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Voltage angle direct torque control of induction machine in field-weakening regime
IET Electric Power Applications
Abstract
This study presents a novel algorithm for the induction motor torque control in field-weakening region. The proposed method insures maximum DC bus utilisation and extends the torque-speed curve up to the system limits. Controller is based on the stator voltage angle control. It provides full DC-link disturbance rejection and offers the torque response time sufficient for most applications. The algorithm is simple, without neither the outer flux loop nor the inner current loop. Dynamic response is preserved over wide speed range by means of gain-scheduling. Simulations and experiments prove an ease of implementation and the robustness of the proposed solution. This study comprises analytical considerations, simulation results, a detailed description of implementation steps and extensive experimental results.
... Adequate tunings of the PI regulator with variable gains for the VATC control are proposed in order to obtain well damped torque response in the flux weakening regime. The proposed torque control scheme is presented in the Section 3. The scheme is based on the approach presented in [22], but upgraded with the low speed IFOC and smooth transition between the IFOC to the VATC modes. It is shown that unique control structure can be used both for the base speed region (when machine operates in the IFOC mode) and in the flux weakening (when machine operates in the VATC mode). ...
... Linearized model is derived from (1-7) assuming that stator dynamics is much faster than rotor and mechanical dynamics. The approximated transfer function is obtained for the least favorable pole position, i.e. for the unloaded IM as given in [22]. The IM transfer function obtained by linearization around operating point "0" is given as: ...
... frame d 2 q 2 in Fig.1). GS block becomes active, and the IFOC scheme from Fig. 2 reduces to the VATC scheme [22] shown in Fig. 3. When speed lower than the base speed threshold is reached, it is presumed that there is sufficient voltage margin for current regulators to operate, and IFOC mode is restored. ...
This paper presents improved torque control scheme for a high speed sensorless induction motor drive. The proposed high speed torque control scheme substitutes the flux oriented control by the voltage angle control in the flux weakening regime. This scheme uses maximum of available inverter voltage, alleviates well known problems of current control schemes in conditions with insufficient voltage margin and avoids the influence of estimated speed error to the achieved flux level. The algorithm uses similar slip control as flux oriented control algorithm, but is applied without an outer flux trajectory reference which is typical for the flux weakening, providing a fast and well damped torque response even if error in estimated speed is present.
Experiments confirm the effectiveness of proposed torque control algorithm, smooth transition from the flux oriented control in the base speed region to the voltage angle control in the flux weakening, superior dynamic performance of the voltage angle torque control, and its robustness to an estimated rotor speed error.
... Adequate tunings of the PI regulator with variable gains for the VATC control are proposed in order to obtain well damped torque response in the flux weakening regime. The proposed torque control scheme is presented in the Section 3. The scheme is based on the approach presented in [22], but upgraded with the low speed IFOC and smooth transition between the IFOC to the VATC modes. It is shown that unique control structure can be used both for the base speed region (when machine operates in the IFOC mode) and in the flux weakening (when machine operates in the VATC mode). ...
... Linearized model is derived from (1-7) assuming that stator dynamics is much faster than rotor and mechanical dynamics. The approximated transfer function is obtained for the least favorable pole position, i.e. for the unloaded IM as given in [22]. The IM transfer function obtained by linearization around operating point "0" is given as: ...
... frame d 2 q 2 in Fig.1). GS block becomes active, and the IFOC scheme from Fig. 2 reduces to the VATC scheme [22] shown in Fig. 3. When speed lower than the base speed threshold is reached, it is presumed that there is sufficient voltage margin for current regulators to operate, and IFOC mode is restored. ...
This paper presents improved torque control scheme for a high speed sensorless induction motor drive. The proposed high speed torque control scheme substitutes the flux oriented control by the voltage angle control in the flux weakening regime. This scheme uses maximum of available inverter voltage, alleviates well known problems of current control schemes in conditions with insufficient voltage margin and avoids the influence of estimated speed error to the achieved flux level. The algorithm uses similar slip control as flux oriented control algorithm, but is applied without an outer flux trajectory reference which is typical for the flux weakening, providing a fast and well damped torque response even if error in estimated speed is present. Experiments confirm the effectiveness of proposed torque control algorithm, smooth transition from the flux oriented control in the base speed region to the voltage angle control in the flux weakening, superior dynamic performance of the voltage angle torque control, and its robustness to an estimated rotor speed error.
