The stator and rotor cage windings of a squirrel cage induction machine may exceed design temperatures due to a number of reasons, some of which are as follows:

a) The machine may lock up, i.e., remain at zero speed with voltage applied to the stator winding, as
might happen in the case of low starting voltage and/or mechanical malfunction.

b) The machine may start but fail to accelerate to its running speed, due to inadequate accelerating torque at some speed lower than the breakdown torque point, causing it to run at that subsynchronous speed.

c) The machine may be overloaded continuously at close to its operating speed.

Under the locked-rotor condition, there is normally no ventilation, and the heat loss from the windings is by conduction and radiation. During acceleration, depending on the speed, the heat loss is both by conduction and by the ventilating effect of air movement. During running overloads, the normal ventilation of the machine is the primary mode of cooling.

Thermal limit curve
A thermal limit curve is a plot of the maximum permissible safe time versus line current in the windings of the machine under conditions other than normal operation. It represents the following three situations:
a) Locked rotor
b) Starting and acceleration
c)      Running overload

Thermal Limit Curve Squirrel Cage Motors
The complete curve, representative of these three conditions for the winding (stator or rotor) with the shortest safe time, may be discontinuous and may consist of up to three segments.

The thermal limit curve is intended to be used in conjunction with the machine time-current curve for a normal start to set the machine protective devices for the thermal protection of the machine during starting and running conditions. The machine time-current curves, when available, are presented on the same plot as the thermal limit curves.

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