Torque and current control of induction motor drives for inverter switching frequency reduction

In this paper, an indirect field-oriented control (FOC) induction motor (IM) drive with instantaneous current and torque control is presented. This proposed control scheme employs hysteresis current and torque controllers to regulate the stator currents. The torque controller is proposed to serve the current controller so that full advantage of the zero voltage vector can be taken to reduce the switching frequency of the inverter. As a result, the actual stator currents can follow the current references as closely as possible, and the current ripple and torque ripple can be greatly decreased compared with the conventional adaptive pulsewidth modulation control method. To verify the feasibility of the proposed scheme, computer simulations and experiment results demonstrate that the proposed method can obtain a high-performance IM drive system.     

A dual two-level inverter scheme with common mode voltage elimination for an induction motor drive

Pulse-width modulated (PWM) inverters are known to generate common mode voltages which cause motor bearing currents in the induction motor drives. They also result in leakage currents which act as sources of conducted electromagnetic interference in the drive system. The common mode voltage generated by a conventional three-level inverter can be eliminated by switching only the voltage space vectors which do not produce the common mode voltage. This paper presents a PWM switching strategy to eliminate common mode voltage using the open-end winding configuration for the induction motor. The switching strategy presented in this paper, does not generate any alternating common mode voltages in the drive system and hence the electrostatic coupling of the common mode voltage, which results in the bearing currents and the leakage currents, is avoided. The proposed scheme is devoid of neutral point voltage fluctuations and does not require neutral point clamping diodes, when compared to the common mode elimination scheme based on the conventional three-level inverter topology. Also, the present scheme uses a single dc-link with half the voltage compared to the conventional three-level inverter based scheme.     

High-power CSI-fed induction motor drive with optimal power distribution based control

In this article, a current source inverter (CSI) fed induction motor drive with an optimal power distribution control is proposed for high-power applications. The CSI-fed drive is configured with a six-step CSI along with a pulsewidth modulated voltage source inverter (PWM–VSI) and capacitors. Due to the PWM–VSI and the capacitor, sinusoidal motor currents and voltages with high quality as well as natural commutation of the six-step CSI can be obtained. Since this CSI-fed drive can deliver required output power through both the six-step CSI and PWM–VSI, this article shows that the kVA ratings of both the inverters can be reduced by proper real power distribution. The optimal power distribution under load requirements, based on power flow modelling of the CSI-fed drive, is proposed to not only minimise the PWM–VSI rating but also reduce the six-step CSI rating. The dc-link current control of the six-step CSI is developed to realise the optimal power distribution. Furthermore, a vector controlled drive for high-power induction motors is proposed based on the optimal power distribution. Experimental results verify the high-power CSI-fed drive with the optimal power distribution control.     

Fast Control of Filter for Sensorless Vector Control SQIM Drive With Sinusoidal Motor Voltage

A pulsewidth-modulated (PWM) voltage applied to a squirrel-cage induction motor (SQIM) can cause high bearing currents, heating of rotor shaft, voltage spike across the motor terminals, etc. Filtering of this PWM voltage to obtain a sinusoidal output voltage can be a solution to this problem. However, a passive L-C filter makes the dynamic performance of the drive poor for high-performance control application. In this paper, a feed-forward control strategy for the L-C filter is proposed to have a good bandwidth for the filter output voltage. This filter control strategy is introduced along with a sensorless vector control strategy for the SQIM drive. This complete strategy retains the high dynamic performance of the drive even with the L-C filter. In this paper, a three-level converter is used as a voltage source inverter for the drive to have a less filter-size requirement. The control strategy is verified on a 7.5-hp SQIM drive with a three-level insulated-gate bipolar-transistor inverter and L-C filter. Experimental results validate the high dynamic performance of the drive with filter.     

