Carefully observe the curve below to have an insight into the situation.
The peak is the Pull-Out torque in the curve or slip torque characteristics. To the right of the peak is the Stable region of operation and to the left is the unstable region of operation i.e. the motor cannot operate in unstable region. The slant lines are the load torque lines taken for demonstration purpose only, they are different for different types of loads.
Under such a scenario, wherever the Motor torque curve and the load torque line meet, in the STABLE REGION, that is the point of operation of the motor i.e. the speed at which the motor operates for a particular value of load. Now, if we go on increasing the load, as we can see from the curve above, after the point of maximum torque, the load torque and the motor torque characteristics do not meet. This shows that there exists no point of operation for the motor. Thus, the motor decelerates and stops.
For this reason, it is called the Pull-Out Torque since the motor pulls out of operation.
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