MICROPROCESSOR CONTROLLED CURRENT SOURCE INVERTER FED INDUCTION MOTOR DRIVE

Abstract

Variable speed drive is the prerequisite of any modern industry. For years, statically controlled dc motors have been the only choice for variable speed operation. These drives are powered from either ac or dc supply and variable dc voltage is applied to armature through converter or chopper. The necessity of mechanical commutator restricts its application in explosive free and dust free atmosphere. Due to these limitations of dc drives, variable speed squirrel cage induction motors, being rugged in nature, have received special attraction in the industry. Induction motors are also more popular and acceptable for variable speed operation as compared to synchronous motors which suffer with the problem of starting and synchronization of rotor with stator field and the use of slip rings and carbon brushes. Although induction motors have shunt type torque speed characteristic when operated from, constant frequency supply, the precise speed control is possible only by varying the frequency of the ac source. To operate the machine at rated flux the terminal voltage is also controlled in accordance with the frequency. A dc link converter, having two converters coupled through a capacitor in voltage source inverter or through an inductor in current source inverter, is the most acceptable mean for variable frequency operation of induction motor. However, the regeneration capability which is the prerequisite of any variable speed drive is easily met in current source inverter with out any extra cost. Another added advantage of current source inverter is its controlled current operation leading to inherent overcurrent protection. The use of typical auto sequentially commutated current source inverter (ASCI) also simplifies the commutation requirement. Due to its merits the drive is expected to make a major impact in industry in near future. Therefore, a need is felt to design and develop a statically controlled current source inverter fed induction motor drive for smooth and stable operation. The current source inverter fed induction motor drive consists of two converters coupled through a dc link inductor. The front end converter acts as a current source and the ASCI converts regulated dc link current into variable frequency ac current. For better dynamic characteristic of the drive PI controllers are used in the feedback loops. The speed PI controller compares the reference speed and actual speed and sets the dc link current reference. The speed PI controller reduces the steady state speed error to zero. The current PI controller operates on dc link current error and adjusts the firing angle of the converter for regulating the dc link current. To maintain the motor flux at rated value a slip regulator is also used in the feedback loop. The synchronous speed is obtained by adding actual speed, and slip speed with proper sign as per the speed error signal: For positive speed error, the machine operates in motoring region while for negative speed error, it operates in generating region. To analyse the performance of the drive under steady state as well as under, dynamic * condition a mathematical model of the drive is developed in terms of d-q variables in synchronously rotating reference frame. The closed form expressions are developed for torque, terminal voltage, dc link voltage, power factor, efficiency etc and performance of the drive is investigated under constant current variable frequency operation, constant load torque operation, fixed frequency constant flux operation and linearly varying load torque, operation. Although current source inverter fed induction motor drive has numerous advantages over other drives, it has a serious problem of low speed torque pulsations which are mainly due to interaction of 5th and 7th harmonic currents with, the fundamental flux. To investigate the sixth harmonic performance of the drive another mathematical model is developed and a method is proposed to compute torque pulsations, speed pulsations, voltage pulsations etc. under different operating conditions, namely - constant current variable frequency operation and linearly varying load torque under constant flux operation. As in voltage source inverter, natural current filtering does not take place in current source inverter which gives rise to harmonics in the output currents and, hence, torque and speed pulsations. These speed pulsations are more pronounced at low speeds. An effective method of reducing torque pulsations is to modulate the output currents at low speeds. The modulation increases with the decrease in speed. A method is proposed to optimize the number of pulses in a cycle as well as pulse widths over drive operation control range. The pulse widths are optimized using Box complex method for minimum harmonic losses, and number of pulses in a cycle are selected for acceptable speed pulsations. The pulse width modulation can be easily implemented through auto sequentially commutated current source inverter. The commutation process of the inverter is discussed in detail to generalize the governing equations during commutation. This generalization is done for digital simulation of the pulse width modulated inverter. A computer program is developed which can be used to study the voltage, current, flux and torque waveforms of the PWM current source inverter fed induction motor drive and for selection of proper rating commutating capacitors, thyristors and diodes. The control of the