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