From the variation of flux with time as shown in the Fig. 19, it can be observed that when the motor speed is increasing (during the transient period), more stator current is required to develop the requisite flux in the air gap. Hence, the flux also starts increasing during the transient period (0 to 0.4 sec) exponentially. Once, the motor attains the set rated speed, the flux required to develop the torque almost remains constant after O 0.4 secs. Once, the flux in the air gap remains constant, the variation of the load torque and speed will not disturb the flux curve. Hence, the IM will be operating at a constant flux.
Torque characteristics for a set reference speed of 100 r/s (955 rpm) is shown in the Fig. 20. From this figure, we arrive at a conclusion that when the motor is operating at lower speeds, the slip is more. Hence, the machine requires more torque to attain the set speed. Once the machine reaches the set speed of 955 rpm the average torque of the machine becomes nearly zero after 0.44s, which is justified from the simulation result in Fig. 21. The terminal voltage of the IM is shown in Figs. 21 (a) & (b) respectively.
(a) Plot of voltage vs. time (normal) (b) Plot of voltage vs. time(zoomed) between t = 0.42 s to 0.45 s
Fig. 21 : Plot of voltage vs. time (normal & zoomed)
The variation of the 3 stator currents (is) with time is shown in the Fig. 22. It can be clearly observed from this figure, 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 stator 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 stator currents decreases exponentially. Once, it attains the set speed at 0.44 secs, it requires a nominal stator current to drive the IM system.