12-12-2012, 01:13 PM
Direct Vector Control of Induction Motor
Direct Torque Control (1).ppt (Size: 1.65 MB / Downloads: 42)
Basic block of DTC
Principle
In principle the DTC method selects one of the six
nonzero and two zero voltage vectors of the inverter on
the basis of the instantaneous errors in torque and stator
flux magnitude.
MAIN FEATURES OF DTC
Decoupled control of torque and flux.
Absence of mechanical transducers.
Current regulator, PWM pulse generation, PI control of flux and torque and co-ordinate transformation are not required.
Very simple control scheme and low computational time.
Reduced parameter sensitivity
Introduction
Three-phase induction motors are the most common and frequently encountered machines in industry
Simple design, rugged, low-price, easy maintenance.
Wide range of power ratings: fractional horsepower to 10 MW .
Run essentially as constant speed from no-load to full load.
Its speed depends on the frequency of the power source.
PWM METHODS
Sine PWM (1)
The main draw back in the so called 6 step VSI is that it mainly comprises of the high magnitude harmonics in its waveform.
The THD is found to be in between 50%-70%
The above value holds good for the different PWM techniques other than SVPWM.
The THD content is SVPWM is 15%-20% only.
Principle of Space Vector PWM
Treats the sinusoidal voltage as a constant amplitude vector rotating
at constant frequency
This PWM technique approximates the reference voltage Vref by a combination
of the eight switching patterns (V0 to V7)
CoordinateTransformation (abc reference frame to the stationary d-q frame)
: A three-phase voltage vector is transformed into a vector in the stationary d-q coordinate
frame which represents the spatial vector sum of the three-phase voltage
CONCLUSION
An improved torque and flux response was achieved with the DTC and ANN_DTC than the conventional DTC.
Fuzzy approach which is the newly evolved technique is proved to be superior over these classical techniques.
Also, a command flux optimization (i.e. hysteresis control) scheme has been proposed to reduce the torque ripple.
Reduction of torque and current ripples in transient and steady state response.
No flux droppings caused by sector changes .circular trajectory.
Fast stator flux response in transient state.