Field-oriented controllers with rotor deep bar compensationcircuits [induction machines]

A rotor deep bar effect compensation circuit for field-oriented controllers is derived from the transient equations of a double-cage induction machine. Torque and flux are decoupled with respect to the airgap flux. Accurate tuning of the field-oriented controller is possible both for steady-state and high-frequency conditions in the rotor. Consequently, an extended static stability region and improved torque dynamics are obtained with the deep bar effect compensated induction motor drive     

Flux and torque decoupling control for field-weakened operation offield-oriented induction machines

One of the primary advantages of field-oriented induction machines for high-performance applications is the capability for easy field weakening and thus full utilization of the voltage and current ratings of the invertor to obtain a wide dynamic speed range. This attribute has not been widely developed, however, because of the difficulty in actually knowing the flux magnitude and the effective rotor time constant at each operating point as needed for indirect field-oriented controllers. A straightforward approach is presented for estimating and fully decoupling the rotor flux and rotor time constant terms in indirect field-oriented controllers. This approach is based on a combination of both the intrinsic magnetic linearity of the field-weakened machine and the fully decoupled stator current and voltage equations which result from the use of delta-modulator current regulators     

Efficiency-optimized flux trajectories for closed-cycle operationof field-orientation induction machine drives

A dynamic programming scheme that optimizes the efficiency of an induction machine drive operated in closed cycle and has both control and state constraints is developed. Application of rotor flux feedforward field orientation control for an induction machine reduces the system equations to contain only three state variables: rotor flux, velocity, and position. Maximum stator current and rotor velocity are set as constraints. Saturation effects are modeled to provide a state-dependent constraint on the rotor flux magnitude. Load is treated as a function of the rotor position, which is appropriate for many mechanical system applications. State trajectories of the system that optimize machine efficiency are found by dynamic programming. Flux trajectories for the optimal solution are found to vary significantly over the machine cycle. The validity of the energy optimization is investigated experimentally on a feedforward, field-oriented induction machine     

Rotor time-constant estimation approaches based on energy function and sliding mode for induction motor drive

To solve the problem of detuning due to parameter variations in the current decoupled control of a direct stator-flux-oriented induction motor (IM) drive, two approaches for the rotor time-constant estimation are presented in this study. The first approach is based on the model reference adaptive system (MRAS) using an energy function, and the second approach is based on the sliding mode technique. The estimated rotor time-constant is used in the current decoupled controller, which is designed to decouple the torque and flux in the stator flux field-oriented control. To increase the accuracy of the estimated rotor time-constant, the estimation methods are implemented using a digital signal processor (DSP). The effectiveness of the proposed estimation methods are demonstrated by some simulation and experimental results.     

A simple and robust adaptive controller for detuning correction infield-oriented induction machines

A novel adaptive controller for correcting the rotor time constant estimate used in the slip frequency calculator of indirect field-oriented controllers (IFOCs) is presented. The controller possesses near digital deadbeat performance and simultaneously maintains its correctness down to zero speed. The robust controller dynamics are based on an operating-point form of digital deadbeat control. The correctness down to zero rotor speed is based on the determination of the rotor flux from an accurate air-gap flux estimate obtained from the third harmonic stator phase voltage component introduced by the saturation of the machine stator teeth. The proposed controller is implemented using a digital signal processor, and the experimental results demonstrate its excellent performance for a wide range of rotor speeds, including locked rotor conditions     

Slip-frequency detection for indirect field-oriented control drives

This paper presents a new technique to detect the slip angular frequency in induction motor drives with indirect field-oriented control (IFOC). Slip frequency is calculated by processing the effects exerted by an additional high-frequency signal on the zero-sequence stator voltage. By measuring such a zero-sequence voltage, it is possible to evaluate the air-gap flux and detect the slip frequency in order to implement an IFOC algorithm. According to the proposed approach, rotor time constant errors are not influent as in conventional field-oriented control schemes, thus, complex tuning or compensation techniques are not required. Simulations and experimental tests are performed to evaluate the consistency of the proposed approach.     

Nonlinear adaptive state-feedback speed control of a voltage-fed induction motor with varying parameters

An adaptive nonlinear-state-feedback speed control scheme of a voltage-fed induction motor has been developed in which the control of torque and flux is decoupled. The inputs to the control algorithm are the reference speed, the reference flux, the measured stator currents, the measured rotor speed, the estimated rotor flux, and estimates of the rotor resistance, stator resistance, and load torque, which may vary during operation. The controller outputs are the reference stator voltages in rotor-flux rotating reference frame. An accurate knowledge of the rotor flux and machine parameters is the key factor in obtaining a high-performance and high-efficiency induction-motor drive. The rotor flux is estimated using the induction-motor rotor-circuit model. Although the estimated rotor flux is insensitive to the stator-resistance variation, it does depend on the rotor resistance. A stable model reference adaptive system (MRAS) rotor-resistance estimator insensitive to stator-resistance variation has been designed. Stable stator-resistance and load-torque MRAS estimators have also been developed. These estimators have been developed to constitute a multi-input-multi-output (MIMO) decoupled-cascade structure control system. This simplifies the design problem of the estimators for a stable operation from a MIMO design problem to a single-input-single-output (SISO) design problem. The continuous adaptive update of the machine parameters and load torque ensures accurate flux estimation and high-performance operation. Simulation and experimental results are presented to verify the stability of the induction-motor drive in various operating modes.     

