FLUX ESTIMATORS FOR SPEED-SENSORLESS
INDUCTION MOTOR DRIVES


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Abstract

This thesis deals with flux estimators for speed-sensorless induction motor drives. To enhance
the stability and the performance of state-of-the-art sensorless drives, new flux estimator
designs based on the standard motor model are proposed. Theoretical and experimental
research methods are both used. The dynamics and stability of flux estimators
are analyzed using linearized models, and the effects of parameter errors are investigated
using steady-state relations. Performance is evaluated using computer simulations and laboratory
experiments. It was found that most sensorless flux estimation methods proposed
in the literature have an unstable operating region at low speeds (typically in the regenerating
mode) and that the damping at high speeds may be insufficient.

Introduction

Three-phase induction motors are the most widely used electrical motors, due to their
ruggedness and low price. The induction motor can be operated directly from the mains,
but variable speed and often better energy efficiency are achieved by means of a frequency
converter between the mains and the motor. A typical frequency converter consists of a
rectifier, a voltage-stiff dc link, and a pulse-width modulated (PWM) inverter. The inverter
is controlled using a digital signal processor.

Space Vectors

The space vector approach by Kov´acs and R´acz (1959a) is commonly used to model the
dynamic behaviour of ac machines. The space vector is a complex variable, whose amplitude
and angle can vary arbitrarily in time. The current is assumed to be distributed
sinusoidally along the air gap. The space vector of the stator current in the stator reference
frame is defined by,

Induction Motor

Two mathematically equal flux linkage models of the induction motor are shown in Fig.
2.1. The conventional T model is commonly used in the literature but the simpler inverse-