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

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.



Figure 1 Three-level diode clamped inverter

Figure 2 Block diagram of the controller


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
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.

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