Multilevel inverters are very popular for medium and high-voltage induction motor (IM) drive applications. They have superior performance compared to 2-level inverters such as reduced harmonic content in output voltage and current, lower common mode voltage and dv/dt, and lesser voltage stress on power switches. To get nearly sinusoidal current waveforms, the switching frequency of the conventional inverters have to be in¬creased. This will lead to higher switching losses and electromagnetic interference. The problem in using lower switching frequency is the introduction of low order harmonics in phase currents and undesirable torque ripple in the motor. The 5th and 7th harmonics are dominant for hexagonal voltage space-vector based low frequency switching. Dodecagonal voltage space-vector based multilevel inverters have been proposed as an improvement over the conventional hexagonal space vector based inverters. They achieve complete elimination of 5th and 7th order harmonics throughout the modulation range. The linear modulation range is also extended by about 6.6%, since the dodecagon is closer to circle than a hexagon. The previous works on dodecagonal voltage space vector based VSI fed drives used voltage controlled PWM (VC-PWM). Although these controllers are more popular, they have inferior dynamic performance when compared to current controlled PWM (CC¬PWM). VSIs using current controlled PWM have excellent dynamic response, inherent short-circuit protection and are simple to implement. The conventional CC-PWM tech¬niques have large switching frequency variation and large current ripple in steady-state. xix As a result, there has been significant research interest to achieve current controlled VSI fed IM drives with constant switching frequency. Two current error space vector (CESV) based hysteresis controllers for dodecagonal voltage space-vector based VSI fed induction motor drives are proposed in this work. The proposed controllers achieve nearly constant switching frequency at steady state operation, similar to VC-SVPWM based VSI fed IM drives. They also have fast dynamic response while at the same time achieving complete elimination of fifth and seventh order harmonics for the entire modulation range, due to dodecagonal voltage vector switching. The first work proposes a nearly constant switching frequency current error space vector (CESV) based hysteresis controller for an IM drive with single dodecagonal voltage space vectors. Parabolic boundaries computed offline are used in the proposed controller. An open-end winding induction motor is fed from two inverters with asymmetrical DC link voltages, to generate the dodecagonal voltage space vectors. The drive scheme is first studied at different frequencies with a space vector based PWM (SVPWM) control, to obtain the current error space vector boundaries. The CESV boundary at each frequency can be approximated with four parabolas. These parabolic boundaries are used in the proposed controller to limit the CESV trajectory. Due to symmetries in the parabolas only two set of parabola parameters, at different frequencies, need to be stored. A generalized next vector selection logic, valid for all sectors and rotation direction, is used in the proposed controller. For this an axis transformation is done in all sectors, to bring the CESV trajectory to the first sector. The sector information is obtained from the estimated fundamental stator phase voltage. The proposed controller is extensively studied using vector control at different frequencies and transient conditions. This controller maintains nearly constant switching frequency at steady state operation, similar to VC-SVPWM inverters, while at the same time achieving better dynamic performance and complete elimination of 5th and 7th order harmonics throughout the modulation range. In the second work the nearly constant switching frequency current hysteresis con¬troller is extended to multilevel dodecagonal voltage space-vector based IM drives, with online computation of CESV boundaries. The multilevel dodecagonal space-vector dia¬gram has different types of triangles, and the previously proposed methods for multilevel hexagonal VSI based current hysteresis controllers cannot be used directly. The CESV trajectory of the VC-SVPWM, obtained for present triangular region, is used as the reference trajectory of the proposed controller. The CESV reference boundaries are com¬puted online, using switching dwell time and voltage error vector of each applied vector. These quantities are calculated from estimated sampled reference phase voltages, which are found out from the stator current error ripple and the parameters of the induction motor. Whenever the actual current error space vector crosses the reference CESV tra¬jectory, an appropriate vector that will force it along the reference trajectory is switched. Extensive study of the proposed controller using vector control is done at different fre¬quencies and transient conditions. This controller has all the advantages of multilevel switching like low dv/dt, lesser electromagnetic interference, lower switch voltage stress and lesser harmonic distortion, in addition to all the dynamic performance advantages of the previous controller. The third work proposes an elegant 5th and 7th order harmonic suppression tech¬nique for open end winding split-phase induction motors, using capacitor fed inverters. Split-phase induction motors have been proposed to reduce the torque and flux ripples of conventional three-phase IM. But these motors have high 5th and 7th order harmonics in the stator windings due to lack of back-emf for these frequencies. A space-vector harmonic analysis of the split-phase IM is conducted and possible 5th and 7th order harmonic sup¬pression techniques studied. A simple harmonic suppression scheme is proposed, which requires the use of only capacitor fed inverters. A PWM scheme that can maintain the capacitor voltage as well as suppress the 5th and 7th order harmonics is also proposed. To test the performance of the proposed