David Nedeljkovic

Predictive Torque Control of Induction Machines Using Immediate Flux Control

In this paper a new concept for control of induction machines by generating the torque at the end of the sampling interval, is presented. The method uses the predictive algorithm and can be split into two parts. The purpose of the first part (PTC - predictive torque control) is to predict the reference flux vector corresponding to the reference torque. The aim of the second part is the optimal tracking of the reference flux by selecting either an active or a zero voltage vector. This method, called immediate flux control (IFC), has already been presented by the authors. There are two possible variants of IFC. The characteristic of the first variant is the selection between an active and zero voltage space vectors to be impressed throughout the entire sampling interval. The minimal flux error is obtained following a simple and fast algorithm. As a consequence, the switching frequency becomes very low, but current and torque ripple are considerable. The second IFC variant enables more accurate flux generation by applying an active vector throughout a calculated time interval within a sampling interval, thus requiring higher switching frequency. Both IFC variants together with PTC require minimal processing time. The method has been tested on a 3 kW induction machine laboratory model controlled by DSP. The experimental results confirm the validity of the approach

Predictive Direct Control Applied to AC Drives and Active Power Filter

This paper presents different applications of a method called direct control. The previously developed approach has been redefined into a generalized form. The method relies on the prediction of either current or flux in discrete-time intervals and, consequently, selects the inverter voltage vector that produces the fastest possible transient. Depending on the task, two possible variants have been developed, offering a compromise between ripple in the controlled variable and switching frequency. A special effort has been made to overcome problems due to various delays (processing time, acquisition, gate driver delay, etc.) in the prediction routine, thus achieving maximum performance. The approach has been upgraded for application in AC drives, which allows additional torque control. The functional versatility of the approach has been demonstrated on different applications of power electronics (active power filter, induction machine, surface-mounted permanent-magnet synchronous machine). All applications have been tested on different laboratory models and have confirmed the validity of the approach.

Direct current control-a new current regulation principle

In this paper, two novel methods for current regulation are proposed. Both methods follow the synchronized on-off principle. In the classical approach, transistors are switched depending on the sign of the current error, which in turn is sampled at equal time intervals. In the first method, the current vector at the end of the interval is predicted for two possible cases when either the active voltage vector pointing toward current error or the zero voltage vector is applied. The one producing the smaller current error at the end of the sampling interval is chosen, thus obtaining drastic reduction of the switching frequency. In the second method, the best fitting active voltage vector succeeds the zero vector during the same time interval. A simple algorithm is used to calculate the duty cycle thus gaining the smallest possible current error. The method is compared with the CRPWM. Both methods were simulated and tested on a laboratory model with passive load. In the last part of the paper, the behavior of the second method is tested for erroneously estimated load parameters. The two methods show very small degradation of performance even when a rather high parameter error is introduced.

Detection of Broken Bars in Induction Motor through the Analysis of Voltage Modulation

The paper presents an application of a novel method for diagnostics of electric and magnetic asymmetry of rotor cage in induction motor due to broken rotor bars. Increasing anomaly in magnetic field distribution results in a degradation of steady-state and dynamic performance of induction motor. It can be determined through the analysis of the average duty cycle of the modulated voltage. Broken rotor bars would cause torque and speed ripple which is compensated by speed control algorithm. Consequently oscillation in duty cycle of the modulated stator voltage appears. This effect can be simply detected without additional hardware. Correct evaluation of faulty motor performance is very significant part of condition monitoring and diagnostic procedure in modern supervision systems of electrical drives

Detection of Broken Bars in Induction Motor Through the Analysis of Supply Voltage Modulation

This paper presents an application of a novel method for the diagnostics of electric and magnetic asymmetries of rotor cage in induction motor (IM) due to broken rotor bars. An increasing anomaly in magnetic field distribution results in degradation of steady-state and dynamic performance of an IM. This degradation can be determined through the analysis of the average duty cycle of the modulated supply voltage. Broken rotor bars would cause torque and speed ripple which is mitigated by an efficient speed-control algorithm. Consequently, specific oscillation in the duty cycle of the modulated stator voltage appears. This effect can be simply detected without additional hardware and therefore provides a correct evaluation of faulty motor performance, which is a very significant part of condition monitoring and diagnostic procedure in modern supervision systems for electrical drives.

