sensorless drive can be enhanced by properly designing the frequency of injection voltage. This paper also includes the experimental comparison between closed-loop encoder-based and closed-loop saliency-based sensorless operation of an interior PM machine drive.

Torque capability improvement of sensorless FOC induction machine in field weakening for propulsion purposes

An electric propulsion system is generally based on torque controlled electric drive and DC series motors are traditionally used for propulsion system. Induction machines, which are reliable, low cost and have less maintenance, satisfy the characteristics of the propulsion and reinstating the DC series motor. Field oriented control (FOC) of induction machines can decouple its torque control from field control which allows the induction motor to act like a separately excited DC motor. In this paper, the characteristic control of induction motor is achieved through appropriate design modification of induction motor by varying magnetizing current to produce maximum torque in field weakening (FW) region. Thus to improve the torque capability of induction machine in FW region by varying machine parameters. The sensorless operation of the induction motor is carried out by adopting model reference adaptive system (MRAS) using sliding mode control (SMC) and a FW algorithm based on the voltage and current constraints. The simulation of the induction motor drive models based on the design options have been carried out and analyzed the simulation results.

Observer-based sensorless speed control of PMSM: A focus on drive’s startup

Sensorless control of permanent magnet synchronous motor has received a lot of research interest due to cost reduction and reliability improvement of the drive. One of the applied techniques is Luenberger observer. However, starting the drive from unknown initial position and from standstill are not sufficiently covered. This paper presents the details of starting the drive from standstill on a fan-type load and roller-type load. The dynamic performance of the drive during homing and open loop acceleration under current control is presented and investigated. The transition from open loop acceleration to observer-based sensorless operation is discussed and the stability of sensorless control against load disturbances is discussed. The paper presents simulation results and verifies them experimentally.

매입형 영구자석 동기기 센서리스 구동부의 개선된 절환 기법

매입형 영구자석 동기기 센서리스 구동부의 개선된 절환 기법

Angle-Sensorless Zero- and Low-Speed Control of Bearingless Machines

Bearingless machines are successfully used for a variety of applications that demand for low mechanical losses, low wear, and low contamination. These machines require exact knowledge of the radial and angular rotor position in order to ensure stable levitation. This information is obtained with different position sensors. Alternatively, a sensorless approach can be used to determine the rotor position. The omission of the angular sensors leads to a reduction in costs and an improvement in reliability and allows to explore new areas of applications for bearingless machines. However, few zero and low-speed sensorless angle estimators are published to this date. Therefore, a novel model-based estimator to determine the rotor angle at zero and at low speeds and, thus, to allow sensorless operation over the whole speed range of the bearingless permanent magnet synchronous machine is proposed in this paper. The observer utilizes the radial position measurements and a model of the bearing force generation to determine the error of the angular position estimation. The method is implemented on a bearingless machine prototype, and the functionality is shown by measurements.

Stand-Still Self-Identification of Flux Characteristics for Synchronous Reluctance Machines Using Novel Saturation Approximating Function and Multiple Linear Regression

Motor characterization has a fundamental role in dynamics, torque accuracy, and efficiency of vector controlled Synchronous Reluctance Machine (SynRM) drives. Control performances and robustness in the whole speed/torque range, including the flux-weakening region, and in sensorless operation strongly rely on the knowledge of machine flux versus current characteristics. A convenient flux saturation approximating function is proposed in this paper, together with an efficient parameters self-identification procedure. The adopted strategy is very simple and can be performed at stand-still by injecting a proper voltage stimulus (current control is not involved), and does not require any additional hardware (motor can be either connected or disconnected from mechanical load). Nevertheless, an excellent fitting for the flux curves on both axes is obtained, using reasonable memory and computational resources. These features make the technique very suitable to motor self-identification in industrial drives. Experimental results based on a commercial drive and two SynRMs are reported to demonstrate the effectiveness of the proposal. Extensions of the method to the evaluation of the whole flux map (including cross-saturation effects) or to interior permanent-magnet machines is also investigated and verified.

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 the 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.

Sensorless Vector Controlled Multilevel Inverter Fed BLDC Motor

BLDC motors are mostly known to be driven by trapezoidal control due to its simple implementation, but this type of control results in pulsating torque ripple which is unwanted in high performance drives. In this paper, vector control is combined with a five-level inverter to minimize the torque ripple of BLDC motor in sensorless operation as well as reducing the total harmonic distortion in the voltage and current waveforms. The MATLAB/Simulink environment is used to simulate and verify the proposed method.