Fig. 22 : Plot of 3 stator current vs. time
The Fig. 23 shows the variation of slip vs. time characteristics for a speed of 100 r/s (955 rpm). From this simulation result, we infer that the IM attains the set reference speed of 955 rpm in 0.44 secs using the ANFIS (neuro-fuzzy) controller. At that instant,the slip being 0.46, can be verified from the result shown. Note that the slip decreases from 1.0 to 0.46 linearly in a time span of just 0.44 secs.
The slip-speed characteristics is shown in the Fig. 24. It can be noted that when the speed is varied from 0 to the rated speed, the slip decreases, i.e., the slip is inversely proportional to the speed,which is the property of the IM. When the speed is zero, the slip is 100 %, while the IM is operating at near the rated speed (180 r/s),the slip is very very low.
The plots of the direct axes (id) & quadrature axes currents (iq) versus time is shown in the Figs. 25 & 26 respectively. From these figures, it can be inferred that the machine reaches the set reference speed of 955 rpm in a time interval of 0.44 secs.
The variation of the 3 rotor currents (ir - abc) with time is shown in the Fig. 27. It can be inferred that at lower speeds, the slip is more, the flux required to develop the suitable torque is also more. Also, the torque required to reach the set speed is also more. Hence, the magnitude of the rotor currents will also be more during the transient periods (starting periods) of the induction motor. When the speed is reaching the set value from zero, the 3 rotor currents decreases exponentially upto 0.44 secs, thereafter it is maintained constant.
Fig. 27 : Plot of rotor current ir abc vs. time
The 3 rotor currents (ir - abc) is transformed to direct axes & quadrature axes currents using the d - q transformation techniques and the variation of the transformed currents with time is shown in the Fig. 28. Here, only two phases (d & q axes) of the currents can be observed in the characteristic curve. In this case, also,once the motor achieves the the set speed at 0.44 secs, it requires a nominal current to drive the IM system. The plot of rotor angle vs. time is shown in the Fig. 29.