15-01-2013, 10:09 AM
An Improved direct torque control for 3-phase inverter fed induction motor sensor less drive
An Improved direct torque.pptx (Size: 1.27 MB / Downloads: 35)
Direct torque control (DTC)
Basic principle of DTC.
Problem statement.
3-level inverter feeder.
Two kinds of scheme proposed for three level DTC they are :
DTC METHOD I.
DTC METHOD II.
Performance enhancement of DTC.
Problem statement
Achieve high performance sensor less DTC.
Overcome neutral point potential balance and smooth vector switching caused by the topology of 3-level inverter.
To overcome the issue of large starting current.
Achieve high-performance sensor less control over a wide speed range.
3-level inverter
The three-level neutral-point-clamped (NPC) inverter is one of the most commonly used multilevel inverter topologies in high-power ac drives.
There are three states for one phase output: “+Udc/2, 0 and Udc /2, with the neutral point as reference.
More output levels causes more sinusoidal output.
Complexity arises with more number of vectors
Problems of neutral point balance and smooth vector switching.
DTC method I
Inserts appropriate intermediate vectors to meet the demand of neutral-point-balance and smoothed vector switching.
The vector is selected according to the demand of the flux and torque.
Fig. 2 shows the space vector diagram for a three level DTC control strategy and its sector division.
The basic principles of the vector selection are shown in Table I.
DTC method II
In DTC method I, by inserting the appropriate intermediate vector, the problems of neutral-point-balance and smooth vector switching were solved.
However, it may degrade the performance of torque and increase the complexity of vector selection, so another scheme is proposed here.
A series of novel synthesizing vectors are produced, which are illustrated in Fig3 and marked by V s1,V s2...V s12.
Speed adaptive flux observer
Sensor less motor drives has more reliability and less complexity.
This paper adopts a speed adaptive flux observer with novel gains to improve the stability of the system.
The mathematical model of the observer can be expressed as
Decreasing starting current
To decrease the starting current and maintain sufficient starting torque, pre-excitation of the stator flux is proposed.
Adaptive flux observer is incorporated in the system, the stator flux can be controlled accurately and achieve the pre-excitation in the true sense.
During the pre-excitation process, when the current exceeds the limitation setting, a zero vector will be selected to reduce the current.
Simulation results
To validate the effectiveness of the two DTC methods, a three-level DTC motor drive was developed.
The sensor less three-level DTC drive is illustrated in
Fig. 9.
The system parameters are listed in Table IV.
Figs. 10 and 11 show the starting response for DTC method I from 0 to 1200 rpm.
Similar results are shown for DTC method II in the figs.12 and 13