... In order to fully utilize DC bus voltage, maximum available voltage is used in the whole flux weakening region and the only one control variable is stator voltage angle. It is shown that IM torque control based on voltage angle control is possible by using the Gain -Scheduling (GS) approach, i.e. by compensating changes in motor dynamics according to speed and slip changes [7][8][9][10][11]. In this paper, the GS approach in DTC of IM in field weakening is implemented on the real drive, and steady state error in torque regulation during the speed changes is explored. ...
... The only one independent control variable is the synchronous speed. When the IM is controlled by stator voltage angle, its dynamics can be described by a transfer function in the form [7,9,11]: T s T T k k s T s T T s T T k k T s T s s T l U T T k k T ...
... In [9] the static gain is adopted for the case when it has the maximal value (when slip is equal to zero) as critical, and IM transfer function (1) is approximated as: ...
In this paper an overview of principle of Direct Torque Control of induction motor in field weakening based on Gain – Scheduling approach is given. The voltage angle is used as the control variable, while its amplitude is kept on maximum value. Robust gain-scheduling control for the optimal torque response in the wide range of motor speed and load is described. Analysis of proposed algorithm’s performance is given, and steady state error is discussed. Obtained results are illustrated by the computer simulation and experiment.
... In order to fully utilize available voltage, it is desirable to maintain its amplitude on the maximum value. Since the only one control variable is the voltage phase angle [4,5], torque and flux contours are coupled both in steady states and transients. ...
... An approximated IM transfer function (torquesynchronous speed) [4,5] is adopted in the form of static gain plus the complex-conjugate pairs of stator-related and rotor-related poles, with finite zeros neglected as: ...
... As the natural frequency of the stator poles exceeds by large the natural frequency of the rotor poles, the dynamic response in the field weakening regime is determined prevalently by the rotor dynamics. Maximal value of static gain appears under no load operation [5] (when relative slip is 0 = s ) and equals to: ...
n this paper an algorithm for Direct Torque Control (DTC) of induction motor (IM) in field weakening is presented. The study includes the analysis of the usual IM control approaches in field weakening which are based on open loop stator or rotor flux reference calculations from steady-state machine equations. Drawbacks of the usual approaches are pointed out and an algorithm for DTC of IM without outer flux reference is proposed. In the proposed algorithm rotor flux is accomplished automatically by closed loop regulation, enabling fast and well damped torque response with full voltage utilization, and maintaining optimal flux level in the whole range of field weakening. Torque control is obtained by voltage angle control. Effectiveness of proposed method is investigated via simulation and experiment on Digital Signal Processor (DSP) development platform.
... Since the IM dynamics is not considered, the DC bus voltage is not fully exploited, the torque per ampere ratio and the peak torque drive capabilities are suboptimal, while the flux and torque loops are coupled both in steady state and in transients. If the machine is supplied by full constant voltage in field weakening, the outer flux trajectory reference is not necessary since rotor flux automatically adjusts by torque control loop in field weakening [23][24][25]. ...
... In this Section, dynamic and static behavior of the linear motor model will be analyzed, and proper approximated model will be derived in order to achieve independent torque and flux control in the base speed region and high performance torque control in the field weakening region. From the state-space linearized model (20)(21)(22)(23)(24), transfer function matrix can be found as: ...
... Dynamic behavior of the motor model (20)(21)(22)(23)(24) is characterized by the positions of the matrix A (22) eigenvalues. These eigenvalues represent poles of the transfer function matrix (30). ...
