Induction motors are the starting point to design an electrical drive system which is widely used in many industrial applications. In modern control theory, different mathematical models describe induction motor according to the employed control methods. Vector control strategy can be applied to this electrical motor type in symmetrical three phase version or in unsymmetrical two phase version. The operation of the induction motor can be analyzed similar to a DC motor through this control method. With the Joint progress of the power electronics and numerical electronics it is possible today to deal with the axis control with variable speed in low power applications. With these technological projections, various command approaches have been developed by the scientific community to master in real time, the flux and the torque of the electrical machines, the direct torque control (DTC) scheme being one of the most recent steps in this direction. This scheme provides excellent properties of regulation without rotational speed feedback. In this control scheme the electromagnetic torque and stator flux magnitude are estimated with only stator voltages and currents and this estimation does not depend on motor parameters except for the stator resistance. In this dissertation report conventional DTC scheme has been described. Induction motor has been simulated in stationary d-q reference frame and its free acceleration characteristics are drawn. Conventional DTC scheme has been simulated with a 50 HP, 460V, 60 Hz induction motor. Literature review has been done to study the recent improvements in DTC scheme which somehow is able to overcome the drawbacks of conventional one. The space vector modulation technique (SVPWM) is applied to 2 level inverter control in the vector control based induction motor drive system, thereby dramatically reducing the torque ripple. Later in this project space vector PWM technique will be applied to DTC drive system to reduce the torque ripple.
Fig.1 Block diagram of conventional DTC scheme for IM drives
EXPECTED SIMULATION RESULTS:
Fig.2 Electromagnetic torque
Fig.3 Rotor speed
Fig.4 Stator current
Fig.5 d-axis stator flux
Fig.6 q-axis stator flux
Fig.7 Electromagnetic torque
Fig.8 Rotor speed
Fig.9 Trajectory of d axis and q axis stator flux in stationary reference frame
Fig.10 Electromagnetic torque
Fig.11 Rotor speed
Fig.12 d-axis stator flux
Fig.13 q-axis stator flux
Fig.14 d-axis stator current
Fig.15 q-axis stator current
Fig.16 Mean value of Phase voltage of inverter
Fig.17 Line voltage output of inverter
Fig.18 Electromagnetic torque
Fig.19 Rotor speed
Fig.20 q-axis stator flux
Fig.21 d-axis stator flux
For any IM drives, Direct torque control is one of the best controllers proposed so far. It allows decoupled control of motor stator flux and electromagnetic torque. From the analysis it is proved that, this strategy of IM control is simpler to implement than other vector control methods as it does not require pulse width modulator and co-ordinate transformations. But it introduces undesired torque and current ripple. DTC scheme uses stationary d-q reference frame with d-axis aligned with the stator axis. Stator voltage space vector defined in this reference frame control the torque and flux. The main inferences from this work are:
1. In transient state, by selecting the fastest accelerating voltage vector which produces maximum slip frequency, highest torque response can be obtained.
2. In steady state, the torque can be maintained constant with small switching frequency by the torque hysteresis comparator by selecting the accelerating vector and the zero voltage vector alternately.
3. In order to get the optimum efficiency in steady state and the highest torque response in transient state at the same time, the flux level can be automatically adjusted.
4. If the switching frequency is extremely low, the control circuit makes some drift which can be compensated easily to minimize the machine parameter variation. The estimation accuracy of stator flux is very much essential which mostly depends on stator resistance because an error in stator flux estimation will affect the behaviour of both torque and flux control loops. The torque and current ripple can be minimized by employing space vector modulation technique.
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