Torque-Slip and Torque-Speed Curves

So far, we have understood that the value of the torque is calculated by the following expression:

With the mentioned value, we can say:

  1. Torque is directly proportional to slip s, which means torque is zero when slip is zero

2.a  When the value of slip ‘s’ is very low, it will state that torque T is proportional to the slip. The condition of the proportion will be realized when rotor resistance R2 is constant. When slip‘s’ is very low, it will lead to smaller value of the term sX2 which becomes negligible as compared to R2. The torque-speed and torque-slip will curve when speed is almost near to the synchronized speed.

      2.b   When the speed decreases in the presence of increasing load, the slip ‘s’ will also increase which leads to rise in Torque. The value of the Torque will reach the maximum value when slip‘s’ will be equal to fraction of R2 and X2, i.e.

When the Torque reaches the maximum value, it is termed as, “pull-out” and “break-down” torque.

2.c When you further increase the slip, it will result into decreasing value of Torque. When the slip’s’ is increased, the motor work under maximum value of torque corresponding to the value of slip. Considering the equation,

This happens when the standstill reactance will become constant

Although we mentioned that torque is independent of the value of rotor resistance, R2 as seen the figures, we can state that the location of Tm is dependent of the value of R2. Therefore, we can say that when R2 increases the value of the slip where the maximum torque is observed also occurs at the same page.

As seen in the 2nd equation and both the figures, the rectangular hyperbola of speed-torque or slip-torque is observed when the speed or slip goes beyond the maximum torque value. In addition to the same, it is also observed in the figures that torque –speed and slip curves with various values of Rotor resistance R2.


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