This paper presents improved high speed Induction Motor (IM) torque control algorithm that achieves full inverter voltage utilization in field weakening and smooth speed transition from the base speed to high speed region. The Stator Voltage Vector Direct Torque Control (SVV DTC) algorithm simultaneously controls IM torque and rotor flux by regulating stator voltage angle and amplitude in the base speed region, and by regulating only the voltage angle in the field weakening region. Fast torque dynamics is obtained in wide speed range by using torque regulator with variable gain. The main benefit from the proposed algorithm is in field weakening range, in which the machine is supplied by maximal available voltage without outer flux trajectory reference. This approach enables full utilisation of both magnetic material of the machine and power capabilities of the inverter, which is not possible with traditional vector drives with current regulators. Transition from base speed region to field weakening is obtained by simply limiting the output voltage. Experimental results gathered on the high speed low cost IM drive confirm the effectiveness of the proposed SVV DTC approach.
... The test problem is an IM of 7.5kW with main parameters given in Table 2. The steel here is used with different types of TATA steel [20]. The comparison of efficiency and weight between conventional and optimal solutions with diferent steel types is shown in Figure 5 and Table 3. ...
... Today several approaches have still been developed and various studies [7][8][9][10][11][12] have been carried out in order to develop the performance of induction motors. Kumar et al. [7] presented a new approach that minimizes copper & iron losses and optimizes the efficiency of a variable speed induction motor drive. ...
In this study, a new design was suggested in order to improve the performance of induction motors. In the proposed design, slits were applied in the middle of stator and rotor teeth. In these slitted models, the depth and width of slits were optimized with Finite Element Method Magnetics (FEMM) software using Finite Elements Method (FEM). To show performance improvement, suggested motor model and a reference motor model were compared at the rated operating point for the values such as input and output power, input current, power factor, efficiency and losses. Because of the decreases in both iron and copper losses, total losses in motor were reduced and the efficiency for rated operating point was improved by 1,869%. In the modelling, 3 kW squirrel-cage induction motor was used.
... Research proposals for DTC based field-weakening operations are given in [7]- [10]. The majority of these proposals focuses on the field-weakening methods of directtorque-controlled induction motors and is not readily applicable for the IPMSM. ...
Direct torque control has been widely accepted as a simple but effective control method for AC motor drives. It is robust to parameter variations since only stator resistance is used in the algorithm. However, to obtain high performance motor drives, the stability and robustness of direct torque control in the field-weakening region must be guaranteed. Some of the existing research proposals are parameter dependent. This paper proposes an improved robust field-weakening control algorithm for direct torque controlled IPM synchronous motors. The effectiveness of the proposed method is validated by simulation results.
... The tip of stator flux space vector moves along the spiral trajectory from the initial to the final position. The spiral motion of the stator flux space vector enables aperiodic response of torque and flux in field weakening [6]. ...
In this paper a space vector model of an induction motor in a field weakening regime is detailed. Stator and rotor flux space vector trajectories are shown in a complex plane for the case when the machine is supplied by limited voltage.
This paper presents a robust model and its simulation to investigate the performance of an AC propulsion system in a rail vehicle for directly returning the regenerative braking power to the feeder substation of an AC traction network. This direct returning method can be an efficient approach for energy recovery if the regenerative braking is reliably applied. However, it is shown that this method can cause undesired voltage fluctuations if the regenerative braking regime or braking location of the rail vehicle change. The load torque on the traction motor (TM) is precisely modelled when pure electrical braking is applied. Different states of the direct torque controlled inverter are modelled when the TM regenerates. A circuit model for the utility grid, load impedances and the traction network is developed to evaluate the network receptivity against the regenerated power. The dynamics of the electromagnetic torque and the fluctuations of the DC-link voltage are investigated for two operational conditions: changes on the regenerative braking regime and changes on the rail vehicle braking location. The results justify how the DC-link voltage dramatically fluctuates with variations of the rail vehicle's operation conditions, whereas the electromagnetic torque is maintained on optimum rates.
This study proposes an adaptive Takagi-Sugeno-Kang-fuzzy (TSK-fuzzy) speed controller (ATFSC) for use in direct torque control (DTC) induction motor (IM) drives to improve their dynamic responses. The proposed controller consists of the TSK-fuzzy controller, which is used to approximate an ideal control law, and a compensated controller, which is constructed to compensate for the difference between the TSK-fuzzy controller and the ideal control law. Parameter variations and external load disturbances were considered during the design phase to ensure the robustness of the proposed scheme. The parameters of the TSK-fuzzy controller were adjusted online based on the adaptive rules derived in Lyapunov stability theory. The ATFSC, fuzzy control, and PI control schemes were experimentally investigated, using the root mean square error (RMSE) performance index to evaluate each scheme. The robustness of the proposed ATFSC was verified using simulations and experiments, which involved varying parameters and external load disturbances. The experimental results indicate that the ATFSC scheme outperformed the other control schemes.
