I'm currently studying the induction motor and I have some troubles to understand which causes this thing.
Let $$w_2 = w_1 - w_r$$ be the slip velocity and $$s = w_2 / w_1$$ be the slip, where w_1 is the speed of the stator magnetic field and w_r the speed of the rotor electromagnetic variables. Let also the amplitude of the three voltages applied to the motor be constant. Why is the characteristic T vs w_2 constant with respect to the frequency of the applied voltages?
I'm more interested in a qualitatively explanation than a proof that involves only calculus but both are welcome.