INVESTIGATIONS ON MULTI-LEVEL INVERTER FED PERMANENT MAGNET SYNCHRONOUS MOTOR DRIVE

Abstract

Electric motors are the prime mover of the growing industries. Motion control with electric drives is always a challenge for researchers and technologists. According to the technological progress adjustable speed drives are preferred over constant speed drives because of several reasons like energy saving, velocity or position control for good transient response etc. DC motors have been used widely in variable speed drive systems for many decades. On the other hand, due to some of the disadvantages, like recurrent maintenance, increased cost due to commutators and brush, motivated researchers to develop AC motors. Although Induction Motors (IM) have been widely used and considered as a workhorse in the industry but slip power loss and lagging power factor limits the use of induction motor drives. Synchronous motor attracts researcher to overcome these limitations. However, the conventional wire wound synchronous motors suffers from necessity of extra power supply for field excitation. To meet the increasing application demand of adjustable speed drive researchers and technologist have been persistent in their efforts to develop new machines like Brushless DC (BLDC) motor, Switched Reluctance Motor (SRM), and Permanent Magnet Synchronous Motor (PMSM). With the advancement of permanent magnet material PMSM becomes the most promising and fastest growing machine for variable speed drive. Permanent magnet synchronous motors gained wide spread popularity and acceptance in adjustable speed drive due to its interesting characteristics like high efficiency, high air gap flux density, high torque-to-inertia ratio, high power density, high acceleration and deceleration rates, lower maintenance cost, simplicity and ruggedness. In a PMSM the excitation is provided by the permanent magnets on rotor which eliminates the external excitation requirements. The elimination of excitation winding reduces the cost and power loss. Compared to DC motor PMSM has some advantages, which includes compact size, less maintenance, longer life. On the other hand it also has some advantages over induction motor in terms of high efficiency, small size and wide range of power factor. The rapid development of power electronic devices and converter topology in the last few decades has made possible to develop energy efficient adjustable speed PMSM drive. In industrial drives, two-level inverters are mostly used to fed the PMSM due to their simple structure, control, reliability as well as cheaper parts. However, in case of two-level inverter, the switching frequency is one of the main concerns regarding to the size of passive ii components, superior control of electric drives and to proliferation power density of inverter. Higher switching frequencies operation reduces the size of the passive components. On contradictory, the higher switching frequencies may lead to have higher switching losses, which cause inferior inverter efficiency. Compare to two-level inverter, Multi-Level Inverter (MLI) produce lower switching losses at higher switching frequencies to get the same dc-link voltage. The other attracting feature is reduction in total harmonic distortion. Lower dv dt also reduces the voltage stress across the power switches. To increase the fundamental output voltage and corresponding power level of the inverter, the DC link voltage also need to be increased substantially. However, high frequency semiconductors have a limitation in the maximum voltage and very high voltage jump will lead to terrible electromagnetic interference and high windings insulation stress. So for the high performance of AC drive systems at increased power level, high quality inverter output with low harmonic loss and generating low torque fluctuation is necessary. The significant reduction in the torque ripple is achieved with use of three-level inverter. Multi-level topology provides some additional benefits compared to conventional two-level inverter which includes superior harmonic spectrums for a given gate switching frequency, a lower over voltage stress at cables and end windings of motors, a lower common-mode voltage, and substantially lower semiconductor switching losses. Satisfying the application demand, PMSM bring the keen attention of the researchers and technologist. The combination of MLI and PMSM allow to operate at increase dc bus voltage. Multi-level inverter is an emerging technology used in efficient permanent magnet synchronous motor drive system with low harmonic distortion, low torque fluctuations and avoid the limitations of two-level Voltage Source Inverter. To drive the machine, multi-level inverter becomes the most preferred choice replacing the conventional two-level VSI. The vector controlled PMSM drives brought renaissance in the variable speed drives, replacing the traditional scalar-controlled drives. In order to overcome the inherent coupling effect and the sluggish response of scalar control and to achieve the high performance, the vector control is employed. To implement the vector control the accurate position sensing is required. The rotor position information is usually obtained by encoders or resolvers attached to rotor shaft. However, the use of these position sensors reduces the noise immunity, and increase the size, cost and weight of the system. Elimination of these position sensors is highly encouraged to increase the reliability and robustness and also reduce the cost of drive. The present work is carried out to improve the performance of PMSM drive. The drive mainly consists of four major components, PMSM, inverter, position sensor and a controller. An iii extensive literature review is carried out which starts with the basic motor modelling and covers speed control in vector controlled drive, possible inverters and control techniques, and different sensorless control of the PMSM drive. Extensive efforts have been made by researchers to meet the challenges associated with PMSM drive. However, software based position sensing approach from the resolver without using Resolver to Digital Converter (RDC), attract attention to reduce the cost of the drive. Vector-control of resolver based RDC less PMSM drive fed by two-level VSI is taken as the investigative objective initially. Then problems associated with conventional two-level voltage source PWM (Pulse Width Modulation) inverter fed PMSM drive is addressed. Their possible solution and recent trends attract further attention to improve the performance of the drive system using MLI topology. Sensorless control motivates to reduce the hardware complexity, increase the mechanical robustness and reliability and also reduce the maintenance requirements. The design of an accurate control system requires the mathematical model of actual system being controlled. In this thesis, complete mathematical model of the PMSM drive is developed in MATLAB/Simulink