This paper describes an SVM-DTC control scheme of an induction motor. Combining the Space Vector Modulation (SVM) and Direct Torque Control (DTC) resulting a minimal torque ripple compare to classical DTC. Investigation of speed and torque performance of the induction motor within base speed and in field weakening region are verified in the experiment. The stator flux in field weakening region is weaken using “1/ ωr” method. The overall experiment is conducted using a drive system base on TMS320F28335 floating point microcontroller and dynamometer as a load of the motor.
In numerous motor drive applications, high rotor speed is the key factor for system cost, performance, and overall energy efficiency. As a result of energy crises and global market competition, the specified rotor speed and fundamental frequency of the induction motor (IM) in many drive applications noticeably go up. For the same cost and efficiency reasons, that increase of inverter fundamental output frequency cannot be followed with the increase of pulsewidth modulation (PWM) frequency. Therefore, a very low ratio between the PWM and motor fundamental frequencies is to be expected in the near future. In this paper, the shaft-sensorless drive performance is investigated at high speeds, with a very low sampling to fundamental frequency ratio. As a result, two main problems with rotor flux estimators were discovered: the integration problem in the current-based rotor flux model and the phase error in the voltage-based rotor flux model. Both problems were addressed, and a proper joint solution is suggested. The effectiveness of the proposed solution is tested in a model-reference-adaptive-system-based high-speed shaft-sensorless IM drive. The experimental results collected from the digitally controlled IM drive with a low frequency ratio validate the proposed solution.
This paper presents a simple overmodulation method employed in direct torque control (DTC)-constant switching frequency (CSF) controller of induction machines. The proposed overmodulation method is utilized to extend a constant torque region and hence produces high torque capability in field weakening region with six-step operation. It is shown that the overmodulation operation using the DTC-CSF scheme can be established by controlling the stator flux locus from circular to the hexagonal shape. This is achieved by modifying the flux error status produced from flux hysteresis controller, before it is being fed to the look-up table. The main benefit in the proposed strategy is its simplicity and it can be implemented in many electric drive applications to obtain maximum torque capability for a wider speed range operation.
A new, deadbeat type of direct torque control is proposed, analyzed and experimentally verified in this paper. The control is based on stator and rotor flux as state variables. This choice of state variables allows a graphical representation which is transparent and insightful. The graphical solution shows the effects of realistic considerations such as voltage and current limits. A position and speed sensorless implementation of the control, based on the self-sensing signal injection technique, is also demonstrated experimentally for low speed operation. The paper first develops the new, deadbeat DTC methodology and graphical representation of the new algorithm. It then evaluates feasibility via simulation and experimentally demonstrates performance of the new method with a laboratory prototype including the sensorless methods.
In this paper a new sensorless Direct Torque Control (DTC) algorithm for induction motor drive is proposed and tested. This algorithm provides decoupled control of the torque and flux with constant inverter switching frequency and a minimum torque and flux ripple. Compared to the other DTC methods, this algorithm is much simpler and has less mathematical operations, and can be implemented on most existing digital drive controllers. Algorithm is based on imposing the flux vector spatial orientation and rotation speed, which defines the unique solution for reference stator voltage. The implementation of the control scheme using DSP-based hardware is described, with complete experimental evidence and the straightforward implementation instructions.
A torque control scheme that utilizes the stator flux components as control variables, applied to a speed-sensorless induction motor drive, is presented. The proposed scheme allows the motor to exploit the maximum torque capability in the whole speed range, and shows a reduced dependence on the motor parameters. It is based on a control algorithm that decreases the d-component of the stator flux as soon as the voltage corresponding to the maximum torque exceeds the available voltage. The control scheme produces a smooth transition into and out of the field weakening region and preserves the torque dynamic. The feasibility of the field weakening technique is confirmed by computer simulations and experimental tests.
