A slip ring motor cannot be started direct on-line with its rotor windings short-circuited, otherwise it would cause unacceptable current peaks.
Resistors must therefore be inserted in the rotor circuit and then gradually short-circuited, while the stator is powered at full mains voltage.
The resistance inserted in each phase is calculated to ascertain the torque-speed curve with strict accuracy. The result is that it has to be fully inserted on starting and that full speed is reached when it is completely short-circuited.
The current absorbed is more or less proportional to the torque supplied at the most only a little greater than the theoretical value.
For example, for a starting torque equal to 2 RT, the current peak is about 2 RC. This peak is thus much lower and the maximum starting torque much higher than with a squirrel cage motor, where the typical values are about 6 RC for 1.5 RT when directly connected to the mains supply. The slip ring motor, with rotor starting, is the best choice for all cases where current peaks need to be low and for machines which start on full load.
This kind of starting is extremely smooth, because it is easy to adjust the number and shape of the curves representing the successive steps to mechanical and electrical requirements (resistive torque, acceleration value, maximum current peak, etc.).