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Abstract
Some essential and important improvements of the direct torque controlled interior
permanent magnet (IPM) synchronous machine drive are presented in this thesis. These
studies, including analysis, modeling and experimental implementations confirm the
possibility of a high performance direct torque controlled IPM synchronous motor drive
without any continuous rotor position and speed sensor and without any current controller.
The direct torque control technique, the comparison between DTC and FOC, and
compensation methods for the problems/limitations associated with DTC have been
investigated in this thesis. A number of important problems that affect the accuracy of the
estimated machine flux linkage on which the DTC technique is built are thoroughly
examined. Estimation of stator resistance variation, analysis and compensation of the
non-linear effects of the inverter such as forward voltage drop and dead-time, speed
sensorless control, and torque and flux ripple minimization for a direct torque controlled
IPM motor drive are of major concern in this thesis. A Proportional-Integral stator
resistance estimator based on stator current has been investigated for the compensation of
any variation in stator resistance. It is shown that the estimator can track the variation of
the stator resistance adequately. The scheme utilizes the error between the actual current
and the reference current and requires no position signal. Modeling and experimental
results will be shown.
The non-linear effects of the inverter affect flux estimation greatly, especially at low
speed. The effects such as forward voltage drop, dead-time and switching delay is
analyzed, they degrade the system performance by introducing error between the
estimated values and the actual values. The effects of the forward voltage drop and deadtime
can be compensated by using a look-up table. The performance improvement of the
drive has been shown in experiments. A speed estimation scheme based on stator flux
linkage estimation is adopted and investigated experimentally. Furthermore, the
possibility of fielding-weakening operation of the speed sensorless control is also
investigated by modeling.
The torque and flux ripples are significant problems of the DTC, and are mentioned
widely. In order to solve this problem, the changes of torque and flux linkage over a
sampling period are derived. Based on the analysis, a modified DTC is proposed to
overcome these significant problems. Modeling and experimental results confirm the
effectiveness of the proposed scheme. The field weakening control and speed sensorless
control scheme is also combined with the proposed scheme. The experimental results
show the new DTC scheme can achieve wider range operation and speed sensorless
control successfully. The torque and flux ripples are reduced greatly under the new
scheme in all experimental results.
These abovementioned studies have clearly established that the DTC technique for the
IPM machine is now much closer to being a viable and cost-effective candidate for a
sensorless PM synchronous motor drive.