At high angular velocity, the induction motor is operated in the field weakening range due to the voltage limit of the inverter. Field oriented vector control (FOC) is unsuitable for this operation due to coupling, non-linearities, and saturation of linear current controllers. A proposed direct torque control space vector modulation (DTC-SVM) scheme using SVM does not use coordinate transforms or current controllers to achieve DTC. Control of the stator flux vector allows for dynamic change in the torque in all regions, including field weakening with the six-step operation. This paper describes the torque control dynamic in the field weakening range, using a step change in stator flux vector magnitude and it's angular velocity. The method is demonstrated experimentally.
A novel control strategy for the induction motor drive, based on the field acceleration method, is presented. The torque is controlled through variations of the stator flux angular velocity. The stator flux is controlled by using a feedforward control scheme, with the stator flux reference vector adjusted so as to obtain the fixed rotor flux amplitude. The applied controller assures a fast torque response, low torque ripple in the steady state, and drive operation with a constant switching frequency. The algorithm includes the improved stator and rotor flux estimation that guarantees the stable drive operation in all operating conditions, even at low speeds. The experimental tests verify the performance of the proposed algorithm, proving that good behavior of the drive is achieved in the transient and steady-state operating conditions.
Field-weakening techniques have been developed which provide
maximum torque capability above rated speed. Unfortunately, most of
these techniques are valid only for steady-state operation and show
significant sensitivity to DC-bus voltage and machine parameters. This
paper analyzes the requirements of dynamically providing maximum torque
under field-weakening operation. Three major issues are addressed:
current regulator design; saturation techniques for current regulators
in order to ensure best performance under voltage constraints; and flux
regulator design to minimize transient errors when varying flux. Maximum
DC-bus utilization through the proper use of saturation techniques,
dynamic response and reduced sensitivity are the advantages of the
proposed solution
The analysis and design of current regulators for multiphase AC
loads in AC machines is presented using complex vector notation. The use
of complex vector notation provides a way of comparing the performance
of controller topologies through their complex vector root locus and
complex vector frequency-response functions. Limitations in the
performance of the synchronous frame proportional plus integral current
regulator are outlined and several ways of improving its performance are
suggested and investigated
A direct induction machine torque control method based on
predictive, deadbeat control of the torque and flux is presented. By
estimating the synchronous speed and the voltage behind the transient
reactance, the change in torque and flux over the switching period is
calculated. The stator voltage required to cause the torque and flux to
be equal to their respective reference values is calculated. Space
vector PWM is used to define the inverter switching state. An
alternative approach to deadbeat control for use in the transient or
pulse-dropping mode is also presented. An alternative modulation scheme
is presented in which transient performance is improved by specifying
the inverter switching states and then calculating the required switched
instants to maintain deadbeat control of the flux while reducing the
torque error during the entire switching interval. A similar approach is
used for a transient in the flux. The implementation of the control
scheme using DSP-based hardware is described, with complete experimental
results given
Digital Control of Electrical Drives offers insight into electric drives and their usage in motion control environment. It provides links among electrical machine and control theory, practical hardware aspects, programming issues, and application-specific problems. The book prepares the reader to understand the key elements of motion control systems, analyze and design discrete-time speed and position controllers, set adjustable feedback parameters, and evaluate closed-loop performances. Basic engineering principles are used to derive the controller structure in an intuitive manner, so that designs are easy to comprehend, modify, and extend. Digital Control of Electrical Drives helps the reader acquire practical skills in designing discrete-time speed and position controllers. Each chapter is followed by a set of Matlab® and Simulink® tools which help readers master the phases of design, tuning, simulation, and evaluation of discrete time controllers, and foresee the effects of control solution on the overall motion control system. Readers will also understand the present performance limits of digital motion controllers. © 2007 Springer Science+Business Media, LLC. All rights reserved.