Analysis and implementation of a real-time predictive currentcontroller for permanent-magnet synchronous servo drives

A real-time current controller for PWM inverter-fed permanent-magnet synchronous motor drives is presented and analyzed. The proposed current control scheme is based on predictive control with a parallel integral loop added to compensate for the inaccuracy of the motor model and for the variations of motor parameters and DC voltage source. The proposed current control scheme is analyzed and its performance is evaluated by computer simulation. An EPROM-based implementation is presented in which calculations and pulsewidth modulation are executed by lookup tables resulting in high-speed operation. The controller performance is evaluated using a prototype l kW PM synchronous servo drive. Experimental results are given and discussed     

A novel control scheme for the multilevel rectifier/inverter

A novel three-level pulsewidth modulation (PWM) rectifier/inverter is proposed: this single-phase three-level rectifier with power factor correction and current harmonic reduction is proposed to improve power quality. A three-phase three-level neutral point clamped (NPC) inverter is adopted to reduce the harmonic content of the inverter output voltages and currents. In the adopted rectifier, a switching mode rectifier with two AC power switches is adopted to draw a sinusoidal line current in phase with mains voltage. The switching functions of the power switches are based on a look-up table. To achieve a balanced DC-link capacitor voltage, a capacitor voltage compensator is employed. In the NPC inverter, the three-level PWM techniques based on the sine-triangle PWM and space vector modulation are used to reduce the voltage harmonics and to drive an induction motor. The advantages of the adopted th-ree-level rectifier/inverter are (1) the blocking voltage of power devices (T1, T2, S_a1-S_c4) is clamped to half of the DC-link voltage, (2) low conduction loss with low conduction resistance due to low voltage stress, (3) low electromagnetic interference, and (4) low voltage harmonics in the inverter output. Based on the proposed control strategy, the rectifier can draw a high power factor line current and achieve two balance capacitor voltages. The current harmonics generated from the adopted rectifier can meet the international requirements. Finally, the proposed control algorithm is illustrated through experimental results based on the laboratory prototype.     

A novel vector control scheme for transistor PWM inverter-fedinduction motor drive

A novel vector control scheme for a transistor pulse-width-modulation (PWM)-inverter-fed induction motor drive is presented. The system is based on a current control loop that consists of two independent nonlinear controllers that regulate the DC (field-oriented) components of the stator current vector. Three-level hysteresis comparators are used as current controllers. The outputs of the comparators select the appropriate inverter output voltage vectors via a switching erasable programmable ROM (EPROM) table. The theoretical principle of this method is discussed. Simulation and experimental results that illustrate the operation of the proposed system and performance in comparison with the other known schemes based on two-level hysteresis comparators are presented     

A practical vector control algorithm for μ-based induction motordrives using a new space vector current controller

In this paper, a very simple vector control algorithm is proposed for μ-based induction motor drives. The proposed method controls the motor torque directly and linearly to achieve instantaneous torque response without oscillation. The merits of this control method include its simple architecture and obviating the coordinate transformation. At the same time, a new space vector-based current controller is proposed to serve as a quick response torque controller of the proposed drive system. In addition to the current error, information on the current error derivative is further employed so that one can take more advantages of adding the zero voltage vector for reducing the switching frequency. As a result, the efficiency can be greatly increased. It is seen that through integration of the above two parts, the proposed drive system is very simple to implement and has very good performance     

Sensorless field-oriented control for double-inverter-fed wound-rotor induction motor drive

A novel control technique for sensorless vector control operation of a double-inverter-fed wound-rotor induction motor is presented. Two current controllers control the stator-side currents based on a vector control algorithm. Another V/f-type flux and frequency controller controls the rotor-side frequency directly. A novel frequency command profile for the rotor-side controller is suggested to make this sensorless drive operation reliable and reduce dependence on motor parameters at any rotor speed. A complete inverter power flow analysis is presented to show that the drive can deliver full torque from 0- to 2-p.u. speed for either direction of rotation. Thus, double the rated power can be extracted from the induction motor without overloading it. The proposed algorithm allows the drive to start on-the-fly without any rotor transducer. Results from a prototype 50-hp drive are presented.     

