In conventional DTC, to regulate the flux and torque value in
comparison with the reference value the appropriate voltage
vector based on the 2-level inverter is selected. However, the 2-
level inverter has some limitation due to the blocking capability of
the power semiconductor switches. Besides that, it can produce
the higher stress on the switching devices because the 2- level
inverter has only six voltage vectors to be selected for torque
demand. Therefore, the Cascaded H-bridge Multilevel Inverter is
chosen to overcome the problem of two-level inverter. The basic
power circuit of Cascaded H-bridge multilevel inverter contains
four high switching devices, i.e. IGBT/MOSEFT to control the
on/off of the gate signal. In ensuring the proper sequence of turn
ON/OFF of the devices, the switch Sa1+and Sa2- are on and
switch Sa1- and Sa2+ are off to produce +Vdc, whereas, in case
of switch Sa1- and Sa2+ are on and switch Sa1+and Sa2- off, the
output voltage is equal to -Vdc. The same switches in similar leg
should not be turned on to prevent short circuit on DC source. The
topology of three phases cascaded H-bridge multilevel inverter of
induction machine shown in Fig.4 is implemented to improve the
performance of the system and provide the excellent power
quality. Each phase motor has four IGBT to produce the current
on winding phase and single DC circuit. By implementing this
topology, a total vector 27 voltage vector can be generated,
including the zero and active voltage vector. However, in this
paper, only 19 had selection of active vectors are used to control
the flux and torque regulation based on the operating condition of
motor as in Fig.5. These voltage vectors have three different
amplitude vectors. There numbers of switching state which is;
Sa1+, Sa1, Sb1+,Sb1-,Sc1+,Sc1-. The each angle between each voltage vector is 30 degrees.