SYNOPSISDIRECT TORQUE CONTROL OF INDUCTION MOTOR USING SPACE VECTOR PULSE WIDTH MODULATION

MAHARAJA AGRASEN INSTITUTE OF TECHNOLOGY

TEAM TEAM MEMBERSMENTOR: DR. S.S DESWAL (DEAN ACADEMICS) RAGHAV KUMAR 07014804911 MR. JITENDER LATHER MANISH DAS 06414804911 ( ASST. PROFFESSOR) SHUBHAM VASHISHT 00214807812

IntroductionOver the past decade DC motors were invariably used for the variable speed applications due to the decoupled nature of torque and flux which could be easily controlled by varying the field and armature currents but DC motors requires great maintainance and are not cheap to operate therefore the focus has shifted to AC motors which are rugged , cost effective and requires less maintainance.Here we are interested in finding a suitable method of controlling the speed and torue of induction motors to make it more efficient and versatile. The DIRECT TORQUE CONTROL method is one of the vector controlled method which gives improved response,decoupled torue and flux control, and a wide range of working speeds.In DTC method estimation of motor torque and stator flux is required for close loop control of induction motor . the main idea behind dtc is that error in torque and flux is directly used to feed the inverter without any intermediate current control loop ad coordinate transformation.

TheoryWhen resistance of stator winding is neglected then the stator flux is integral of applied voltage. Hence in short duration the flux is proportional to applied voltage. The forward active space vector is applied when the required torue is to be increased and this causes the torque angle to increase. Similarly backward or zero space vector is applied when the torue requirement is less and this causes the torque angle to reduce.Hence it can be concluded that the torque can be controlled directly by rotating the stator flux linkage space vector to the appropriate position this is why the contol scheme is called DTC .

ADVANTGES OF DTC Direct control of flux and torque Coordinate transformation is not required Current controller is not required It is a sensor less control It is sensitive to changes in stator resistance only.

DISADVANTAGES OF DTC Torque and flux estimators are required Inherent torque and flux ripples are present Possibility of problem in starting.

Principle of DTC schemeThe basic principle of dtc is to directly select torque and stator flux errors which are the differences between the reference inputs and their actual values. The governing interaction for torque is due to the interaction of stator and rotor fields. Torque and stator flux linkages are computed from measured stator voltage and current. An optimum voltage vector for the switching of VSI is selected from the six non zero voltage and two zero voltage vectors by the hysteresis control of stator flux and torque.SPACE VECTOR PULSE WIDTH MODULATION FED INDUCTION MOTOR DRIVEThe conventional PWM technique is suitable for open loop control of AC drives for closed loop control the SVPWM .In this technique the switching pattern of the bridge inverter are generated from the knowledge of stator phase voltage vectors.a reference voltage vector is generated to generate a field synchronous with the rotating voltage vector by utiilising the different switching states of the three phase voltage vector.

Theory of SVPWMWhen a three phase supply is given to a induction motor a rotating magnetic field is produces which generates a voltage lagging by 90 degrees. This field can also be realized by a logical switching of the inverter which is the basis of SVPWM.

Conclusion and future scope

The DTC scheme can be further improved using hybrid space vector PWM technique which will be helpful in reducing switching losses and ripple in torque flux and current. This method can be used for controlling drives of small and medium rating. Double three phase inverter can also be used for smoothing out the ripples. The torque ripple can also be smoothened out by discrete pulse width modulation technique. The ripples in output torque can be minimized by using neural network and metahueristic algorithms.