The direction of the rotating magnetic fi eld in an induction motor (or any other type of ac motor) can be reversed by changing the supply sequence of the phases to the stator poles. This is termed phase-switching. An induction motor can be decelerated quickly in this manner. This is known as "plugging" an induction motor. The slip versus torque relationship of an induction motor may be expressed as
Show that the same relationship holds under plugged conditions, except that k(S) has to be replaced by -k(2 - S). Sketch the curves k (S), k(2 - S), and -k(2 - S) from S = 0 to S = 2. Using these curves, indicate the nature of the torque acting on the rotor during plugging.