Improved Neural Network Based Direct Torque Control of Double Star Induction Machine Using Different Multilevel Inverter. The Direct Torque Control (DTC) method is one of the highest performance control strategies for AC machines to provide fast torque and flux control.
137 Torque Ripple Reduction in DTC Induction Motor Drive
Introduction
However, this arrangement has some drawbacks such as variable switching frequency, size and complexity of the switching tables and the strong ripple torque. One of the most common examples of polyphase machines is the double star induction machine (DSIM) [5].
DSIM model
To do this, one of the solutions is to use multiphase machines thanks to their advantages, such as power segmentation and the minimization of torque ripples (elimination of the sixth order harmonic torque). For any control, DTC has advantages and disadvantages, and among these advantages, the stator resistance is theoretically the only machine parameter that interferes with the control.
Modeling of three-level inverter
The floating diodes provide the application of the different voltage levels at the output of each arm. The output voltages relative to the neutral point of the DC source are expressed by.
Modeling of five-level inverter
These switches should not be opened or closed simultaneously, to avoid shorting the DC source in the input. The floating diodes (six per arm) ensure the application of the different voltage levels at the output of each arm.
Direct torque control based on neural networks
- Neural network strategy
The direct torque control of a DSIM is based on the direct determination of the control sequence applied to the contactors of a voltage converter. The amplitude of the stator flux is thus estimated from the components along the axes (α,β):.
Simulation results
Figures 5 and 6 show the simulation results of three- and five-level DTC control for DSIM. Figures 7 and 8 show the simulation results of three- and five-level DTC-ANN control for low-speed operation.
Conclusion
Sensorless fuzzy sliding-mode control of the double-star induction motor using a sliding-mode observer. Direct torque control of six-phase induction motors using three-phase matrix Improved direct torque control based on neural network of double-star induction.
Model of induction motor dedicated for direct torque control
The first method is a conventional direct torque control (C-DTC) where torque and flux are regulated by hysteresis controllers. The second is direct torque control by space vector modulation (SVM-DTC) strategy where torque and flux are adjusted by PI controllers.
Two-level voltage source inverter (VSI) model
The third one is fuzzy SVM-DTC with adaptive fuzzy-PI speed controller where the torque and flux are regulated by fuzzy logic controllers. The main feature of the proposed (SVM-DTC) strategy is the reduction of torque and flux ripples.
Principles of direct torque control
It achieves a decoupled control of stator flux and electromagnetic torque in the stationary frame (α, β), and allows induction machines to have an accurate and fast electromagnetic torque response. The choice of switching states is directly related to the change in stator flux and machine torque.
Estimation of stator flux and electromagnetic torque 1 Control of stator flux
Estimation of stator flux and electromagnetic torque5.1 Control of stator flux 5.1 Control of stator flux. Given that the control of the inverter contacts is done by control period (or sampling). In order that the conditions Sa, Sb and Scare remained constant in each of these periods, the method of numerical integration of the rectangles makes it possible to obtain an expression for sample k + 1 of the stator flux in the following form:
We can neglect the stator resistance voltage drop compared to Vs for high speed ranges. It allows to easily drop the extremity of the flux vector within the boundaries of the two concentric circles of close radius. The choice of the hysteresis bandwidth depends on the switching frequency of the inverter. Figures 2 and 3. Development of stator flux vector in the complex plane.
From expression (14), it is clear that the electromagnetic torque is controlled by the amplitude of the stator and rotor flux. If these quantities remain constant, the torque can be controlled by adjusting the load angle δ.
Estimation of stator flux and electromagnetic torque 1 Stator flux estimation
- Electromagnetic torque estimation
Switching table construction and control algorithm design
The torque error is defined by the difference between the reference values of the torque and the actual estimated values:. Orϕsαandϕsβ are estimated by eq. 4) which requires knowledge of the components of the stator current vector isαand isβand that of the vector stator voltage vsαand vsβ. The produced electromagnetic torque of the induction motor can be determined by using the cross product of the stator quantities (i.e. stator flux and stator currents).
To maintain decoupled regulation, a pair of hysteresis comparators receives the stator flux and torque errors as inputs. However, the choice of voltage vector depends not only on the output of hysteresis regulators, but also on the position of the stator flux vector. Thus, the trajectory of the circular stator flux vector will be divided into six symmetrical sectors (Table 1).
For each sector, the vectors (Viand V3+i) are not considered because they can both increase or decrease the torque in the same sector according to the position of the flux vector on the first or second sector. Direct Torque Control Strategies for Induction Machine: Comparative Studies DOI: http://dx.doi.org/10.5772/intechopen.90199.
