Control Characteristics of Inverter-Fed Induction Motor

Efficient use of an inverter-fed induction motor drive necessitates a knowledge of the characteristics of the induction motor from a " control" point of view. The characteristics of both the voltage -and the current-source inverter-fed induction motor drive are investigated, neglecting the filter time constants in the link and feedback loops. Of particular interest is the role played by the voltage/current and frequency inputs. Their effects on variables that are not often discussed such as torque angle, stator real power, airgap flux- linkages and stator and rotor voltages/currents are studied. A by- product of this aspect of the study leads to the nature of feedback signals and their suitability for control purposes. It is also found that the link filter has little effect on relative suitability of these signals. A study of the current source induction motor drive dynamics for torque angle feedback is included.     

Variable-speed control of brushless half-speed synchronous motor byvoltage source inverter

The variable-speed control of a voltage source inverter-fed cylindrical rotor type of brushless half-speed synchronous motor with feedback of the rotor position is described. On the basis of an approximate analysis of the steady state, an expression for torque is derived, and two methods of speed control are discussed. The step response of speed in the drive system is almost the same as that in a speed control method based on a slip frequency control in an induction motor drive, and a relatively high torque is obtained even in a low speed region. The motor has the capability of line start and change over from an induction motor (Gorges phenomenon) to a self-controlled half-speed synchronous motor by using a simple position sensor     

TORQUE VECTOR CONTROL (TVC) OF AXIALLY-LAMINATED ANISOTROPIC (ALA) ROTOR RELUCTANCE SYNCHRONOUS MOTORS

Abstract

Axially-laminated anisotropic (ALA) rotor reluctance synchronous motors (RSMs) have been proved recently capable of higher torque density, higher power factor and efficiency compared to their induction motor counterpart with the same stator. Vector current control systems have been proposed for such drives, based on maximum available torque constant current angle or minimum current for a given torque constant current angle or combined current-voltage control. In an effort to do away with the current controller limitations, especially in the high speed region, the paper proposes a torque vector control (TVC) system which is a direct stator flux and torque control through a table of optimum switchings in the voltage-source PWM inverter. A sliding mode speed controller is added. Extensive digital simulations are presented. They demonstrate the TVC fast response, wide speed range and load rejection performance.     

TORQUE VECTOR CONTROL (TVC) OF AXIALLY-LAMINATED ANISOTROPIC (ALA) ROTOR RELUCTANCE SYNCHRONOUS MOTORS

Abstract

Axially-laminated anisotropic (ALA) rotor reluctance synchronous motors (RSMs) have been proved recently capable of higher torque density, higher power factor and efficiency compared to their induction motor counterpart with the same stator. Vector current control systems have been proposed for such drives, based on maximum available torque constant current angle or minimum current for a given torque constant current angle or combined current-voltage control. In an effort to do away with the current controller limitations, especially in the high speed region, the paper proposes a torque vector control (TVC) system which is a direct stator flux and torque control through a table of optimum switchings in the voltage-source PWM inverter. A sliding mode speed controller is added. Extensive digital simulations are presented. They demonstrate the TVC fast response, wide speed range and load rejection performance.     

TORQUE PRODUCTION IN ac MOTORS WITH CONSTANT CURRENT SUPPLY

ABSTRACT

Basic theory is presented concerning the production of torque in synchronous motors and induction motors under conditions of constant current supply. Simple methods are described for treating the effects of magnetic saturation, which are important at high currents. It is shown that, for a given stator current, a synchronous motor produces more torque than an induction motor, albeit with higher air-gap flux. Examples illustrate the variation, due to saturation, of the optimum torque angle of synchronous motors with cylindrical and salient rotors.     

The effect of link current ripple on the torque pulsations in a current source inverter-fed induction motor

This paper analyses the effect of link current ripple on the torque pulsations of a current source inverter-fed induction motor. The torque pulsations produced by a ‘pure’ DC link current are compared with those produced by a link current containing both DC and a superimposed ripple. It shows that if the average values of link current in the two cases are the same, the magnitudes of the torque pulsations with and without ripple are approximately equal in the time domain, although the magnitudes of the individual torque harmonics are affected. It also investigates the effects of different inverter frequencies on the torque pulsations.     

MICROCOMPUTER BASED EFFICIENT AUTOSYNCHRONOUS MOTOR DRIVE

ABSTRACT

The paper presents the development of a microcomputer based digital control system for the current source load-commutated inverter fed synchronous motor drive. The control system comprised two loops: (i) Fast acting dynamic speed and torque angle control loop to ensure the self synchronous operation of the drive system, disturbance rejection and robustness of the complete motor drive system. (ii) Slow acting quasi steady-state efficiency maximization loop that ensures that the drive load is met at minimum source power input.     

