DESIGN AND DEVELOPMENT OF MODIFIED SELFCOMMUTATING CSI-FED INDUCTION MOTOR DRIVE

Abstract

Variable speed drive is the main requirement ofany industry. Among the various AC/DC motors, the statically controlled d.c. motor easily achieves this capability and hence it was a probable choice for the industry. The motor is powered from both d.c.or a.c. supply, and variable d.c. voltage is applied to the armature through chopper or converter, but because of mechanical commutators, high cost, noise and frequent maintenance of carbon brushes; hence these motors are not reliable and economical for industrial applications. Due to these limitations of d.c. drives, a.c. drives have gained preference over the d.c. drives. Among the a.c. drives, variable speed squirrel cage induction motor drive is preferred over the synchronous motor drive, because ofits robust construction without slip ring and carbon brushes. Induction motor is having hard torqueslip characteristics; hence only asmall range speed control is possible when it is operated from aconstant frequency supply. The precise wide range speed control is possible only with the variable frequency a.c. source. With the latest development in the semi conductor technology, it is easily achieved with the variable frequency power semi controllers. To operate the machine at rated flux, the terminal voltage is also controlled in accordance with the frequency. Variable frequency operation of the induction motor is possible with the help of either voltage source inverter or current source inverter. The current source inverter is having the regeneration capability without an extra cost and it is having inherent over current protection because of its controlled current operation. The conventional CSI-fed induction motor drive line currents are stepped in nature and have dominant low order harmonics. These harmonics give rise to torque pulsation, however the line to line voltage is nearly sinusoidal with transient voltage spikes. To overcome the problem associated with the conventional CSI-fed induction motor drive, lots of work has been carried out such as: implementation of PWM technique in front-end current source as well as in motor- end current source inverter, for better input displacement factor and line currents respectively. PWM operation in frontend converter also reduces the size ofd.c.link inductor required. In the light ofabove works, a modified self commutating CSI-fed induction motor drive is designed and developed so that the motor line voltages and currents are nearly sinusoidal over the wide range of the speed control, with smooth drive operation. The input supply voltage and current offront-end rectifier is also sinusoidal and hence the size of d.c. link inductor is drastically reduced. The modified current source inverter fed induction motor drives consists of two converters coupled through a d.c. link inductor. The front-end converter is equal PWM converter and it acts as a current source, with d.c link current feed back. The current source inverter is space vector PWM inverter, and it regulates d.c. link current into a variable frequency a.c. current. Both converter and inverter make use ofMOSFET as selfcommutating devices. At the terminals of induction motor, a 3-phase capacitor is connected, which removes the harmonics from the machine voltages and currents at almost every operating frequency. For better dynamic characteristics of the drive two PI controllers are used, one in outer speed feed back loop and other in inner current feed back loop. The speed PI controller compares the reference speed with actual speed and sets the reference slip speed (©„*). To maintain the motor flux at rated value a slip regulator is also used in the feed back loop. The reference value of stator active current (lac) and the stator reactive current (Ireact) are obtained, from slip regulator characteristics. These characteristics are obtained experimentally at rated voltage and frequency, by 11 loading the motor till rated load. Two characteristic curves (Iac, vs. ©,,) and (Ireact vs. ©,,) are drawn. These characteristic curves are defined as slip regulator, characteristics in the present work. The synchronous speed is obtained by adding actual speed and reference slip speed. The desired capacitor current (Ic) is calculated at the operating frequency for constant (V/f) control of the drive. The reference values of stator active current, reactive current and capacitor current are used to obtain the reference d.c. link current (Iref). The current PI controller acts on d.c. link current error between reference d.c. link current and actual d.c. link current. The current PI controller sets the Ton/Toff periods ofPWM pulses, ofthe front-end converter and hence d.c. link voltage for regulating the d.c. link current. To analyze the performance of drive in steady state and dynamic conditions, a mathematical model of complete drive system is developed in synchronously rotating (qe - de) reference frame, by considering only fundamental component and neglecting the effect of core loss, saturation and switching transients. To study the steady state performance, mathematical expressions are derived for torque developed, power output, stator voltage per phase, stator current per phase, power loss, efficiency, d.c. link voltage. The performance characteristics of the drive are drawn for different operating conditions; (i) Variable capacitance at the motor terminals, fixed operating frequency, fixed d.c. link current, (ii) Fixed capacitance, variable frequency, fixed d.c. link current, (iii) Fixed capacitance, fixed frequency, variable d.c. link current, (iv) Without capacitance at the motor terminals, fixed d.c. link current, variable operating frequency, (v) Fixed capacitance, fixed d.c. link current, fixed load torque, (vi) Fixed capacitance, fixed d.c. link current, linearly varying load torque. The simulated results and practical waveforms decide the value ofcapacitor, which is to be connected across the motor terminals. With the selected value ofcapacitance, simulated steady state curves shows that at some value in of frequency resonance is taking place, and hence at the resonance frequency and in the vicinity of resonance frequency experimentation is not possible. To improve the performance of the front-end converter a 3-phasePWM converter of equal pulse width modulation type is applied, which gives almost ripple free d.c. link current with small d.c. link inductance, and input power factor is almost unity. Firing signals for the converter are generated through a dedicated 8031 micro controller based