Refer to Figure 2.1 where
T,, = starting torque or breakaway torque.
T, = minimum, pull-in or pull-up torque.
Tpo = pull-out, breakdown or maximum torque, obtainable over the entire speed range.
In a good design this should occur as close to the rated slip as possible to ensure that the motor runs safely, even during momentary overloads, load fluctuations exceeding the load torque, or abrupt voltage fluctuations, without harmful slip losses.
In some specially designed rotors, however, to achieve a high starting torque sometimes the pull-out torque Tpo may not be available on the speed-torque curve.
It is possible that in such cases the T,, may be the highest torque developed by the motor in the entire speed range (Figure 2.2).
T, = rated or the full-load torque and should occur as near to the synchronous speed as possible to reduce slip losses.
S = rated slip at which occur the rated torque and current.