scheme, an open-loop v/f control is used on an open-end winding split-phase induction motor under no-load condition. Synchronized PWM with two samples per sector was used, for frequencies above 10 Hz. The har¬monic spectra of the phase voltages and currents were computed and compared with the traditional SVPWM scheme, to highlight the harmonic suppression. The concepts were initially simulated in Matlab/Simulink. Experimental verifica¬tion was done using laboratory prototypes at low power. While these concepts maybe easily extended to higher power levels by using suitably rated devices, the control tech¬niques presented shall still remain applicable. TMS320F2812 DSP platform was used to execute the control code for the proposed drive schemes. For the first work the output pins of the DSP was directly used to drive the inverter switches through a dead-band circuit. For the other two works, DSP outputs the sector information and the PWM signals. The PWM terminals and I/O lines of the DSP is used to output the timings and the triangle number respectively. An FPGA (XC3S200) was used to translate the sector information and the PWM signals to IGBT gate signal logic. A constant dead-time of 1.5 µs was also implemented inside the FPGA. Opto-isolated gate drivers with desaturation protection (M57962L) were used to drive the IGBTs. The phase currents and DC bus voltages were measured using hall-effect sensors. An incremental shaft position encoder was also connected to the motor to measure the angular velocity. The switches were realized using 1200 V, 75 A IGBT half bridge modules.

Current-Controlled Pulse Width Modulated (CC-PWM) Voltage Source Inverters (VSIs) are extensively employed in high performance drives (HPD) because of the considerable advantages offered by them, such as, excellent dynamic response and inherent over-current protection, as compared to the voltage-controlled PWM (VC-PWM) VSIs. Amongst the different types of CC-PWM techniques, hysteresis current controllers offer significant simplicity in implementation. However, conventional type of hysteresis controllers (with independent comparators) suffers from some well-known drawbacks, such as, limit cycle oscillations (especially at lower speeds of operation of machine), overshoot in current error, generation of sub-harmonic components in the current, and random (non-optimum) switching of inverter voltage vectors. Common problems associated with the conventional, as well as current error space phasor based hysteresis controllers with fixed bands (boundary), are the wide variation of switching frequency in the fundamental output cycle and variation of switching frequency with the change in speed of the load motor. These problems cause increased switching losses in the inverter, non-optimum current ripple, excess harmonics in the load current and subsequent additional machine heating. A continuously varying parabolic boundary for the current error space phasor is proposed previously to get the switching frequency variation pattern of the output voltage of the hysteresis controller based PWM inverter similar to that of voltage controlled space vector PWM (VC SVPWM) based VSI. But the major problem associated with this technique is the requirement of two outer parabolas outside the current error space phasor boundary for the identification of sector change which gives rise to some switching frequency variations in one fundamental cycle and over the entire operating speed range. It also introduces 5th and 7th harmonic components in the voltage causing 5th and 7th harmonic currents in the induction motor. These harmonic currents causes 6th harmonic torque pulsations in the machine. This thesis proposes a new technique which replaces the outer parabolas and uses current errors along orthogonal axes for detecting the sector change, so that a fast and accurate detection of sector change is possible. This makes the voltage harmonic spectrum of the proposed hysteresis controller based inverter exactly matching with that of a constant switching frequency SVPWM based inverter. This technique uses the property that the current error along one of the orthogonal axis changes its direction during sector change. So the current error never goes outside the parabolic boundary as in the case of outer parabolas based sector change technique. So the proposed new technique for sector change eliminates the 5th and 7th harmonic components from the applied voltage and thus eliminates the 5th and 7th harmonic currents in the motor. So there will be no introduction of 6th harmonic torque pulsations in the motor. Using the proposed scheme for sector change and parabolic boundary for current error space phasor, simulation study was carried out using Matlab-Simulink. Simulation study showed that the switching frequency variations in a fundamental cycle and over the entire speed range of the machine upto six step mode operation is similar to that of a VC-SVPWM based VSI. The proposed hysteresis controller is experimentally verified on a 3.7 kW IM drive fed with a two-level VSI using vector control. The proposed current error space phasor based hysteresis controller providing constant switching frequency is completely implemented on the TI TMS320LF2812 DSP controller platform. The three-phase reference currents are generated depending on the frequency command and the controller is tested with drive for the entire operating speed range of the machine in forward and reverse directions. Steady state and transient results of the proposed drive are presented in this thesis. This thesis also proposes a new hysteresis controller which eliminates parabolic boundary and replaces it with a simple online computation of the boundary. In this proposed new hysteresis controller the boundary computed in the present sampling interval is used for identifying next vector to be switched. This thesis gives a detailed mathematical explanation of how the boundary is computed and how it is used for selecting vector to be switched in a sector. It also explains how the sector in which stator voltage