Robust direct torque and flux control of adjustable speed sensorless induction machine drive based on space vector modulation using a PI predictive controller

A Direct torque and flux control design for a sensorless induction motor, following a Lyapunov-based stator flux observer is presented. In this control scheme, the torque error signal and the amplitude of the stator flux reference vector are delivered to a PI predictive controller. The predictive controller also uses information on the amplitude and position of the actual stator flux vector and measured stator currents to determine the voltage command vector for space vector modulation inverter. In addition, a conventional PI speed controller is used to generate the torque reference signal. Using the fifth order model of the three-phase induction machines in a stationary two axes reference frame, a nonlinear stator flux observer is developed in order to estimate the rotor speed, rotor and stator resistances simultaneously. The stability of this observer is proved by Lyapunov theory. It is shown that if the persistancy of excitation condition is satisfied, the estimated quantities converge to their real values. The effectiveness of the proposed control scheme is verified by computer simulation and experimental results.     

Selection of Flux Level in Field-Oriented Induction Machine Controllers with Consideration of Magnetic Saturation Effects

The magnitude of rotor flux is a primary design and application variable for field-oriented induction machine controllers. Optimization of a nonsaturating machine controller for peak torque/A will lead to operation at the peak of the constant current torque-versus-slip curve. It is shown how flux level selection for both standard and high efficiency motors is altered by including saturation characteristics. Theoretical and experimental approaches for calculation of flux-producing current in the presence of magnetic saturation are presented. The theoretical method includes a straightforward two-parameter saturation model, which is demonstrated to possess characteristics very close to experimental data. This approach contributes substantial insight to saturated machine analysis while retaining relative simplicity as a design tool. The experimental approaches developed are straightforward and general.     

Optimal rotor flux regulation for fast-accelerating inductionmachines in the field-weakening region

Quality and reliability of field-oriented controlled induction machines depend strongly on the capability of the inverter switching strategy to control the current space phasor according to the desired torque value. Especially at high speed, when DC-link voltage is fully utilized, the rotor flux level has to be chosen carefully to enable sufficient dynamic capability of the drive. This paper presents optimal speed- and load-dependent amplitudes of the rotor flux to achieve constant dynamic behavior. A combination of an adaptive machine model with a rotor flux controller maintains excellent flux management, especially during rapid acceleration in the field-weakening area. The advantageous function of the scheme is illustrated by experimental results     

Analysis of rotor slotting saliency in induction motor sensorless control

This paper deals with induction machines study by Finite Element Analysis (FEA) for position sensorless control drives based on rotating voltage carrier injection. It focuses the analysis of the rotor slotting saliency, under the variations of: (1) supply of the machine (both the high frequency rotating voltage carrier and the fundamental frequency supply), (2) reference flux of the drive, (3) load torque, and (4) rotor geometry design. The relative spatial harmonic of the stator inductance space-vector responsible for the rotor slotting effect has been chosen as an index for evaluating the response to the high frequency carrier. A methodology for retrieving the stator inductance space-vector from a FEA is proposed as well. Results of the variation of the rotor slotting spatial harmonic of the stator inductance with the carrier frequency, the rotor flux linkage, the load torque and the rotor geometry are presented.     

High torque induction motor with rotating magnets in the rotor

This paper describes a new type high torque induction motor which has the rotating magnets in the rotor. The motor basically consists of a usual stator, cylindrical rotor, and inner cylinder of which the surface is covered by a set of magnets. The rotor turns at somewhat less than synchronous speed. The inner cylinder with magnets can revolve freely against a rotor shaft. The magnets revolve synchronized by the rotating magnetic field induced by the stator current. The magnets make the flux in the rotor. Then we can expect torque increase by the increase of the flux. The results of magnetic field analysis indicate the flux is increased. In the experimental results of a test motor which is a 400 W prototype machine, we have obtained the torque increase by approximately 20 percent as anticipated in the magnetic field analysis. Moreover, test results show improvements of efficiency and power factor in the motor operations. The efficiency of the test motor is obtained as high as 10 percent at the rated output over those of a same size conventional induction motor. Although power factor of conventional induction motors is lagging at all times, the test motor can be operated with near unity, leading or lagging by adjusting the ac supplied source voltage.     