Steady-State and Stability Analysis of Induction Motor Driven by Current Source Inverter

Steady-state characteristics and instability of an induction motor driven by current source inverter are studied in comparison with the system driven by voltage source inverter. It is shown that as the speed is increased the electromagnetic torque takes three extremities a maximum torque is close to the standstill and a minimum and a second maximum torques are near to the synchronous speed. Thus the magnitude of the starting torque is slightly less than the breakdown torque and larger than that for the voltage source type. Operation close to the synchronous speed is impractical, because of the higher copper loss and lower efficiency due to the increase of current and the decrease of the power factor with increasing speed. As this fact prevails also in the negative slip region, a dynamic brake action rather than a regenerative braking action will be obtained. Over a wide range of slip, higher torques than those for the voltage source type are obtained. Instability appears usually as a sustained oscillation on armature voltage and current. Applying the Routh-Hurwitz criterion, several instability boundaries for various parameters are described. Some experimental results are presented which show essential agreement with the theory.     

ANALYSIS OF A SERIES-DELTA CONNECTED TANDEM INDUCTION MOTOR

Abstract

A tandem induction motor comprised of two independently housed stator windings, one housing stationary and the other able to be physically rotated, and a squirrel-cage rotor common to both stators is described. When the motor is fed from a single source, altering the position of the moveable stator changes shaft speed in a similar manner to operating a normal induction motor at variable terminal voltage

This paper presents an equivalent circuit model of the tandem induction motor for series-delta connected stator windings. Using the equivalent circuit stator current, power factor, and torque are calculated and then compared to laboratory test results obtained from a 3 kW tandem motor test rig     

COMPUTER CONTROL OF THYRISTORIZED INDUCTION MOTORS AND ON-LINE PERFORMANCE PREDICTIONS

ABSTRACT

This paper describes a method of modelling the dynamic behaviour of thyristor controlled induction motors. Real time operational verification of the model is achieved by interfacing the drive system to a computer. The steady state torque produced by the motor is described as a function of firing angle, extinction angle and speed. The model concentrates on the open loop transfer functions of induction machines for small perturbations in motor speed and firing angles. The non-linear characteristics of the open loop transfer function has been discussed. The solutions presented here can lead to a comprehensive closed loop analysis for the dynamic behaviour of induction motors in industrial applications.     

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

This paper deals with sensorless vector control of pulsewidth-modulated 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.     

Evaluation of the energy characteristics of an asynchronous inverter-fed induction motor at different approximations of the magnetization curve

Results of research on the effect of magnetization-curve approximation of the electrotechnical steel and excitation frequency of an asynchronous inverter-fed induction motor (IFIM) on its energy performance with extremal control algorithms based on energy-saving criteria are presented in this article. An IFIM is a variant of a double-fed motor with static frequency converters in the stator and rotor circuits. Its application in a variable-speed drive meets to the greatest extent the main requirements for an ac machine in terms of speed regulation and providing high energy performance. It is shown that application of piecewise linear approximation by three sections and the spline function provides improved values of energy performance relative to the simple approximation by two linear sections due to becoming close to a real magnetization curve. It provides, under otherwise equal values of electromagnetic torque, a higher flux linkage with decrease in magnetization current. Control of the excitation frequency of the asynchronous inverter-fed induction motor has a significant influence on the steel losses of the machine. In the region of low loads, they are particularly important relative to the active losses in the motor windings.     

Control Algorithm for the Inverter Fed Induction Motor Drive with DC Current Feedback Loop Based on Principles of the Vector Control

ABSTRACT

This paper brings out a control algorithm for VSI fed induction motor drives based on the converter DC link current feedback. It is shown that the speed and flux can be controlled over the wide speed and load range quite satisfactorily for simpler drives. The base commands of both the inverter voltage and frequency are proportional to the reference speed, but each of them is further modified by the signals derived from the DC current sensor. The algorithm is based on the equations well known from the vector control theory, and is aimed to obtain the constant rotor flux and proportionality between the electrical torque, the slip frequency and the active component of the stator current. In this way, the problems of slip compensation, Ri compensation and correction of U/f characteristics are solved in the same time. Analytical considerations and computer simulations of the proposed control structure are in close agreement with the experimental results measured on a prototype drive.     

异步电动机低转矩脉动直接转矩控制研究

针对异步电动机直接转矩控制中存在的转矩脉动问题,提出了一种基于多扇区的转矩脉动抑制策略.通过减小磁通在旋转过程中的空间角度,使磁通与电压矢量间的夹角趋向于连续,从而降低电压矢量步进式切换造成的脉振转矩,同时提取电压空间中的主矢量,以对称地改变定子磁通幅值并消除电流畸变引起的转矩脉动.仿真结果表明,该方案能有效降低异步电动机低速运行时的转矩脉动,提高直接转矩控制系统的响应特性.     

