Direct Torque Control (DTC) has become a favorite method for the control of induction motor drives as it supply a fast dynamic torque reaction and strength to machine parameter difference. Hysteresis band control is the one of the simplest and most popular method used in DTC of induction motor drives.
still the normal direct torque control has a variable switching density which causes serious problems in DTC. This paper now the DTC of induction motor with a constant switching density torque controller. By this method constant switching density operation can be produce for the inverter.
Also the torque and flux ripple will get decreased by this method. The practicability of this method in minimizing the torque ripple is validation through some simulation results.
- Direct torque control(DTC)
- Constant switching frequency
- Induction motor
- Three phase inverter.
Fig. 1. Block diagram of conventional DTC
EXPECTED SIMULATION RESULTS
Fig. 2. Step response of torque (a) hysteresis based (b) modified torque controller
Fig. 3. Response of torque and speed for squre wave torque reference in (a) hysteresis based (b)modified torque controller
Fig. 4.(a) Hysteresis based controller (b) modified torque controller
Fig. 5. flux waveform for (a) hysteresis based (b)modified torque controller
Fig. 6. flux locus for (a) hysteresis based (b)modified torque controller
Fig. 7. Frequency spectrum of the switching pattern Sb for (a) hysteresis based (b) modified torque controller
This paper now a constant switching density torque controller based DTC of induction motor drive. By using the modified torque controller the switching density of the inverter also becomes constant at 10 kHz. As a result, the harmonic contents in the phase currents are very much decreased. So the phase current distortion is decreased.
The torque ripple is also decreased by replacement the torque hysteresis controller with the modified torque controller. Moreover, with the modified torque controller, an almost circular stator flux position is get. Without give up the dynamic work of the hysteresis controller, the modified scheme gives constant switching density. This work can be achieve using DSP.
The work can be carry on by increasing the switching density above audible range, i.e. more or equal to 20 kHz. This is an creative way to shift the PWM harmonics out of human audible density range. With high switching density the harmonic content of stator current will be decreased easily.
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- Tang, L.Zhong, M.F.Rahman, Y.Hu,(2002)“An Investigation of a modified Direct Torque Control Strategy for flux and torque ripple reduction for Induction Machine drive system with fixed switching frequency”, 37th IAS Annual Meeting Ind. Appl. Conf. Rec., Vol. 1, pp. 104-111.
- J-K. Kang, D-W Chung, S. K. Sul, (2001) “Analysis and prediction of inverter switching frequency in direct torque control of induction machine based on hysteresis bands and machine parameters”, IEEE Transactions on Industrial Electronics, Vol. 48, No. 3, pp. 545-553.
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