TorqueSlip and TorqueSpeed CharacteristicsThe Torqueslip curve of an induction motor shows the variation of torque with the slip. We have If R_{2} and X_{20} are kept constant, then the torque τ will depend upon the slip s. The torqueslip characteristic curve is divided into three regions:
(a) Lowslip regionAt synchronous speed s = 0, the torque will be zero. When the speed is very near to synchronous speed, the slip is very low and (sX_{20})^{2} is negligible in comparison with R_{2}. Therefore, When k_{2} = k_{1}/R_{2} From the above relation, we can see that the torque is proportional to the slip. Hence, when the slip is small, the torqueslip curve is a straight line. (b) Mediumslip regionAs slip increases, the term (sX_{20})^{2} becomes large, so that may be neglected in comparison with (sX_{20})^{2} and Thus, the torque is inversely proportional to slip towards standstill conditions. We can represent the torqueslip characteristic by a rectangular hyperbola. For intermediate values of the slip, the graph changes from one form to another. In doing so, it passes through the point of maximum torque when R_{2} = sX_{20}. The maximum torque developed in an induction motor is called the pullout torque or breakdown torque. This developed torque is a measure of the shorttime overloading capability of the motor. (c) Highslip regionThe torque decreases beyond the point of maximum torque, and the result is that the motor slows down and then stops. At this stage, we should immediately disconnect the motor from the supply to prevent the damage due to overloading. The motor operates for the value of the slip between s=0 and s = s_{M}, where s_{M} is the value of the slip corresponding to maximum torque. For a typical induction motor, the pullout torque is 2 to 3 times the rated fullload torque. Thus, the motor can handle short time overload, without stalling. The starting torque is 1.5 times the rated fullload torque. The figure drawn below shows the torqueslip curves and torquespeed curves: Figure: Torqueslip Curves Figure: Torquespeed Curves
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