A multilevel inverter system for an induction motor with open-end windings

In this paper, a multilevel inverter system for an open-end winding induction motor drive is described. Multilevel inversion is achieved by feeding an open-end winding induction motor with two two-level inverters in cascade (equivalent to a three-level inverter) from one end and a single two-level inverter from the other end of the motor. The combined inverter system with open-end winding induction motor produces voltage space-vector locations identical to a six-level inverter. A total of 512 space-vector combinations are available in the proposed scheme, distributed over 91 space-vector locations. The proposed inverter drive scheme is capable of producing a multilevel pulsewidth-modulation (PWM) waveform for the phase voltage ranging from a two-level waveform to a six-level waveform depending on the modulation range. A space-vector PWM scheme for the proposed drive is implemented using a 1.5-kW induction motor with open-end winding structure.     

基于预前控制异步电机直接转矩控制方案的研究

在传统异步电动机直接转矩控制方案中，由于负载的变化规律不可预测，因此其常会带来较大的开关频率的变化，为此提出了一种基于预前控制的异步电动机直接转矩控制的方法。该方法依据前一个周期的磁链和转矩误差，对下一个开关周期所应施加到异步电动机的定子电压矢量进行预测，然后借助空间矢量PWM的方法，合成此开关电压矢量。样机实验结果表明，该方案不但能维持逆变器的开关频率基本恒定，而且还具有比传统直接转矩控制更为优良的动静态特性。     

A space vector modulated CSI-based AC drive for multimotorapplications

A space vector controlled channel state information (CSI) drive for multimotor applications is investigated. The multimotor operation of the drive is achieved by integrating the proposed active damping control, inverter-side DC link voltage feedforward control and pulse width modulation (PWM) index control into the conventional V/f control. The main function of the active damping control is to suppress possible LC resonances caused by the inverter filter capacitor and motor inductances. This function is essential in achieving stable operation of the drive, especially in the multimotor drive where multiple LC resonant modes exist. An additional advantage provided by the active damping control is that it makes the control system less sensitive to motor parameters. The inverter-side DC link voltage feedforward control and the adjustable PWM modulation index control are developed to improve the dynamic performance of the drive system. In addition, the proposed space vector PWM pattern features a low switching frequency (500 Hz), which makes the proposed drive system suitable for high power applications. The system stability is investigated by means of eigenvalue analysis. The theoretic analysis is verified by experiments on a digital signal processing (DSP) controlled CSI multimotor drive     

Digital control of induction motor current with deadbeat responseusing predictive state observer

For a high-power induction motor drive, the switching frequency of the inverter cannot become higher than one kilohertz, and such a switching frequency produces a large current ripple, which then produces torque ripple. To minimize the current ripple, a method based on deadbeat control theory for current regulation is proposed. The pulsewidth modulation (PWM) pattern is determined at every sampling instant based on stator current measurements, motor speed, current references, and rotor flux vector, which is predicted by a state observer with variable poles selection, so that the stator currents are controlled to be exactly equal to the reference currents at every sampling instant. The proposed method consists of two parts: (1) derivation of a deadbeat control and (2) construction of a state observer that predicts the rotor flux and the stator currents in the next sampling instant. This paper describes a theoretical analysis, computer simulations and experimental results     

Control of the single-phase three-leg AC/AC converter

This paper investigates the control of a single-phase three-leg ac/ac reversible converter in which a leg is shared by both the grid and the load side. Pulsewidth-modulation (PWM) techniques based on scalar and vector approaches are developed, introducing the concept of local and general apportioning factor and, also, a complete equivalence between scalar and space vector PWM. A hysteresis current controller capable of taking into account the shared leg is developed and a zero current error linear controller is presented. Furthermore, a control strategy to obtain maximum utilization of the dc-bus voltage is proposed. In addition, several relevant characteristics of the converter are addressed, such as voltage rating, harmonic distortion, shared leg and capacitor currents, and power rating. The converter is compared to four-leg and two-leg converters. Experimental results are presented.     