vector is present is determined. The most important part of this proposed hysteresis controller is the estimation of stator voltages along alpha and beta axes during active and zero vector periods. Estimation of stator voltages are carried out using current errors along alpha and beta axes and steady state equivalent circuit of induction motor. Using this estimated stator voltages along alpha and beta axes, instantaneous phase voltages are computed and used for finding individual voltage vector switching times. These switching times are used for the computation of hysteresis boundary for individual vectors. So the hysteresis boundary for individual vectors are exactly calculated and used for vector change detection, making phase voltage harmonic spectrum exactly similar to that of constant switching frequency VC SVPWM inverter. Sector change detection is very simple, since we have the estimated stator voltages along alpha and beta axes to give exact position of stator voltage vector. Simulation study to verify the steady state as well as transient performance of the proposed controller based VSI fed IM drive is carried out using Simulink tool box of Matlab Simulation Software. The proposed hysteresis controller is experimentally verified on a 3.7 kW IM drive fed with a two-level VSI using vector control. The proposed current error space phasor based hysteresis controller providing constant switching frequency profile for phase voltage is implemented on the TI TMS320LF2812 DSP controller platform. The three-phase reference currents are generated depending on the frequency command and the proposed hysteresis controller is tested with drive for the entire operating speed range of the machine in forward and reverse directions. Steady state and transient results of the proposed drive are presented for different operating conditions.

Variable-speed Induction motor drives are nowadays used for various kinds of industrial processes, transportation systems, wind turbines and household appliances in the world. The majority of drives are for general purpose speed control applications where accurate speed control is not required for entire speed range. But for high dynamic drive application, very precise and fast control of induction motor drive is essential. For such applications, sophisticated and well-performing control design is a key issue. Precise and accurate torque control of the Induction Motor (IM) can only be accomplished by vector control and direct torque control. In terms of space vector theory, vector control implies that the instantaneous torque is controlled by way of the stator current vector that is orthogonal to the rotor flux vector. Precise knowledge of the rotor flux angle is therefore essential for a vector controlled IM. IMs do not allow the flux position to be easily measured, so most modern vector controlled IM drives rely on flux estimation. This means that the flux angle is derived from a flux estimator, using the dynamic model of the IM. Given that the rotor speed of the IM is measured by a mechanical shaft sensor. Flux estimation is a fairly easy task. However, vector control of IM without mechanical shaft speed sensor is of current interest in industrial environment. The driving motivations behind the development in sensorless control are lower cost, improved reliability and operating environment. In this thesis, a sensorless vector control scheme for rotor flux estimation using current error space phasor based hysteresis controller is proposed including the method for estimation of leakage inductance, Ls. For frequencies of operation less than 25 Hz, the rotor voltage and hence the rotor flux position is computed during the inverter zero voltage space vector using steady state model of IM. For above 25 Hz, active vector period and steady state model of IM is used. The whole rotor flux estimation scheme is dependent on current error space phasor and the steady state motor model, with rotor flux as a reference vector. Since no terminal voltage sensing is involved, dead time effects will not create problem in rotor flux sensing at low frequencies of operation. But appropriate device on-state drop are compensated at low frequencies (below 5 Hz) of operation to achieve a steady state operation up to less than 1 Hz. A constant switching frequency hysteresis current controller is used in inner current control loop for the PWM regulation, with smooth transition of operation to six-step mode operation. A simple Ls estimation based on current error space phasor is also proposed to nullify the deteriorating effect on rotor flux estimation. The parameter sensitivity of the control scheme to changes in the stator resistance Rs is also investigated. The drive scheme is tested up to a low frequency operation less than 1 Hz. The extensive simulation and experiment results are presented to show the proposed scheme’s good dynamic performance extending up to six-step operation. In contrast to vector control, direct torque control (DTC) method requires the knowledge of stator resistance only and thereby decreasing the associated sensitivity to parameters variation and the elimination of speed information. DTC as compared to vector control does not require co-ordinate transformation and PI controller. DTC is easy to implement because it needs only two hysteresis comparators and a lookup table for both flux and torque control. This thesis also investigates the possibilities in improvement of direct torque control scheme for high performance induction motor drive applications. Here, two schemes are proposed based on the direct torque control scheme for IM drive using 12-sided polygonal voltage space vectors for fast torque control. The torque control scheme based on DTC algorithm is proposed using 12-sided polygonal voltage space vector. The basic DTC scheme is used to control the torque. But the IM drive is open-end type. For torque control, the voltage space vectors orthogonal to stator flux vector in 12-sided polygonal space vector structure are used as hexagonal space vector based DTC scheme. The