Stator flux orientation: A robust control technique for induction machines

A traditional low cost, open loop, constant volts per hertz ac drive usually yields poor control characteristics and may provide poor utilization of the machine as well as the inverter. On the other hand, a rotor flux oriented (RFO) induction machine drive offers good control performance but often requires additional sensors on the machine to locate the rotor flux. This adds to the cost and complexity of the system hardware. In many cases, an off-the-shelf machine can not be directly used.;The research reported in this thesis is aimed at providing a compromise between cost and performance. To avoid using sensors on the machine, machine terminal quantities (voltages and currents) are used to estimate fluxes in the machine. Therefore, all the sensors are contained in the drive and an off-the-shelf machine can be easily adopted.;Estimation of rotor flux from the terminal quantities depends on parameters such as the stator resistance and the leakage inductance. A study of parameter sensitivity shows that leakage inductance can greatly affect system performance such as stability, dynamic response and utilization of the machine and the inverter. To reduce the sensitivity of the system, a stator flux orientation (SFO) control strategy is proposed. Since stator flux estimation is only sensitive to the stator resistance, which is insignificant except for very low frequencies, SFO yields more robust steady state control characteristics than RFO. Dynamic response of an SFO system is improved by a feedforward decoupling compensator. A comparison between SFO and RFO in the field weakening region shows that SFO achieves better overall machine utilization.;Experimental results obtained from a versatile digital signal processor (DSP) based control system show that the SFO system achieves good control performance over a wide speed range from 30 rpm to 3000 rpm. A low cost stator flux oriented drive using only the dc link current is also proposed and tested and satisfactory performance is obtained.