Global scheme of conventional direct torque control
Constant switching frequency direct torque control using SVM The conventional direct torque control has several disadvantages, among which
- Space vector modulation algorithm
- Principle of fuzzy direct torque control
It makes it possible to improve the speed control efficiency by eliminating the winding phenomenon caused by the saturation of the pure integrator [20]. This strategy consists of correcting the integral operation based on the difference between the control signal and the saturation limit. Although the selected values are of reduced bandwidth, ripples are still significant due to the discrete nature of hysteresis regulators.
The main disadvantages of DTC-SVM using PI controllers are the sensitivity of performance to system parameter variations and inadequate external rejection. The principle of SVM is to predict the inverter voltage vector from the projection of the reference vector Vs∗ between adjacent vectors corresponding to two non-zero switching states. The position of the reference voltage vector relative to the stator flux vector must be chosen so as to maintain the stator flux and electromagnetic torque in an optimal error band around their reference value.
These are used by the block fuzzy control rules after defuzzification; the value of (ψ) which must be added to the angle of the stator flux [30–32] (Figure 12). It enlarges the operating range of the linear controller (PI) so that it also works with a non-linear system [33, 34].
Simulation results
Shown is the start-up and steady states of a controlled motor with load input. For the classic DTC, the bandwidths of the hysteresis controllers are selected�0.01 Wb for flow and�0.1 Nm for torque. This section presents the starting condition of an induction motor with respect to a reference speed level of 1000 rpm.
The results in Figure 16 show that the conventional DTC technique provides good startup dynamics. It is clear that the current ripple of the conventional DTC has exceeded the hysteresis limit. The magnitude and trajectory illustrate that the flux takes several steps before reaching the reference value (1.2 Wb) at the initial stage due to the area change.
The simulation in Figure 17(b) shows that the SVM-DTC-fuzzy has better performance than that obtained by both other DTC strategies (conventional and SVM-PI). There is a significant decrease in the initiation reaction time; we can notice that the speed regulation loop rejects the applied load disturbance very quickly which proves the performance of adaptive fuzzy PI controller as well as a significant weakening of the ripples of the torque and of the sinusoidal current without any ripple in the steady state.
Conclusion
The conventional DTC in Figure 16 shows a clipped sinusoidal current waveform showing a high harmonic level, while the SVM-DTC in Figure 17(a) shows a smoother sinusoidal waveform. After that, the results show the magnitude of the evolution of the stator current and the circular rail.
IM motor parameters
- A case study: solar panel positioning 1 Problem heading
- Mathematical model of induction machines
- Voltage source inverter
- Basic concept of DTC based position control
- Concept of DTC-SVM with hysteresis controllers based position control The implementation scheme of the DTC-SVM strategy with hysteresis control-
- Simulations and discussions: A comparative study
- Concept of PI DTC-SVM based position control 1 Computing of flux reference coordinates
- Computing of voltage reference coordinates
- Position control loop
- Concept of sliding mode DTC-SVM based position control
- SM controllers with adaptive parameters estimation
- Simulation results investigated SM DTC-SVM approach based position control
- Performance criteria
- Conclusion
- Operation principle of FRPMM
- Sizing equation of FRPMM 1 Magnetic circuit model
- Airgap flux density equation
- Torque equation
- Influence of design parameters on key performances 1 Average torque performances
- Pulsating torque performances
- Power factor performances
- PM demagnetization performances
- Geometric design of stator and rotor 1 Stator design
- Design methodology and evaluations 1 Design procedure
The implementation scheme of Takahashi basic DTC strategy applied to position control of an induction motor drive is shown in Figure 1. Locus of the extremities of Φs, with (a) basic Takahashi DTC strategy, (b) modified Takahashi DTC strategy, and (c) ) DTC- SVM strategy with constant commutation frequency. Average commutation frequency of the inverter circuit breakers, (a) basic Takahashi DTC strategy, (b) modified Takahashi DTC strategy.
Average commutation frequency of the inverter power switches, (a) basic Takahashi DTC strategy, (b) modified Takahashi DTC strategy. It should be noted that the application of the following nonlinear feedback represents a nonlinear compensator:. The idea is based on the decoupling between the amplitude and argument of the stator flux reference vector.
The idea is based on the decoupling between the amplitude and the argument of the stator flux reference vector. The first criterion is the average total harmonic distortion (THD) of the stator current, which is defined in the appendix. Legend: (a) evolution of the position and its reference and (b) motor speed and its reference.
Moreover, it can be seen that the knee point of the equivalent magnet load increases with the number of stator slots. When the rotor slot opening ratio continues to increase, the flux modulation effect of the rotor teeth becomes weaker and weaker, thus the smaller modulated flux and lower back EMF. Thus, during the design process, the optimum rotor slot opening ratio can be applied directly without considering the risk of PM demagnetization.