Synchronization of active power filter current reference to the network

This paper presents an improved method of current reference determination for a parallel active power filter. To insure sinusoidal line currents, thus reducing reactive power, an integrative method of reference calculation is applied. As a result, satisfactory response of line current to the load change is achieved. One of the major problems, the synchronization to the net voltage, is also considered. Classical solution is based on zero-cross detection of line voltage. Since this approach is very sensitive to the voltage distortions, a new method is suggested, relying on voltage fundamental harmonic calculation. These ideas were tested on a three-phase active power filter, controlled by 32-bit microcontroller.

Direct Current Control of a Synchronous Machine in Field Coordinates

This paper presents an implementation of a modified method for direct current control (DCC) in a permanent-magnet synchronous motor (SM). The control is performed using a predictive algorithm in rotor field coordinates due to its simpler implementation. Its task is to minimize the final current error at the end of the sampling interval. Depending on the desired performance, two variants are proposed, yielding either lower switching frequency or lower current ripple. Both variants of DCC, named DCC I and DCC II, have been rearranged for applications in SMs with either surface-mounted permanent magnets or interior (buried) permanent magnets. A comparison with field-oriented control using space-vector modulation and synchronized on/off modulation shows the advantages and drawbacks of the proposed method. Simulations and measurements on a laboratory model with SM having low inductances (thus expecting high ripple) and very high number of poles (high stator supply frequency) confirm the validity of the approach. Additional considerations regarding practical problems, some of which can be found in similar predictive methods, are also presented.

Predictive Torque Control of Induction Machines using Immediate Flux Control

In this paper a new concept for control of induction machines by generating the torque at the end of the sampling interval, is presented. The method uses the predictive algorithm and can be split into two parts. The purpose of the first part (PTC - predictive torque control) is to predict the reference flux vector corresponding to the reference torque. The aim of the second part is the optimal tracking of the reference flux by selecting either an active or a zero voltage vector. This method, called immediate flux control (IFC), has already been presented by the authors. There are two possible variants of IFC. The characteristic of the first variant is the selection between an active and zero voltage space vectors to be impressed throughout the entire sampling interval. The minimal flux error is obtained following a simple and fast algorithm. As a consequence, the switching frequency becomes very low, but current and torque ripple are considerable. The second IFC variant enables more accurate flux generation by applying an active vector throughout a calculated time interval within a sampling interval, thus requiring higher switching frequency. Both IFC variants together with PTC require minimal processing time. The method has been tested on a 3 kW induction machine laboratory model controlled by DSP. The experimental results confirm the validity of the approach

Cost-Effective Three-Phase PMSM Drive Tolerant to Open-Phase Fault

This paper presents a low-cost fault-tolerant system for open-phase fault (OPF) in a power-converter-fed permanent-magnet synchronous machine. The proposed fault-tolerant system is based on field orientation control with additional fault tolerance functionality. A current predictive method for OPF detection is presented, together with an estimation of the threshold level for detection. The proposed method is based on the prediction of stator current for the next sampling interval. Furthermore, a new method for postfault operation of the machine is proposed. For optimal performance of the complete drive, a prefiring angle is introduced in order to avoid the temporary generation of negative torque. This improvement increases the average generated postfault electromagnetic torque, and consequently, it reduces the mechanical stress on various machine parts. The proposed fault detection and postfault operation solutions were simulated in MATLAB, and they were also tested on an experimental setup. The results show several advantages of the proposed fault-tolerant solution, like its short fault-detection time, substantial robustness against variation of machine parameters or load fluctuations, and negligible implementation costs, since no hardware modifications are needed. The fault detection algorithm does not require high computing power, and it operates well even during transients.

Direct current control of active power filter without filter current measurement

The paper presents a three-phase parallel active power filter without filter current measurement. Only supply grid voltage sensors, filter capacitor voltage sensor and supply line current sensors are used to provide filterpsila proper operation, aiming to reduce reactive power and line current distortion, caused by a non-linear load. Due to filterpsilas inverter-like topology with known parameters in filter branch, the predictive direct current control (DCC) method has been successfully applied, thus obtaining good dynamic response, reducing the commutation frequency of the active power filterpsilas switches and consequently decreasing the filter branch losses.