Direct torque control of squirrel cage induction motor for optimum current ripple using three-level inverter Best Electrical Engineering Projects

ABSTRACT:
Three-level neutral point clamped (NPC) inverters have been widely used in medium voltage applications. In this study, direct torque control strategy for sensorless squirrel-cage induction motor (SQIM) using NPC three-level inverter is presented. The proposed control strategy optimises the current ripple in steady state and also gives a high dynamic performance of torque in transient state. The advantage of having low dv/dt for three-level inverter output voltage is retained. The control strategy is verified on a 7.5 hp SQIM drive with three-level insulated-gate bipolar transistor (IGBT) inverter. Experimental results validate the steady state and the dynamic performance of the proposed control strategy.

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:


Figure 1 Three-level diode clamped inverter


Figure 2 Block diagram of the controller

EXPECTED SIMULATION RESULTS:


Figure 3 Trajectory of the stator flux in stationary α –β Plane


Figure 4 Experimentally estimated rotor flux (Ѱrsα, Ѱrsβ) at a frequency of 5 Hz

Figure 5 Three-level motor line voltage


Figure 6 Torque reference and actual torque ripple

Figure 7 Steady state speed and line current waveforms (a) Experimental waveform of speed (ωe)
and line current (is) at steady state (b) Zoomed waveform of line current showing the ripples


Figure 8 Experimental waveforms of speed (ωe) and line current (is) at steady state at 14 Hz

Figure 9 Torque reference (m*d) and actual torque (md) during transient
CONCLUSION:
This paper has introduced a direct torque and flux control for SQIM using neutral point clamped three-level inverter. The operation of the drive is divided into two regions – low frequency operation and high-frequency operation. Separate switching strategies are proposed for these two regions of operation. The proposed strategy optimises the switching to have minimum dv/dt in the motor line voltages. It also optimises the current ripple in steady state and gives a fast torque response during transient. There is an effective reduction in the dv/dt applied to the motor terminals. This in turn reduces the bearing current and shaft voltages [14, 15] of the motor thereby increasing the life of the motor. Experimental results using SQIM and three-level inverter validate the proposed strategy.

REFERENCES:
[1] NABAE A., TAKAHASHI I., AKAGI H.: ‘A new neutral point clamped inverter’, IEEE Trans. Ind. Appl., 1981, IA-17, (5), pp. 518–523
[2] LAI J.-S., PENG F.Z.: ‘Multilevel converters – a new breed of power converters’, IEEE Trans. Ind. Appl., 1996, 32, (3), pp. 509–517
[3] MCGRATH B.P., HOLMES D.G., LIPO T.A.: ‘Optimized space vector switching sequences for multilevel inverters’, IEEE Trans. Power Electron., 2003, 18, (6), pp. 1293–1301
[4] BUJA G.S., KAZMIERKOWSKI M.P.: ‘Direct torque control of PWM inverter-fed AC motors – a survey’, IEEE Trans. Power Electron., 2004, 51, (4), pp. 744–757
[5] CASADEI D., SERRA G., TANI A., ZARRI L.: ‘Assessment of direct torque control for induction motor drives’, Bull. Pol. Acad. Sci., Techn. Sci., 2006, 54, (3), pp. 237–254

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