Induction Motors

Induction Motors

Figure 1: The graph of Torque against Power input

The graph in figure 1 above shows the variation of torque in the motor with gradual increase in power input. It is evident from the graph that there is a rapid reduction in torque of the motor as the power input into the motor gradually increases. However, this gradual reduction in torques eventual levels. Further increase in power input into the motor does not result in any change in the motor torque (Lang & Das, 2009). The relationship depicted by the graph above is common to most squirrel induction motors currently in use. The reduction in torque may be attributed to effects of slip.

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Figure 2: The graph of Line voltage against Torque.

A shown in the graph in Figure 2 above, the torque of the motor reduces rapidly initial with an increase in line voltage. After some time, the reduction in torque is gradual as line voltage is increased. As suggested earlier in figure 1 above, the reduction in the rate of reduction of torque may be attributed to effects of slip in the system. The variation of torque with line voltage can be considered as linear in the beginning but afterwards, the trend is not linear.

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Figure 3: Graph of Speed against Torque.

The graph shown in figure 3 above represents the variation of torque produced in a squirrel cage motor with speed. Initially, the speed of the motor gradually increases as the torque of the motor reduces. The reduction in speed as discussed earlier is attributed to an effect called slip. According to Yadamale (2003), it has also be found theoretically that this effect also depends on the frequency of the AC power used to power the induction motor.

Induction Motors 3

Figure 4: The graph of Torque against speed

The graph shown above represents the variation of Torque produced by the motor against speed. As shown in the graph, the variation between the two parameters are not linear. This is attributed to the fact that the motor has to change its polarities in a more rapid manner to counteract the increasing level of slip. The increasing number of times that the polarities has to change means that there is increased frequency in the system. Therefore, the frequency increases in the system is meant to increase current into the system and thus help counteract the effect of increase in slip. All these effects makes the variation of current to be non-proportional to slip within the motor. Thus the nature of the graph.


Yedamale, P. (2003). Brushless DC (BLDC) motor fundamentals. Microchip Technology Inc20, 3-15.

Lang, J. H., & Das, S. (2009). Motors and Generators. In Multi-Wafer Rotating MEMS Machines (pp. 325-404). Springer US.