advantages achieved due to 12-sided polygonal space vector are mainly fast torque control and small torque ripple. The fast transient of torque with precise control is achieved using voltage space vector placed with a resolution of ±15. The torque ripple will be less as 6n±1 (n=odd) harmonic torque is totally eliminated from the whole range of PWM modulation. The comparative analysis of proposed 12-sided polygonal voltage space vector based DTC and conventional hexagonal space vector based DTC is also presented. Extensive simulation and experiment results are also presented to show the fast torque control at speeds of operation ranging from 5 Hz to the rated speed. The concept of 12-sided polygonal space vector based DTC is further extended for a variable speed control scheme using estimated fundamental stator voltage for sector identification. The conventional DTC scheme uses stator flux vector for identification of the sector and the switching vector are selected based on this sector information to control stator flux and torque. However, the proposed DTC scheme selects switching vectors based on the sector information of the estimated fundamental stator voltage vector and its relative position with respect to the stator flux vector. The fundamental stator voltage estimation is based on the steady state model of IM and information of synchronous frequency which is derived from computed stator flux using a low pass filter technique. The proposed DTC scheme utilizes the exact position of fundamental stator voltage vector and stator flux vector position to select optimal switching vector for fast control of torque with small variation of stator flux within hysteresis band. The present DTC scheme allows the full load torque control with fast transient response to very low speeds of operation below 5 Hz. The extensive simulation and experiment results are presented to show the fast torque control for speed of operation from zero speed to rated speed. However, the present scheme will have all the advantages of DTC scheme using stator flux vector for sector identification. All the above propositions are first simulated by MATLAB/Simulink and subsequently verified by an experimental laboratory prototype. The proposed control schemes are experimentally verified on a 3.7 kW IM drive. The control algorithms of the sensorless vector control using current error space phasor as well as DTC using 12-sided polygonal voltage space vector are completely implemented on a TI TMS320LF2812 DSP controller platform. These are some of the constituents for chapters 2, 3 and 4 in this thesis. Additionally, the first chapter also covers a brief survey on some of the recent progresses made in the field of sensorless vector control, direct torque control and current hysteresis controller. The thesis concludes with suggestion for further exploration.

No description available.

In DC to AC power conversion, voltage source inverters (VSI) based current controllers are usually preferred for today’s high performance AC drive which requires excellent dynamic and steady state performances at different transient and load conditions, with the additional advantages like inherent short circuit and over current protection. Out of different types of current controllers, hysteresis controllers are widely used due to their simplicity and ability to meet the requirements for a high performance AC drives. But the conventional hysteresis controllers suffers from wide variation of PWM switching frequency, overshoot in current errors, sub-harmonic components in the current waveform and non-optimum switching at different operating point of the drive system. To mitigate these problems, particularly to control the switching frequency variation, which is the root cause of all other problems, several methodologies like ramp comparison based controller, predictive current controller, etc. were proposed in the literature. But amplitude and phase offset error in the ramp comparison based controllers and complexities involved in the predictive controllers have limited the use of these controllers. Moreover, these type of controllers, which uses three separate and independently controlled tolerance band (sinusoidal type or adaptive) to control the 3-phase currents, shows limited dynamic responses and they are not simple to implement. To tackle the problem of controlling 3-phase currents simultaneously, space vector based hysteresis current controller is very effective as it combines the current errors of all the three phases as a single entity called current error space vector. It has a single controller’s logic with a hysteresis boundary for controlling this current error space vector. Several papers on space vector based hysteresis controllers for 2-level inverter with constant switching frequency have been published, but the application of the constant switching frequency based hysteresis current controllers for multi¬level inverter fed drive system, has not been addressed properly. Use of multi-level inverter in modern high performance drive for medium and high voltage levels is more prominent because of multi-level’s inherent advantages like good power quality, good electromagnetic compatibility (EMC), better DC link voltage utilization, reduced device voltage rating, so on. Even though some of the earlier works describe three-level space vector based hysteresis current controller techniques, they are specific to the particular level of inverters and does not demonstrate constant switching frequency of operation. This thesis proposes a novel approach where nearly constant switching frequency based hysteresis controller can be implemented for any general n-level inverter and it is also independent of inverter topology. In this work, varying parabolic boundary is used as the hysteresis current error boundary for controlling the current in a multi-level space vector structure. The computation of the parabolic boundary is accomplished offline and all the necessary boundary parameters at different operating points are stored in the look-up