DESIGN AND PERFORMANCE INVESTIGATIONS ON STATIC CONVERTER FED SYNCHRONOUS MOTOR DRIVE

Abstract

Due to ease in control, smooth and stable operation quality, dc motor has been a popular choice for industrial drives. Variable speed drives, in most cases, use motors powered with dc current fed by static converter using rectifier bridge and choppers. These drives have a number of mechanical and electrical limitations caused by the necessity of fitting a mechanical commutator. The use of dc machine is absolutely impossible in explosive, corrosive and dust filled atmosphere. AC motors such as induction and synchronous type have robust rotor construction, which permits reliable and maintenance free operation at high speed. Unfortunately, the a.c. type motors are inflexible in speed, when operated on standard constant frequency a.c. supply. A.C. motor drives using static converter fed synchronous machine provide some features that make them preferable to the induction motor drives, i.e., where the precise position control and precise simultaneous speed control of number of motors are required. A prerequisite for loss free speed control of ac motor is a power source whose frequency and voltage can be varied in accordance with the particular operating state of motor. A dc link converter can fulfill the requirement in which two converters are coupled by a link containing a capacitor or reactor depending on the type of converter operation. Recently, due to the advantages of simplicity, greater controllability, regenerative capability and ease of the protection, the current link converter is gaining popularity for speed control of a.c. motors. The same operating quality and simple controllability is obtained by a static converter fed synchronous motor drive as in dc motor drives. For high power rating and high speed operation, such drive are now used in industry. For smooth and stable operation, similar to dc drives, it also operates in closed loop manner. The static converter fed synchronous motor has a major advantage towards commutation of inverter thyristors by the machine terminal voltage, when synchronous motor operates under over excited condition. This drive is considered as a substitute of dc motor drive in industries where the dc machine can not be used. Visualizing the utility of this drive as a substitute of dc drive, it was felt to design and develop a fast response static converter fed synchronous motor drive. The drive system consists of two fully controlled converters coupled with link inductor and an over excited synchronous motor. A simple and effective mathematical model is developed for the steady state analysis of the drive in d-q variables (Park's variables). Similar to dc motor drives simple expression for torque and speed is obtained showing the same operating characteristics as dc drives. Since both field and armature voltage control can be employed, similar to dc drives, to control the speed of operation, the effect of voltage and field variation is also studied on the operating characteristics. The dynamic characteristics of a drive improves when it operates in closed loop condition and is affected by the controllers used in feed back loops. Two feed back loops, namely, current and speed feed back loops, are incorporated. The outer feed back loop maintains the speed of operation and internal feed back loop maintains the torque and protect thyristors from over current. Both the controllers used in system are of PI type. The use of PI controllers in two feed back loops provides the fast rosponso of drive to overcome the disturbances. Since the response of drive is widely affected by the controller parameters and in general no straight forward method is available to design the controller parameters, a method is developed to design the controllersused. To study the dynamic performance and designAthe controller, a dynamic model of^system is developed. Using the system model equations the controllers are designed to ensure that system comes in steady state quickly after the disturbance is imposed. Under dynamic condition the equations are non-linear and are linearised for small perturbations about the initial steady state operating point. The characteristic equation of system is developed and used to design the controllers used in system. The D-decomposition technique is employed to determine the probable stable zone in parametric plane. The point check for the stability is carried out by frequency scanning technique. This technique is faster than other conventional techniques and requires less computation efforts and fewer logical operations. Both continuous and sample data analysis approach is attempted to design the controller and study the dynamic behaviour of drive system. Since the drive behaves like a dc motor drive, for sample data analysis, the drive is considered as an equivalent dc motor for simplicity and the controllers are designed to ensure sufficient damping and degree of stability. The final selection of controller parameters is decided by the transient response of the drive in terms of settling time and overshoot. Using state space model of drive system, the transient analysis is carried over and responses are computed under different conditions. Visualizing the response of the drive in terms of settling time and overshoot, the final ntroller parameters are selected to make the system faster CO The development of microcomputer reduces the complexity of the hardware drastically. A microcomputer based static converter fed synchronous motor drive is designed and fabricated. The link converter used. consists of two fully controlled converters connected through a dc link inductor. The variable dc voltage is applied to the machine side converter (CM) by varying the triggering angle of supply side converter (CS). Both the converters are naturally commutated. The voltage input to the motor and its frequency are controlled by the leading angle of commutation of CM operating as an inverter. As the commutation of CM is achieved by the machine voltage and frequency of inverter depends on the speed of motor, the synchronous motor retains its synchronous quality. In the speed and current feed back loops, PI type controllers are employed. During closed loop operation, the speed error is processed through speed controller which decides the current reference. Having compared the current reference with actual current, the current error is processed through current Controller to decide the triggering angle of supply side converter CS. The leading angle of commutation is modified as per the required input current for successful inverter commutation. The inverter triggering frequency is based on terminal voltage sensing which ultimately decides the operating frequency of synchronous motor. The speed and current feed backs are inputted to the microcomputer through analog to digital converter. The operation of drive is realised through software and implemented by an 8085 based microcomputer. The hardware of the developed system consists of two fully controlled converters CS and CM, circuits for the synchronization and zero crossing detection for both the converters as well as speed and current sensing circuits and amplifier circuit for triggering the thyristors from the control signal obtained via different port bits of programmable peripheral interface. The developed software of the system is divided into five parts, namely, four ISS routines and one main program. The detail of software in flowchart form is included. The flowchart starts with initialization of I/O ports according to the requirement. A speed limit is set to operate the drive in LCI mode. The actual operating frequency is always compared with the reference frequency command. In the closed loop operation, two PI controllers, namely, speed and current controllers, are used. The speed and current feed backs are continuously sensed and inputted to the microcomputer through ADC for processing and finally the triggering of thyristors are decided. As the leading angle of commutation is affected by the loading of machine, its value is also modified accordingly. Four interrupts are used for the operation of drive. The first two interrupts are used to fire CS and CM group thyristors during starting and for CS only in LCI mode of operation. The third interrupt is used to give control signal to the CM group thyristors during LCI mode of operation. A detailed flowchart is given to explain the operation of drive. As such three modes of operation are classified as starting mode, crawling mode and the third LCI mode of operation of drive. When the machine attains a speed more than a set speed limit, it enters in LCI mode and closed loop operation is performed. During the operation of drive, for the commutation of CM group thyristors, the field of synchronous motor is kept ON in all modes of operation. Extensive experimentation is carried out to study the performance of drive under steady state and transient conditions. The effect of variation in excitation and leading angle of commutation, is also considered. The effect of controller parameters is visualized by imposing the disturbances in system parameters. To summarise a prototype model of static converter fed synchronous motor drive is designed and developed. The D-decomposition technique is employed to coordinate controller parameters (PI type). The steady state, transient and dynamic analyses are done. Extensive experimentation is carried out to get performance of the drive system which is compared with the performance obtained analytically. The results obtained are in good agreement with the analytical results. Detailed conclusion on the developed system and analysis are given. The D-decomposition technique, frequency scanning technique and detail about the specific hardware components are given in appendices.