Vectorial torque control: a novel approach to torque and fluxcontrol of induction motor drives

A general approach to torque and flux control of an induction motor fed by a voltage-source inverter is described in this paper. It is independent of whether rotor or stator flux is controlled. Based on this, an algorithm can be set up which allows the generation of the inverter switching patterns as a function of the reference values, both of the electromagnetic torque and of the controlled flux. Experimental comparisons with traditional direct torque control and field-oriented control techniques confirm the possibility of achieving very high dynamic performance with the proposed strategy     

Design and implementation of a closed-loop observer and adaptivecontroller for induction motor drives

This paper discusses the implementation and experimental results of a closed-loop rotor flux observer and model reference adaptive system (MRAS) of a direct field-oriented control (FOC) of an induction motor drive. The motor was supplied from a high-frequency (20 kHz) AC resonant link via a MOS-controlled-thyristor (MCT)-based bidirectional converter. Hardware and software implementations of the various motor control functions are presented. The closed-loop observer combines the current and voltage models via a speed-dependent gain (SDG). The current model was formulated to operate in the rotor reference frame and requires only an encoder angle and not the actual rotor speed for implementation. The closed-loop observer permits the use of a pure analog integrator to calculate an adequate stator flux. The use of an AC resonant link further complicated an all-digital calculation of the stator flux. The observer and adaptive controller were tested on a 400-Hz 2-hp induction motor for low and high speeds. The closed-loop observer showed sensitivity at low speeds to the rotor circuit time constant which attributed to the current model rotor flux estimation. At high speed, the closed-loop observer followed the voltage model rotor flux estimation attributes. The MRAS was able to improve the complete speed response by correcting the current model rotor flux observer for errors in estimation of its parameters     

Mixed control strategy of a doubly fed induction machine

In this paper the doubly fed induction machine (DFIM) is supplied by two pulse width modulation (PWM) voltage inverters. The mixed control strategy proposed is divided into two parts: a variable structure control (VSC) as far as the electrical modes are concerned, and a general PI controller for the mechanical modes. It is linked with active power distribution between stator and rotor circuits. An original DFIM mathematical model is defined in a stator flux reference frame. Experimental results of transient and steady-state behaviour are presented. In this paper we focus our study to three operating ranges: the magnetizing range, step and steady-state speed response, load torque impact.     

Adaptive sliding mode observer for induction motor using two-time-scale approach

This paper presents an original method for the design of a robust adaptive sliding mode current and flux observer for induction motor drive using two-time-scale approach. This approach, based on the singular perturbation theory, decomposes the original system of the observer error dynamics into separate slow and fast subsystems of lower dimensions and permits a simple design and sequential determination of the observer gains. For the proposed observer, the rotor speed signal is assumed to be available. The stator currents and rotor flux are observed on the stationary reference frame using sliding mode concept, and the adaptive rotor time-constant is derived from Lyapunov stability theory using measured and estimated currents and estimated rotor flux. The control algorithm is based on the indirect field-oriented sliding mode control to keep the machine field oriented. The control-observer scheme seeks to provide asymptotic tracking of speed and rotor flux in spite of the presence of an uncertain load torque and unknown value of the rotor resistance. The effectiveness of this control algorithm has been successfully verified through computer simulations.     

A novel stator resistance estimation method for speed-sensorless induction motor drives

A method for online estimation of the stator resistance of an induction machine is presented and a speed-sensorless field-oriented drive equipped with the proposed estimator is built. The drive is particularly suitable for low-speed operation. Resistance estimation is based on a two-time-scale approach, and the error between measured and observed current is used for parameter tuning. The simple full-order observer in use allows for direct field orientation in a wide range of operation. The system can drive active load and generate stall torque, as confirmed by numerical simulations and experimental tests on a general-purpose 7.5 kW induction machine.     

异步电机并联运行磁场定向控制

针对两台电机参数和负载不一致的情形,在原有单台电机磁场定向矢量控制模型的基础上,推导出适合一个逆变器控制两台电机的磁场定向控制模型.该方法考虑了电机参数的不同及电机定子电流的不一致,基于转子平均磁链进行定向,以两台电机的定子平均电流为控制目标,定子电流的d轴分量控制平均转子磁链,q轴分量控制平均电磁转矩.理论分析、仿真结果和实验结果表明了该控制方法的正确性和可行性.     

Dynamic analysis and experimental evaluation of delta modulatorsfor field-oriented AC machine current regulators

Implementation of field-oriented control requires regulation of the temporal orientation of flux and current vectors in AC machines. The orientation is generally achieved via instantaneous control of stator current relative to either the measured flux or the indirectly estimated flux terms. In either case, the instantaneous control of current is critical. A novel approach for current regulation is presented, based on the use of a nonlinear discrete-time delta modulator which offers the potential for reduced parameter sensitivity and more accurate command tracking than either classical or state-space linear system controllers. A power electronic topology, the resonant DC link is ideally suited to implement this controller. Experimental data on a field-oriented induction machine controller using a resonant DC link inverter topology demonstrate the command tracking performance of this induction-machine current regulator     

Consideration about problems and solutions of speed estimation method and parameter tuning for speed-sensorless vector control of induction motor drives

A classical model-based speed-sensorless field-oriented control method for a general-purpose induction machine is considered. Improved versions of both speed and stator resistance online estimators are presented with the aim of extending control capabilities down to zero speed. Only electrical stator measurements are needed, making the method suitable for general-purpose inverter applications. Online rotor resistance tuning is also included to fully compensate for thermal drift. Performance of the proposed drive is investigated by simulation and experiments.