Answer / ashhad khan

if armature wdg is opened then rotor will start deaccelerating normally and motor will stop eventually. you can think it as a small DC permanent magnet motor supply disconnected suddenly. it will jst stop rotating. because in our case also field winding is connected to supply so magnetic field will be there continuously but our armature is disconnected from supply so it will stop generating magnetic field which would have forced rotor to rotate.

question should be what would happen if fiel supply is disconnected suddenly in a running motor.
because in this case concept of back emf comes .As field vanishes so back emf becomes zero and we know eq. v=Eb+IR so current will become large enough to burn out the armature winding of motor

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