Indirect Field Orientation Control of Induction Machine with Detuning Effect

: Field orientation control (FOC) methods of an induction machine achieve decoupled torque and flux dynamics leading to independent control of torque and flux as for separately excited DC motor, but they are sensitive to motor parameter variations. The has present work selects the indirect field orientation control (IFOC) as an effective method for eliminating the coupling effect. The results show how well the drive performance has been improved by this control strategy. However, to which extent the control strategy can perform the decoupling relies on the accuracy of the slip frequency calculation. Unfortunately, the slip frequency depends on the rotor time constant that varies continuously according to the operational conditions and, then, the coupling effect may again arise. This paper investigates the improvement in the performance of the induction machine dynamics as the IFOC technique is utilized, also, it investigatesthe degradation in dynamic performance when the rotor resistance is deviated from its nominal value.


Introduction:
The fundamentals of vector control implementation can be explained with the help of Fig. (1), where the machine model is presented in a synchronous rotating reference frame.The inverter is omitted from the figure, assuming that it has unity current gain, that is, it generates currents a   For decoupling control, one can make a derivation of control equations of indirect vector control with the help of e e q d − dynamic model of induction machine (IM) [   Thus, the above analysis shows that the vector control strategy can provide the same performance as is achieved from a separately excited DC machine; this is done by formulating the stator current phasor, in the twoaxis synchronously rotating reference  To what extent this decoupling is actually achieved will depend on the accuracy of motor parameters used.It is easy to be noted that the calculation of the slip frequency in Eq.( 14) depends on the rotor resistance.Owing to saturation and heating, the rotor resistance changes and hence the slip frequency is either over or under estimated.
Eventually, the rotor flux e dr λ and the stator-axis current e qs i will be no longer decoupled in Eq.(10) and the instantaneous torque control is lost.Furthermore, the electromechanical torque generation is reduced at steady state under the plant parameter variations and hence the machine will work in a low-efficiency region.Finally, the variation of the parameters of moment of inertia J and the friction constant B is common in real applications.For instance, the bearing friction will change after the motor has run for a period of time [11].
Since the values of rotor resistance and magnetizing inductance are known to vary somewhat more than the other parameters, on-line parameter adaptive techniques are often employed to tune the value of these parameters used in an indirect field-oriented controller to ensure proper operation [2,11,12].The detuning effect,  (volt).One can observe the oscillatory response of the rotor flux linkage before it builds up to its steady state value (172.56V.).The flux response shows a large change when the load is suddenly changed.

Simulation of IFOC developed in Stationary Reference Frame:
This simulation is implemented to be familiar with indirect fieldoriented control and to observe the variables at every stage of the control.
It is easy to build the SIMULINK modeling for a current regulated PWM IFOC IM of Fig. (3) [13,14,15].In this simulation, reference dq currents are obtained according to the reference load torque and speed waveform.These dq reference currents are transformed into abc reference currents to be compared with the actual motor currents and the errors are fed to three hysteresis controllers to obtain reference voltages.In the next study, the machine is subjected to the same sequence of step changes in load torque as previously applied in perfect tuning,

5.Conclusion:
The implementation of IFOC technique has been performed and the following observations could be concluded: addition, the unit vector assures correct alignment of e ds i with the r λ ′ and e qs i perpendicular to it, as shown.The transformation and inverse transformation including the inverter ideally do not incorporate any dynamics and therefore, the response to

Fig. ( 2 )
Fig. (2) Phasor diagram explaining indirect vector control .Vol.26.No.1,2008Indirect Field Orientation Control of Induction Machine with Detuning Effect 268 frame, to have two components: magnetizing current component and torque producing current component; the generated motor torque is the product of two components.By keeping the magnetizing current component at a constant rated value, the motor torque is linearly proportional to the torque-producing component, which is quite similar to the control of a separately excited DC motor [7,8,9].Figure (3) shows an indirect fieldoriented control scheme for a current controlled PWM induction machine motor drive.The command values for the abc stator currents can then be computed as follows

Fig.( 3 )
Fig.(3) Indirect field-oriented control of a current regulated pwm inverter induction motor drive 3IndirectField Orientation Detuning:The success of FOC is based on the proper division of stator current into two components.Using the above d-q axis orientation approach, these two currents components are * ds i and The model equations of the IM, Eq.(2), in the stationary qd reference frame are modeled using SIMULINK[13,14].The simulation is set up for simulating the dynamic behavior of the motor with fixed-step type of step size (2e-6 sec).The results from these openloop operations will later be used as a benchmark to compare the performance of the same motor operated with fieldoriented control.In the model, three-phase voltages of base frequency ( b f =60 Hz) applied to the input are converted into two-phase stationary reference frame voltages.Once d-q phase voltages obtained, the associated flux and current are calculated and then applied to electromechanical and mechanical torque equations to obtain torque-speed responses.Based on the stationary reference frame model, Fig.(4) shows the waveforms of stator a-phase voltage, ag v , the quadrature stator current, qs i , the developed torque, em T , and the rotor speed r ω at no-load for a 20-hp motor.The figure shows that the speed is settled at approximately 0.2 sec., which is the same settling time for the stator current and developed torque.Also, the speed has a steady state value of 188.5 rad/sec., as shown.Torque vs. speed curve obtained from the same model is shown in Fig.(5) for no-load condition.

The proportional gain p k and integral gain ikFig. ( 7 ) 4 . 2
Fig. (7) Startup and load transients with field-oriented control Fig.(7).As compared to the perfect tuning case, the increased value of rotor resistance (1.5 r r′ ) could cause the responses of flux linkages, torque and current to be distorted, especially, at time of load exertion, as shown in Fig.(14).Also, at this time the speed deviation from its steady state value is larger than the case with kr=1.The situation with the decreased rotor resistance is shown in Fig.(15).The responses of Fig.(15) are run with kr=0.625; the minimum allowable value below which fluctuations will appear at the developed toque response at load rotor flux linkage, speed and current responses as compared to the perfect tuning case.exertion times.One can easily observe the amount of deviation in