A New method for rotor flux field orientation of induction motor via stator fluxes

ABSTRACT Ph.D.Thesis A NEW METHOD FOR ROTOR FLUX FIELD ORIENTATION OF INDUCTION MOTOR VIA STATOR FLUXES Erhan AKIN Fırat Universitiy Graduate School of Natural and Applied Sciences Department of Electrical and Electronics Engineering 1994, page: 178 Although scalar control methods for induction motor control is low-cost, they have poor control characteristics and restrict inverter-motor performance. On the other hand, these restrictions dissappear when field orientation is used. In this approach, motor transient performance becomes superior to other control methods, for example, output torque can be controlled to follow its reference with minimum delay and without fluctuations. In general, a disadvantage of field orientation method is the necessity of speed or flux sensors on the motor. The purpose of this thesis has been to develope a method which can achieve vector control without such sensors and also to eliminate the need for estimation of rotor resistance (or time constant) which require complex and lengthy calculations. For this purpose anVI extensive literature research has been done. This investigation has led to the development of a new approach for rotor flux orientation via stator flux with satisfied the requirements. A mathematical model of the system has been developed, d-q model for squirel cage induction motor has been used. A simulation program based on this model has been prepared. So, a method using stator flux as control variables is brought up measuring stator currents and voltages. The stator flux calculations and hence the performance of the method is affected by stator resistance variations due to the temperature changes during operations and due to other effects. To avoid these negative affects, a new method has been developed to update stator resistance values without interrupting the control system. When the stator resistance correction algorithm is used, the controlability of torque is shown to improved with true computer simulations. The developed field orientation algorithm generates bang-bang control signals to control the inverter. For obtaining an acceptable output waveform at reasonable switching rate, rrdnimum switching time, average switching frequency and total harmonic distortion factors affecting inverter performance is investigated for various hysteresis bands. The findings are presented in this thesis. It is also shown in this work that the speed of the motor can be calculated during operation with good accuracy. TMS320E14 DSP processor is used for implementation of the method. Open integration problem involved during stator flux calculations is solved with a new software algorithm. To discuss the results from the study, the units of the control system are tested seperately and then measurements of the system performance are compared with the results from the simulation program. KEY WORDS : Induction motor, direct vector control, rotor flux field orientation, torque control, bang-bang control, stator resistance correction, open integration, speed calculation.