Implementation of a direct control algorithm for induction motorsbased on discrete space vector modulation

The basic concept of direct torque control of induction machines is investigated in order to emphasize the effects produced by a given voltage vector on stator flux and torque variations. The low number of voltage vectors which can be applied to the machine using the basic DTC scheme may cause undesired torque and current ripple. An improvement of the drive performance can be obtained using a new DTC algorithm based on the application of the space vector modulation (SVM) for prefixed time intervals. In this way a sort of discrete space vector modulation (DSVM) is introduced. Numerical simulations and experimental tests have been carried out to validate the proposed method     

Analysis of Output Current Ripple rms in Multiphase Drives Using Space Vector Approach

Multiphase variable-speed drives, supplied from two-level voltage source inverters (VSIs), are nowadays considered for various industrial applications. Although numerous pulsewidth modulation (PWM) schemes for multiphase VSIs, aimed at sinusoidal output voltage generation, have been developed, no detailed analysis of the impact of these modulation schemes on the output current ripple has ever been reported. This paper presents a comprehensive analytical analysis and comparison of the output current ripple caused by the application of three different continuous PWM schemes, using a five-phase VSI as an example. Main properties of sinusoidal PWM, fifth harmonic injection PWM, and space vector PWM are elaborated and analyzed using the harmonic flux concept. Space vector theory is applied in the analysis. As a result, harmonic distortion factors are obtained for each PWM scheme. Theoretical considerations are verified by simulations and experimental investigation using a custom-designed five-phase VSI-fed induction motor drive.      

A New Hybrid Random PWM Scheme

This paper proposes a new hybrid random pulsewidth modulation (PWM) scheme based on a TMS320LF2407 DSP, in order to disperse the acoustic switching noise spectra of an induction motor drive. The proposed random PWM pulses are produced through the logical comparison of a pseudorandom binary sequence (PRBS) bits with the PWM pulses corresponding to two random triangular carriers. For this reason, the PWM pulses of the proposed scheme possess the hybrid characteristics of the random pulse position PWM and the random carrier frequency PWM. In order to verify the validity of the proposed method, the simulations and experiments were conducted with a 1.5-kW three-phase induction motor under the 2.5-A load condition. The DSP generates the random numbers, the PRBS bits with a lead-lag random bit (8 bit) and the three-phase reference signals. Also, a frequency modulator MAX038 makes the randomized frequency triangular carrier (3 $pm$ 1 kHz). From the results, the proposed scheme shows good randomization effects of the voltage, current, and acoustic noise of the motor as compared with conventional scheme (3 kHz).      

A new scheme for speed-sensorless control of induction motor

Various control algorithms have been proposed for the speed-sensorless control of an induction motor. These sensorless algorithms are mainly based on the speed feedback with the flux and speed estimations. This paper proposes a new scheme for the speed-sensorless control of an induction motor. The proposed scheme is based on the current estimation without the flux and speed estimations, in which the controlled stator voltage is applied to the induction motor so that the difference between stator currents of the mathematical model and motor may be forced to decay to zero. The performance of the proposed scheme is verified through simulation and experiment.     

Three-Level Inverter Scheme With Common Mode Voltage Elimination and DC Link Capacitor Voltage Balancing for an Open-End Winding Induction Motor Drive

A dc link capacitor voltage balancing scheme along with common mode voltage elimination is proposed for an induction motor drive, with open-end winding structure. The motor is fed from both the ends with three-level inverters generating a five level output voltage space phasor structure. If switching combinations, with zero common mode voltage in the pole voltage, are used, then the resultant voltage space vector combinations are equivalent to that of a three-level inverter. The proposed inverter vector locations exhibit greater multiplicity in the inverter switching combinations which is suitably exploited to arrive at a capacitor voltage balancing scheme. This allows the use of a single dc link power supply for the combined inverter structure. The simultaneous task of common mode voltage elimination with dc link capacitor voltage balancing, using only the switching state redundancies, is experimentally verified on a 1.5-kW induction motor drive     

High efficiency, unity power factor VVVF drive system of aninduction motor

A high-efficiency, unity-power-factor VVVF (variable voltage, variable frequency) drive scheme for an induction motor is presented. A unity-power-factor PWM (pulsewidth modulated) converter regulates DC voltage. An inverter circuit with the magnetic flux control PWM method generates VVVF PWM waveforms. The modulation factor of the inverter PWM control with controllable DC link DC voltage is studied. As a result, the distortion factor and the switching frequency are reduced by over-modulation with low DC link voltage. A high-efficiency and unity-power-factor VVVF induction motor drive has been achieved using the control strategy