This is the first comprehensive book on sensor less high performance a.c. drives. It is essential reading for anyone interested in acquiring a solid background on sensor less torque-controlled drives. It presents a detailed and unified treatment of sensor less vector-controlled and direct-torque controlled drive systems. It also discusses the applications of artificial intelligence to drives. Where possible, space vector theory is used and emphasis is laid on detailed mathematical and physical analysis. Sensorless drive schemes for different types of permanent magnet synchronous motors, synchronous reluctance motors, and induction motors are also presented. These include more than twenty vector drives e.g. five types of MRAS-based vector drives, and eleven types of direct-torque-controlled (DTC) drives, e.g. the ABB DTC drive. However, torque-controlled switched reluctance motor drives are also discussed due to their emerging importance. The book also covers various drive applications using artificial intelligence (fuzzy logic, neural networks, fuzzy-neural networks) and AI-based modelling of electrical machines. Finally, self-commissioning techniques are also discussed. This is a comprehensive thoroughly up-to-date, and self-contained book suitable for students at various levels, teachers, and industrial readership. Peter Vas is a Professor at the Department of Engineering at the University of Aberdeen, UK, where he is also the Head of the Intelligent Motion Control Group. His previous books published by Oxford University Press are extensively used worldwide.
This paper presents a new simple overmodulation strategy in direct torque control (DTC) of induction machines. The strategy increases the inverter voltage limit and therefore extends the constant torque region. The minor modification on the conventional flux weakening method is also introduced to improve the torque capability in the field weakening region and hence improving the operation over a wide speed range. These strategies are implemented with a DTC-constant switching frequency (CSF) based system. To perform the DTC-CSF in overmodulation mode, the flux error status is modified before it is being fed to the voltage vector selection table. With this proposed strategy, a single-mode overmodulation based on stator flux vector control with smooth PWM to six-step transition is developed. This strategy also proposes a dynamic overmodulation to obtain fast torque transient with six-step operation including the field weakening region. The proposed strategy has been verified through computer simulations a
An approach to induction-machine control which ensures maximum torque per ampere over the entire field weakening region is presented. The relation of the output torque capability to the leakage inductance of the machine is examined. By adjusting the base speed for the field weakening operation according to the flux level, the current limit, and the voltage limit, smooth and precise transition into the field weakening operation can be achieved. The proposed scheme is verified through simulation and experiment for a 5 hp laboratory induction motor drive system
Direct torque control (DTC) is an induction motor control technique that has been successful because it explicitly considers the inverter stage and uses few machine parameters, while being more robust to parameter uncertainty than field-oriented control (FOC). This paper presents a formal derivation of DTC based on singular perturbation and nonlinear control tools. The derivation elaborates an explicit relationship between DTC performance and machine characteristics; low-leakage machines are expected to perform better under DTC. It is shown that DTC is a special case of a sliding-mode controller based on the multiple time-scale properties of the induction machine. The known troublesome machine operating regimes are predicted and justified. Explicit conditions to guarantee stability are presented. DTC is shown to be a suboptimal controller because it uses more control effort than is required for flux regulation. Finally, compensation strategies that extend DTC are discussed. The derivation does not require space vector concepts thus, it is established that the traditional link between DTC and space vectors is not fundamental
In this paper, a dead-beat type digital controller has been introduced to overcome the problems of a conventional direct torque controller. The proposed induction motor drive with a digital dead-beat controller shows good transient response and negligible steady-state error even at a low switching frequency, which is needed for high power machines used for transportation. Including the rotor dynamics, the stability condition and steady-state error of the proposed control system have been examined in the z-plane. In addition, the good performance has been verified through the simulation and experiment. The flux and torque controllers have been designed with only stator voltage equations in the stator flux reference frame in order to take advantages of the direct torque control. Therefore, the proposed flux and torque controllers have simple forms and can be easily designed and implemented.
Field-oriented control and direct torque control are becoming the industrial standards for induction motors torque control. This paper is aimed at giving a contribution for a detailed comparison between the two control techniques, emphasizing advantages and disadvantages. The performance of the two control schemes is evaluated in terms of torque and current ripple, and transient response to step variations of the torque command. The analysis has been carried out on the basis of the results obtained by numerical simulations, where secondary effects introduced by hardware implementation are not present.