CHARACTERISTICS OF AN ASYNCHRONIZED SYNCHRONOUS GENERATOR

ABSTRACT

Theoretical steady state and transient characteristics of an asynchronised synclironous ( a.s.) generator, having excitation with full compensation for the emf induced in the field windings, are presented. Also transient characteristics of an a.s. generator, having excitation with field current feedback and slip feedback, are presented taking into consideration the ceiling voltage of the exciter

Finally a novel experimental excitation scheme using cyclo-converters and field current feedback is described, using which the steady state power, reactive power versus torque angle curves of a laboratory a.s. generator are obtained for the entire range ( 0 - 180° elec) of the torque angle at synchronous speed.     

SPEED AT BREAKDOWN TORQUE OF A SINGLE-PHASE INDUCTION MOTOR

ABSTRACT

The purpose of this paper is to present an “exact” analytical expression to calculate speed at breakdown torque and the breakdown torque of single-phase induction motors. From the theoretical expression for torque developed by a single-phase motor, a sixth order polynomial in speed at breakdown torque is obtained, and solved, using a well known method. A real root for per unit speed, i.e. the ratio of speed to the synchronous speed, that lies between 0 and 1 is selected and the actual breakdown torque is determined by substituting it in the torque equation. The breakdown torque, also termed as maximum torque, so obtained has been found to yield better correlation between the predicted and the tested results than the only existing empirical relation. Steps to calculate speed at maximum torque are also suggested. For an arbitrarily selected single-phase induction motor, a curve of speed at breakdown torque as a function of r₂/X is also included for the purposes of illustration and understanding.     

SINGLE-PHASE MOTOR INDUCTION-RUN PERFORMANCE

ABSTRACT

Based on the rotating-field theory and the ‘exact’ equivalent circuit of a single-phase induction motor operating on its main winding only, accurate solutions are derived for the salient features of its torque to speed performance at pull-out and zero torque, to enable the torque-speed plot to be readily compiled with sufficient accuracy for preliminary-design purposes. The torque, current, power factor and efficiency at any speed are also derived for use in the final-design stage, again on the assumption that the airgap mmf is sinusoidally distributed. However, in order to be strictly applicable to many modern designs, the analysis is extended to cover in a simple manner the general case of significant space-harmonic mmfs being present in the airgap.     

变工况下感应电机全速域效率优化控制研究

为了进一步降低感应电机在变工况运行条件下的电能损耗,提出了一种基于模型法的全速域节能运行控制策略。该策略在考虑漏感影响的损耗模型基础上,建立可控损耗受转速与电流影响的机理关系,并对感应电机在全速域范围运行的转速变化进行区域划分,利用库恩卡克条件确定在不同速度划分区域运行负载波动时的最优节能控制策略;在弱磁区利用电流轨迹图对励磁电流和转矩电流进行合理分配来提高感应电机最大转矩。仿真分析表明,所提出的节能控制策略能够有效提高感应电机的运行效率,并提高弱磁区的最大输出转矩,使系统达到最佳运行状态。     

Hardware Implementation of Direct Load Angle Controlled Induction Motor Drive

In this article, a direct load angle (DLA) control method for induction motor drive is proposed to reduce the torque and flux ripple. It is based on control of incremental change in load angle. The torque control is achieved without losing the benefits of conventional direct torque control (DTC) by reference stator flux angle, which is the sum of load angle and rotor flux angle. In this proposed DTC scheme, coordinate transformation is not required in torque control unit and reference voltage sector number and angle are not required in PWM unit, which reduces the control complexity. Proposed DTC control method is applied to two level voltage source inverter fed induction motor drive and hardware results are presented. From the hardware results, it is found that the proposed DTC control scheme impressively reduces the torque and flux ripples when compared with conventional space vector modulation (SVM) DTC.     

Torque ripple minimization with optimized <Emphasis Type="Italic">i</Emphasis><Subscript><Emphasis Type="Italic">sx</Emphasis></Subscript> estimation in vector control of current source inverter induction motor

In variable speed applications of high-power induction motors, current source inverters are usually used instead of voltage source inverters. Power switches delay and switching losses in high-power ranges, prevents inverters to operate properly in high frequencies. So the voltage and current waveforms of the motor supply consist of high harmonic content causing a high-torque ripple. One of the parameters that influences the torque ripple most is the stator flux reference value. Motors do not usually operate in their nominal load. Therefore, the optimum value of the flux is not equal to the nominal value. In this paper, an optimal i _sx reference for vector control system of current source inverter fed induction motor is produced by applying a simple estimator. Simulation results show that torque ripple has been reduced, especially in the loads less than their nominal power, as significantly as power factor has been improved. Also flux reduction causes to noticeable reduction in core and switching losses.