III. FUZZY LOGIC BASED SYSTEM
In the classical DTC Induction motor drive there are torque
and flux ripples because none of the inverter switching vectors
is able to produce changes in both the stator flux and torque.
However here we use duty ratio control scheme, which do
not involve inverters using large no. of switches.
By using fuzzy logic based system DTC system, it is possible
to perform a fuzzy logic based duty-ratio controller, where
optimum duty ratio is determined every switching period. The
suggested technique is based on applying to the inverter the
selected active states just enough time to achieve the torque
and flux reference values[7]. The rest of the switching period a
null state is selected which won’t almost change the torque and flux reference values. Therefore a duty ratio has to be
determined each switching time.
By varying the values of duty ratio between the extremes (0 or
1) it is possible to apply any voltage to the motor.
In order to obtain a better reduction in torque and stator flux
ripples, stator flux has to be adopted to an optimum value,
which should be large enough to produce the desired torque.
The reason is that which this optimum value the increase in
active state is large enough and the slight reduction in null
state is lower[8].Moreover, the reduction in reactive power
consumption from supply mains is achieved.