ABSTRACT
Direct Torque Control (DTC) has become a popular
technique for the control of induction motor drives as it provides a fast
dynamic torque response and robustness to machine parameter variations. Hysteresis
band control is the one of the simplest and most popular technique used in DTC
of induction motor drives. However the conventional direct torque control has a
variable switching frequency which causes serious problems in DTC. This paper
presents the DTC of induction motor with a constant switching frequency torque
controller. By this method constant switching frequency operation can be achieved
for the inverter. Also the torque and flux ripple will get reduced by this
technique. The feasibility of this method in minimizing the torque ripple is verified
through some simulation results.
KEYWORDS
1.
Direct torque control(DTC)
2.
Constant switching frequency
3.
Induction motor
4.
Three phase inverter.
SOFTWARE: MATLAB/SIMULINK
BLOCK DIAGRAM:
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
CONCLUSION
This
paper presents a constant switching frequency torque controller based DTC of
induction motor drive. By using the modified torque controller the switching
frequency of the inverter also becomes constant at 10 kHz. As a result, the harmonic
contents in the phase currents are very much reduced. So the phase current
distortion is reduced. The torque ripple is also reduced by replacing the torque
hysteresis controller with the modified torque controller. Moreover, with the
modified torque controller, an almost circular stator flux locus is obtained.
Without sacrificing the dynamic performance of the hysteresis controller, the
modified scheme gives constant switching frequency. This work can be
implemented using DSP. The work can be extended by increasing the switching frequency
above audible range, i.e. more or equal to 20 kHz. This is an effective way to
shift the PWM harmonics out of human audible frequency range. With high
switching frequency the harmonic content of stator current will be reduced
significantly.
REFERENCES
[1] John
R G Schofield, (1995) “Direct Torque Control – DTC”, IEE, Savoy Place,
London WC2R 0BL, UK.
[2] L.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.
[3] 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.
[4] D.Casadei,
G.Gandi,G.Serra,A.Tani,(1994)“Switching strategies in direct torque control of
induction machines,” in Proc. Of ICEM’94, Paris (F), pp. 204-209.
[5] J-K.
Kang, D-W Chung and S.K. Sul, (1999) “Direct torque control of induction
machine with variable amplitude control of flux and torque hysteresis bands”, International
Conference on Electric Machines and Drives IEMD’99, pp. 640-642
