Magnetic field in a wound rotor induction motor
Move the slider "STATOR ELECTRIC ANGLE" and see how the stator and rotor magnetic fields  and  rotate at the same speed. The rotor lags behind them (you can change the lagging by the slider SLIP, the higher the slip the slower the rotor. The rotor stops for . 
Take notice that rotor current frequency is  times the stator current one (see sinusoids at the left). For  the rotor rotates backwards with respect to the magnetic field (dissipative braking operation), for  the rotor rotates faster than magnetic field ().