drive is implemented through personal computer based on 8088 processor. The use of personal computer reduces the complexity of the hardware and makes the implementation of control more flexible. The complete drive is designed and fabricated. The reference speed is fed through keyboard. The rotor speed and dc link current are measured through 12-bit analog to digital converter for processing. The speed error PI Processing is carried out during 20 msec while the current error PI processing is done every 3.33 msec. The speed and current PI controllers are implemented through software. The slip regulator characteristic, stored in the the memory, is used to read the slip speed corresponding to actual dc link current. The firing pulses for converter and PWM inverter are generated through software. A single zero crossing signal is used for synchronization of firing pulses for converter. The complete firing angle range 0° to 180° is divided in three, zones of 60° interval. The firing pulses are issued without sensing the line to line voltage logic waves even if the firing angle zone changes. A method is proposed to Implement the change in firing angle In any 60 interval. The frequency of the inverter is controlled by controlling the rate at which thyristors of ASCI are triggered. As the frequency is reduced below a particular level, pulse width modulation is introduced. A novel method is proposed to generate the firing pulses for the PWM inverter with smooth variation in frequency. In the proposed method only few memory locations are required to store the firing commands since firing commands for only 60° interval of PWM inverter need be stored. The firing pulses for other intervals can be obtained by rotating the firing command. The method is very effective even if the pulse per cycle increases or decreases. The complete control is implemented through four interrupts, namely - zero crossing interrupt, 60° interrupt, converter firing interrupt and inverter firing interrupt. The software for the main program and the subroutines are written in C language. The software is presented in the form of flowcharts and discussed in detail. The use of PI controllers, speed controller and current controller, in the feedback loops, improves the dynamic performance of the drive. However, the proper design of controller parameters makes the response fast and enables the drive to overcome external disturbances. Since no general method is available for design of controllers, a method is proposed to design the controller parameters. The inner current controller is designed first and outer speed controller later on as the electrical time constant is much smaller than mechanical time constant. The system governing equations are basically non linear in nature and, therefore, these equations are linearized using small perturbation method around steady state operating point. The characteristic equation is derived in terms of controller parameters for synthesis. The D-partition technique is employed for finding the probable stable operating region in parametric plane. The stability of the region is confirmed using frequency scanning technique. The final selection of the current and speed controller parameters is carried out by the transient response of the current loop and complete drive. To investigate the transient response state models of the current loop as well as the complete drive are developed and transient response is computed for step input. The implementation of the control is through processor which makes the system discrete in nature. Therefore, speed and current controller parameters are also designed using z-transform in discrete data system. Again, the D-partition technique Is applied in z-domain for finding the probable stable region in parametric plane and frequency scanning technique for checking the stability of this zone. The parameters are selected finally by comparing the transient response of the drive in discrete data. The steady state and dynamic performance of the developed drive is investigated in stages. Extensive experimentation is carried out to study the performance of current source. The operation is checked in various zones and input output waveforms are recorded. The current source inverter is operated in different PWM modes and various voltage and current waveforms are recorded and compared with the computed ones. Later on ,the performance of the complete drive is investigated in open loop and closed loop operating conditions. The transient response of the drive is recorded for step change in the speed reference as well as for load change. To summarize, a prototype model of statically controlled current source inverter fed induction motor drive is designed and developed. The pulse width modulation technique is employed in low speed operation to reduce torque pulsations. A method is proposed to obtain the pulse widths and pulses per cycle for complete operating range in speed. A generalised program is developed for the analysis of the PWM drive. D-partition technique is applied in continuous time domain as well as in discrete time domain for designing the controllers. The control of the drive is implemented through personal computer. The steady state and sixth harmonic analysis of the drive is carried out. The performance of the drive is experimentally obtained and compared with the analytical result. The dynamic performance of drive is investigated experimentally with the designed controller parameters. The conclusion of the developed system is given in detail.