tables. The varying parabolic boundary for the multi-level space vector structure depends on the sampled reference phase voltage values which are estimated from stator current error information and then using the equivalent circuit model of induction motors. Here, a mapping technique is adopted to bring down all the three phase references to the inner- most carrier region, which results in mapping any outer triangular structure where tip of the voltage space vector is located, to one of the sectors of the inner most hexagon of the multi-level space vector structure. In this way, the required mapped sector information is easily found out to fix the correct orientation of the parabolic boundary in the space vector plane. This mapping technique simplifies the controller’s logic similar to that of a 2-level inverter. For online identification of the inverter switching voltage vectors constructing the present outer triangle of the multi-level space vector structure, the proposed controller utilizes the sampled phase voltage references. This identification technique is novel and also generic for any n-level inverter structure. This controller is having all the advantages of a space vector based hysteresis current controller and that of a multi-level inverter apart from having a nearly constant switching frequency spectrum similar to that of a voltage controlled space vector PWM (VC-SVPWM). Using the proposed controller, simulation study of a five-level inverter fed induction motor (IM) drive scheme, was carried out using Matlab-Simulink. Simulation study showed that the switching frequency variations in a fundamental cycle and over the entire speed range of the linear modulation region, is similar to that of a VC-SVPWM based multi-level VSI. The proposed hysteresis controller is experimentally verified on a 7.5 kW IM vector control drive fed with a five-level VSI. The proposed current error space vector based hysteresis controller providing nearly constant switching frequency is implemented on a TI TMS320LF2812 DSP and Xilinx XC3S200FT256 FPGA based platform. The three-phase reference currents are generated depending on the frequency command and the controller is tested with the drive for the entire operating speed range of the machine in forward and reverse directions. Steady state and quick transient results of the proposed drive are presented in this thesis. This thesis also proposes another type of hysteresis controller, firstly for 2-level inverter and then for general n-level multi-level inverter, which eliminates the parabolic boundary and replaces it with a boundary which is computed online and does not use any look up table for boundary selection. The current error boundary for the proposed hysteresis controller is computed online in a very simple way, using the information of estimated fundamental stator voltages along α and β axes of space vector plane. The method adopted for the proposed controller to compute the boundary does not involve any complicated computations and it selects the optimal vector for switching when current error space vector crosses the boundary. This way adjacent voltage vector switching similar to VC-SVPWM can be ensured. For 2-level inverter, it precisely determines the sector, in which reference voltage vector is present. In multi-level inverter, this controller also finds out the mapped sector information using the same mapping techniques as explained in the first part of this thesis. In both 2-level and multi-level inverter, the proposed controller does not use any look up table for finding individual voltage vector switching times from the estimated voltage references. These switching times are used for the computation of hysteresis boundary for individual vectors. Thus the hysteresis boundary for individual vectors is exactly calculated and the boundary is similar to that of VC-SVPWM scheme for the respective levels of inverter. In the present scheme, the phase voltage harmonic spectrum is very close to that of a constant switching frequency VC-SVPWM inverter. In this thesis, at first, the proposed on line boundary computation scheme is implemented for a 2-level inverter based controller for the initial study, so that it can be executed as fast as 10 µs in a DSP platform, which is required for accurate current control. Then the same algorithm of 2-level inverter is extended for multi-level inverter with the additional logic for online identification of nearest switching voltage vectors (also used in the parabolic boundary case) for the present sampling interval. Previously mentioned mapping technique for multi-level inverter, is also implemented here to bring down the phase voltage references to the inner-most carrier region to realize the multi-level current control strategy equivalent to that of a 2-level inverter PWM current control. Simulation study to verify the steady state as well as transient performance of the proposed controller for both 2-level as well as five-level VSI fed IM drive is carried out using Simulink tool box of MATLAB Simulation Software. The proposed hysteresis controllers are experimentally verified on a 7.5 kW IM vector control drive fed with a two-level VSI and five-level VSI separately. The proposed current error space vector based hysteresis controller providing nearly constant switching frequency profile for phase voltage is implemented on the TI TMS320LF2812 DSP and Xilinx XC3S200FT256 FPGA based platform. The three-phase reference currents are generated depending on the frequency command and the proposed hysteresis controllers are tested with drive for the entire operating speed range of the machine in forward and reverse directions. Steady state and transient results of the proposed drive are also presented for different operating conditions, through the simulation study followed by experimental verifications. Even though the simulation and experimental verifications are done on a 5-level inverter to explain the proposed hysteresis controller, it can be easily implemented for any general n-level inverter, as described in this thesis.