Small point of clarification:
An induction motor under load _never_ runs at synchronous speed. Slowing an induction motor down by producing larger slip is not necessarily an overload condition.
An induction motor _requires_ slip to induce current in the rotor conductors, and so will always operate below synchronous speed under load. (Or, if the load is overhauling, the machine will operate above synchronous speed, but will produce negative torque and act as a generator.)
-Jon