card, which relives the personal computer for processing the speed and current errors and generation of pulses for the inverter. The control to the pulse width modulated rectifier is applied through the personal computer, which serially communicates with the 8031 micro controller. The personal computer serially send the pulse width information to control the output voltage of the rectifier, and hence current. The 3-phase inverter connected to the PWM rectifier through d.c. link is also pulse width modulated type and firing commands for the PWM operation of inverter are generated using a space vector technique for the different operating frequency ranges. At low value of operating frequency number of crossing points of modulating signal and reference signals are more and at high value of operating frequency the number of crossing points are less, due to the switching frequency consideration of power devices. The firing pulses for pulse width modulated inverter is generated through personal computer using Add, on cards. The reference speed is given through keyboard of the personal computer. The d.c. link current is measured through the Hall effect current sensor, and rotor speed is measured through incremental rotary pulse encoder. The d.c. link current is digitized through A/D converter for processing. The speed error PI processing is carried out at every 10 ms, while the current error PI processing is done at every 1 ms. A timer is used to generate the interrupt at an interval of lm sec for current and 10 m sec for speed iv PI processing respectively. Both PI controllers are implemented through software. The slip regulator characteristics are stored in the memory and used to read the stator active and reactive currents. The use of PI controllers in the speed loop and current feed back loop improves the dynamic performance of the drive. With the proper design of controller parameters, the response of the system is fast and over shoots within the defined limits of (5% tolerance). Proper design of controllers also enable the drive to over-come external disturbances. For the design of controller parameters D- partitioning technique is used. The inner current loop is designed first and outer speed controller later as the electrical time constant is much smaller than mechanical time constant. The system governing equations are basically non-linear in nature; therefore these equations are linearized using small perturbation method around a steady state operating point. The characteristic equations are derived in terms of the current controller parameters for synthesis. The probable stable region in parametric plane is obtained with the help of D-partitioning technique. The stability of the region is confirmed by frequency scanning. To get the region ofbetter stability, relative stability boundaries are drawn for the different value of (a and $). As these values increase from their minimum values of zero, the better stability regions are achieved which keep on shrinking. After certain values of (a and fc) relative stability in parametric plane regions completely vanish. The final selection of current controller parameters is carried out by the transient response of current loop. With the pre-estimated parameters of current controllers, parameters of speed controllers are designed with the help of D-partitioning technique, followed by frequency scanning. Final decision of controller parameters is obtained by the transient response of the complete drive. The steady state and dynamic performance ofthe drive is thoroughly investigated in steps. The performance investigation ofPWM rectifier is carried out first by operating it alone with R-L load. Various signals, such as zero crossing, quantizers, and firing pulses ofswitching devices are recorded. Output voltage, output current, input voltage and input current waveforms of rectifier are also recorded at the different duty cycles with R- L loads. Fundamental current is found to be in phase with the input voltage. Performance of PWM inverter is investigated by recording the firing pulses of the devices at the different operating frequencies. Inverter output current, motor current, terminal voltage waveforms, with/without capacitor at the motorterminals are also recorded. With the selected value of capacitance, extensive experimentation is carried out in open loop as well as closed loop. The open loop experiment is carried out at the various operating frequencies at a fixed value of d.c. link current, and closed loop experiment is carried out at linearly varying load torque on the motorusing a coupled dc generator. The experimental open loop and closed loop results are compared with the simulated results. They are quite close to each other, except at the resonance frequency, and in the vicinity of the resonance frequency. The transient performance of the drive is also investigated experimentally for linearly varying load torque on the machine, and it is found to be quite satisfactory. To summarize a practical model of modified CSI-fed induction motor drive is designed and developed. The equal pulse width modulated technique is employed in the front-end rectifier and space vector PWM technique is employed in the inverter. Firing signals for the converter is generated through a dedicated 8031 micro controller based which serially communicates with personal computer. The open loop experiment is carried out at the different operating frequency for a fixed value of d.c. link current and at VI the different values ofcapacitance connected across the motor terminals and the value of capacitor is selected. Various voltages and current waveforms of the inverter and converter are recorded. The line voltages and currents ofmotor are nearly sinusoidal at each operating speed. The input power factor of the converter is almost unity and the size ofcoupling d.c. link inductor is drastically reduced. The mathematical model of complete drive system is developed and steady state analysis of the drive is carried out at different operating conditions. D-partitioning technique is applied to continuous-data drive model to obtain the controller parameters. The closed control of the drive is implemented through personal computer. The performance of the drive is investigated experimentally with the designed controller parameters. Extensive experiments are carried out in the open loop and closed loop. The performance characteristics are drawn at various operating conditions. The experimental results are compared with the simulated results over the wide range of operating speed and they are quite close to each other.