The curve assigned to the electric torque against slip can be divided into approximately three regions: The first is the region of small slip of the curve, in which it develops with increasing load and the mechanical speed of the rotor decreases with the load. The power factor of the rotor will be almost unitary, even if the rotor current rises with slipping. The normal operating range of the induction motor is within this linear region of low slip. The second region of the induction motor curve may be called the moderate slip region in which the rotor frequency is higher than before and the rotor reactance is of the same order as the rotor resistance. The maximum torque of the motor lies at the point where the increase in the rotor current is balanced exactly with the reduction of the rotor power factor.
The third region of the induction motor curve is of great slip, in which the induced torque actually softens with the increase of the load, because the increase in the rotor current is not remarkable due to the decrease in the power factor of the rotor.