25-08-2017, 09:32 PM
SENSORLESS VECTOR CONTROL OF INDUCTION MOTOR DRIVE BASED ON DIRECT FIELD ORIENTATION METHOD
SENSORLESS VECTOR CONTROL OF INDUCTION MOTOR.docx (Size: 501.88 KB / Downloads: 32)
Abstract:
The aim of this project is to develop a vector controlled induction motor drive operating without a speed sensor but having a dynamic performance equivalent to a sensored vector drive. Sensorless vector control is a variable frequency drive (VFD) control strategy that improves motor performance by regulating the VFD output based on mathematical determination of motor characteristics and operating conditions.The idea of induction motor control with the principle of Field Oriented Control (FOC) was a big breakthrough which enabled a decoupled control of rotor flux and electromagnetic torque. The field-oriented control as the vector control technique is mainly implemented in two ways: indirect field oriented control and direct field oriented control.The field to be oriented may be rotor, stator, or airgap flux-linkage. In the indirect field-oriented control no flux estimation exists. In this project the speed estimation for sensorless closed-loop speed control of an induction machine is based on direct field-oriented control technique.
Several methods has been proposed for speed estimation for sensorless induction motor drive. The two major categories are :-
1) First one is the Technique which estimates the rotor speed based on non ideal phenomenon, such as rotor slot harmonics and signal injection method.The disadvantage of this method is spectrum analysis, which has time consuming procedures and they allow narrow band of speed control.The advantage of the saliency technique is that the saliency is not sensitive to actual motor parameters, however this method does not have sufficient performance at low and zero speed level. Also, when applied with high frequency signal injection, the method may cause torque ripples, vibration and audible noise.
2) Second method is utilization of induction motor model which is characterised by its simplicity.
In this project the proposed method is the second one which has less drawbacks. In this method the information on motor speed is determined by measured stator current and voltage at motor terminals. Dynamic performance and steady state speed accuracy in low speed range can be achieved by exploiting parasitic effect of the machine. This technique requires the knowledge of the stator resistance along with the stator-leakage, and rotor-leakage inductances and the magnetizing inductance. The flux observer relies on both current and voltage models of the machine to improve the dynamic performance of the estimator.
For long time, open-loop V/f control which adjusts a constant volts-per-Hertz ratio of the stator voltage is used, however, dynamic performance of this type of control methods was unsatisfactory because of saturation effect and the electrical parameter variation with temperature. Recent improvements with lower loss and fast switching semiconductor power switches on power electronics, fast and powerful digital signal processors on controller technology made advanced control techniques of induction machine drives applicable for closed-loop speed control of system, torque, flux and speed producing control loops are tuned by the help of PI regulators. The performance of the closed-loop speed control is investigated by simulations and experiments. TMS320F2812 DSP controller card and the Embedded Target for the TI C2000 DSP tool of MATLAB are utilized for the real-time experiments.
EXISTING SYSTEM IS SENSOR CONTROLLED INDUCTION MOTOR DRIVE
Introduction:
AC Induction motors are the most widely used motors in industrial motion control systems,as well as in home appliances thanks to their reliability, robustness and simplicity of control. Until a few years ago the AC motor could either be plugged directly into the mains supply or controlled by means of the well-known scalar V/f method. When power is supplied to an induction motor at the recommended specifications, it runs at its rated speed. With this method, even simple speed variation is impossible and its system integration is highly dependent on the motor design (starting torque vs maximum torque, torque Vs inertia, number of pole pairs). However many applications need variable speed operation. The scalar V/f method is able to provide speed variation but does not handle transient condition control and is valid only during a steady state. This method is most suitable for applications without position control requirements or the need for high accuracy of speed control and leads to over-currents and over-heating, which necessitate a drive which is then oversized and no longer cost effective. Examples of these applications include heating, air conditioning, fans and blowers.
Application:
The AC machine, with a speed/position sensor coupled to the shaft, acquires every advantage of a DC machine control structure, by achieving a very accurate steady state and transient control, but with higher dynamic performance. Sensor based motors are most often used in applications where the starting torque varies greatly or where a high initial torque is requiredthe DSP-based micro functions used in this application in order to speed-up the execution time.
Controller used:
PID has a fundamental importance in the industrial controller field due to the simplicity ofapproach, the robustness of control and the immediacy of calibration. The Proportional term (P) of the controller is formed by multiplying the error signal by a P gain, causing the PID controller to produce a control response that is a function of the error magnitude. As the error signal becomes larger, the P term of the controller follows and grows to provide more correction. The effect of the P term tends to reduce the overall error as time elapses. In most systems, the error of the controlled parameter gets very close but does not converge so it remains a small steady state error. The Integral (I) term of the controller is used to eliminate small steady state errors. The Iterm calculates a continuous running total of the error signal. Therefore a small steady state error accumulates into a large error value over time. This accumulated error signal is multiplied by an I gain factor and becomes the I output term of the PID controller. The Differential (D) term of the PID controller is used to enhance the speed of the controller and responds to the rate of change of the error signal. The D term input is calculated by subtracting the present error value from a prior value. This delta error value is multiplied by a D gain factor that becomes the D term of the controller. The D term of the controller produces more control output the faster when the system error is changing
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Drawback:-
Scalar control is somewhat simple to implement but the inherent coupling effect give sluggish response and the system is more prone to instability because of high order system effect. For example, if torque is increased by implementing the slip that is frequency, the flux tends to decrease. As the flux variation is always sluggish, the decrement in flux is compensated by sluggish flux control loop feeding in additional voltage. This temporary dipping in flux reduces the torque sensitivity with slip and lengthens the response time. This is valid for both voltage and current fed inverter drives.