Speed sensor-less control of induction machine based on carrier signal injection and smooth-air-gap induction machine model

by Wang, Guanghui.

iii The standard induction machine model will lose its observability at DC excitation, so the rotor speed can not be estimated if only based on the fundamental frequency variables. Many speed estimation methods that are still effective at DC excitation either use second order effects or require modification of the rotor structure of the induction machine. This thesis presents one speed estimation scheme that can work at fundamental DC excitation based on the standard smooth-air-gap induction machine model and carrier signal injection. The carrier signals used for speed estimation are selected to rotate in the opposite direction of the fundamental frequency signals at a sufficiently high frequency, so even if the fundamental exciting frequency is zero, the rotor speed can still be estimated based on the injected carrier signals. In the stator flux reference frame, the locus of steady-state stator currents as a function of rotor speed is a circle. Using the difference between the stator current and the center of this locus as an auxiliary vector, we can define a correction term for the rotor speed as the cross product of the vector based on measured stator current, which is related with the actual rotor speed, and the estimated stator current vector, which is related to the estimated rotor speed. The stability of the scheme is analyzed using the two-time-scale method and classic control stability theory. This estimation is implemented in the stator flux reference frame of the carrier frequency. The estimated rotor speed is then used in the torque controller, which is at fundamental frequency. iv Simulation and experiments are carried out on a 3-phase, 4-pole induction machine rated at 1.5 HP, 60 Hz, 230 V line-line, and 4.7 A to verify the feasibility of the scheme. The carrier signal will tend to cause torque ripple. The magnitude of the carrier signal can be selected relatively small compared to the fundamental frequency signals to minimize the ratio of the torque ripple to the rated torque. However, one can use different methods to reduce or even eliminate this torque ripple. Experimental results are given to illustrate these ideas.