environment. PMSM, inverters, position sensor and controller are the four main constituents of the drive model. A Mathematical model of PMSM with saliency and without saliency in synchronously rotating reference frame is developed. This model consists of dynamic equations of PMSM in terms of the actual parameters and load performance. The equation relating to switching states of the two-level inverter devices with the d-axis and q-axis voltage of the PMSM are derived. The initial step to implement the different control strategies is to simulate the system and to find out the response of the system. To obtain a closed loop operation of PMSM drive mathematical model of resolver sensor, speed controller and load are also developed. The basic idea of the vector control algorithm is to decompose a stator current into a magnetic field-producing component and a torque-producing component. After decomposition both components can be separately controlled like dc machine. The vector control scheme is developed for PMSM drive in MATLAB/ Simulink environment. Implementation of vector control needs accurate position and speed estimation. In usual practice Resolver to Digital Converter (RDC) are used for position and speed estimation from resolver. But a software based position estimation using the output signal of the resolver has been implemented to eliminate the RDC, which reduce the cost of the drive. In addition, a demodulation based algorithm is implemented to calculate the speed and rotor position. Depending on the placement of the permanent magnet PMSM can be categories as Interior Permanent Magnet Synchronous Motor (IPMSM) and Surface Permanent Magnet Motor (SPMSM). Extensive iv simulation study is carried out in MATLAB/ Simulink environment to investigate both type of PMSM. Vector control in terms zero d-axis current control and Maximum Torque Per Ampere (MTPA) control is investigated in IPMSM. A comparative analysis also has been presented for both the control techniques. Further to achieve the wide speed range of operation, a MTPA based flux-weakening control algorithm is developed for IPMSM drive. SPMSM drive with zero d-axis current control and its performance is investigated. Simulation study is carried out in MATLAB/ Simulink environment. The effects of load variation and speed change on the performance of drive are extensively studied. To achieve the high performance of PMSM drive systems at increased power levels the multi-level inverter fed drive is considered. The analytical model of the multilevel inverter is presented. Mathematical models are based on the determination of state equations for dynamic variables. These models are conspicuous by their extreme simplicity in front of other previous analytical models presented in the literature. Cost effective RDC less software based position estimation is implemented for proposed PMSM drive. A detailed study on Neutral Point Clamped (NPC) three-level inverter fed PMSM drive with vector control is presented. But the disadvantage with this inverter is capacitor voltage balancing. To maintain the voltage across the capacitor, a capacitor voltage balancing technique using Three-Level Boost Converter (TLBC) is developed for the proposed drive. The additional advantage of this balancing circuit as compared to the other carrier based PWM technique or SVM technique is the boosting feature, which is most demanding now a day for the electric vehicle application. This proposed MLI fed PMSM drive reduces the voltage THD and torque pulsation. Moreover, performance of the proposed balancing scheme is examined by carrying out extensive simulations studies on a surface-mounted permanent magnet synchronous machine with vector control in terms of zero d-axis current control. The exhaustive simulation study is carried out in MATLAB/ Simulink environment for the proposed drive system. The comparative study between a conventional two-level PMSM drive and proposed drive is also presented. The enhanced performance is achieved with the proposed drive. Furthermore the proposed drive is also investigated for sensorless control operation. Sensorless controls in terms back-EMF based position estimation and Model Reference Adaptive System (MRAS) based estimation have been implemented. Back-EMF based estimation of rotor speed and angular position ensure accurate speed regulation. This method is satisfactory at medium and high-speed. Among the existing sensorless approaches adaptive control seems to be the most promising. The MRAS based estimators provide the desired state from two different models, one is reference and the other one is adjustable. The MRAS is v simple and easier to implement. Exhaustive simulation is carried out using MATLAB/ Simulink and the performance results are obtained under different operating condition to verify the effectiveness and feasibility of the MRAS based estimator in proposed PMSM drive system. In order to validate the simulation, the following prototypes have been developed in the laboratory and experimentation is performed: (i)two-level VSI fed PMSM drive system, (ii) three-level NPC inverter fed PMSM drive system and (iii) capacitor voltage balancing circuit with three-level boost converter. The different hardware component as required for the proper operation of experimental set up such as driver circuit, isolation and dead band circuit, dc power supply, voltage and current sensor circuits designed, developed and interfaced with dSPACE controller. A DSP DS1104 of dSPACE has been used for the real-time implementation of control algorithms of two different inverters topology in MATLAB/ Simulink environment. Extensive experimentation has been carried out to investigate the performance of the drive. Software based RDC less position estimation is implemented using the resolver based position estimation algorithm to estimate the accurate speed and position. The experimental results show the complete 360o rotation of the drive. The accurate position and speed estimation is achieved. In the second part of the experimental work, a two-level inverter fed PMSM drive system is tested in different operating conditions. The vector control in terms of zero d-axis current control is implemented. Further experimentation is carried out in three-level NPC inverter fed PMSM drive. To maintain the voltage across the two dc-link capacitors in three-level NPC inverter a TLBC circuit is used. The exhaustive experimentation is carried out on the proposed drive in different operating condition. The experimental results show the validity and effectiveness of the proposed drive in accordance with the simulation results. A comparative analysis is made between two-level inverter and three-level NPC inverter fed PMSM drive. Further, the experimentation is also carried out in sensorless control mode with three-level NPC inverter fed drive. The MRAS based estimation is implemented and the results show the validity of the drive under different operating conditions.