In the overmodulation region, the operation of the electrical
drive system with a current controller is characterized by a rapid
deterioration of motor torque and speed. It is desirable to use the
overmodulation strategy, which guarantees the fast response even in
transient state and satisfies the overall closed-loop control
performance. In this paper, in order to improve the dynamic
characteristics of the electrical drive, a new overmodulation technique
is proposed. Considering the current transition characteristics, an
efficient overmodulation strategy is introduced to achieve better
transient performance through an adequate voltage selection. With the
help of a new overmodulation strategy, required electrical torque can be
directly produced as quickly as possible, and stable drive
characteristics can be achieved in the transient condition. The proposed
method has been implemented on an actual inverter system and thoroughly
tested on a 900-W interior permanent magnet synchronous machine (IPMSM)
to confirm its feasibility
A control scheme for robust flux-weakening operation of direct-torque-control induction motor drive is proposed. The basic idea is to adjust the flux reference on the basis of the torque error, thus determining a spontaneous flux weakening. To exploit the maximum torque capability, it is necessary to estimate the maximum torque that the induction machine is able to generate at any speed. Initially, a basic version of the algorithm, requiring a simple off-line parameter tuning, is presented. Then, the algorithm is improved and completed with the online estimation of the maximum torque, hence avoiding the initial tuning process. The main features of the proposed methods are a little dependence on machine parameters and a smooth transition into and out of the flux-weakening operation mode. Experimental tests demonstrate the effectiveness of the control schemes.
Overmodulation enhances the power utilization of the installed
capacity of a voltage-source inverter. A space-vector strategy is used
for constant-switching-frequency inverters. In order to achieve the
overmodulation, a modified reference signal with nonuniform angular
velocity is generated using a preprocessor. Such a reference wave
produces low-frequency harmonics in currents. The presence of current
harmonics restricts the bandwidth of the synchronous proportional plus
integral current controller in the overmodulation range. A compensating
current control is presented to allow for high-bandwidth current control
in synchronous coordinates during overmodulation and six-step. The
proposed scheme allows for an easy upgrade of a conventional vector
control scheme to include overmodulation and, thus, reduce the
design-to-market time
The torque-maximizing field-weakening control scheme proposed by
Kim and Sul is developed further. The performance under imperfect field
orientation conditions is investigated, and it is shown that an
overestimated-rather than an underestimated-model leakage inductance
should be used. A slightly modified algorithm, which offers better
robustness and reduced computational complexity, is presented. The
importance, for good performance, of combining the scheme with current
and speed controllers featuring antiwindup and improved disturbance
rejection is emphasized. The dynamics of the resulting closed-loop
system are analyzed. Obtained in the process, are rules for selection of
all controller parameters, allowing tuning without trial-and error
steps. Good performance of the resulting system is verified
experimentally
In a high-speed operation of a vector-controlled induction motor,
coupling between d-q current dynamics impairs the characteristics of
torque response. The feedforward decoupling scheme does not perform well
if an error exists in the motor parameter estimation. We derive a
dynamic decoupling condition when the two additional proportional
integral current controllers are used. A great advantage of this dynamic
decoupling controller is the robustness to the motor parameter
estimation errors. Further, we observe that overmodulation methods lead
to the violation of the decoupling condition, thereby yielding a poor
performance in the high-speed high-power operation. As a method of
resolving this problem, we propose a decoupling preserving
overmodulation algorithm which also enhances the torque transient
response. Through simulation and experimental results, we demonstrate
the improved performance of the proposed controller
A control technique, which utilizes the stator flux components as control variables, has been applied to a speed-sensorless induction motor drive. The scheme may be regarded as a development of a direct torque control scheme, aimed at achieving a constant-switching-frequency operation. At each sampling period the required voltage vector is calculated on the basis of the error between the reference and the estimated stator flux vector. The problems related to the voltage-source inverter dead time and the stator flux estimation at low speed have been analyzed, and an efficient solution has been proposed. The performance of the drive system has been verified by experimental tests, and good results have been achieved in both steady-state and transient operating conditions.
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
A method of actively maintaining voltage margin in field-oriented
induction machine controllers is proposed. A voltage-margin controller
is developed that rejects DC-link and load disturbances, such that
current regulation and field orientation is maintained. In addition, the
voltage-margin controller coupled with rotor-flux-oriented control is
shown to provide maximum torque capability equivalent to
stator-flux-oriented control
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