Zero sequence braking of induction motor

[cnh1995]

I recently read about zero sequence braking of induction motor.
In zero sequence braking of induction motor, the stator phases are connected in series and a single phase ac supply is connected across the series combination. Thus, the currents in all three phases are co-phasal, which are called zero sequence currents. The resultant magnetic flux linking with the rotor creates a braking torque and the motor stops.

But how can there be a magnetic field in the air gap if all three phase currents are co-phasal? Their intantaneous magnetic fields will be equal and 120 degrees apart in space. So if three equal phasors are 120 degrees apart in space, their resultant comes out to be zero.
Am I missing something?

Thanks a lot in advance!

 

[anorlunda]

Where will the kinetic energy of the rotor go when braking? Only one place I can think of; $I^2R$ losses in the squirrel cage.

So the individual bars in the squirrel cage at any moment in time will be closer to one winding than the others. So the three windings won't cancel locally. A current will be induced, causing $I^2R$ losses. It matters not if the currents in different places reverse themselves seemingly aimlessly. Any current in any direction for any duration causes $I^2R$