Free Fall In EOT Crane

An induction motor produces torque in proportion to "slip," the difference between line frequency and motor frequency. Torque is produced equally well in either of the two conditions: motor faster than line frequency (braking torque); motor slower than line frequency (driving torque).

Induction motors are called asynchronous, meaning the supply frequency and the rotor induced frequency are not the same when the motor is producing torque.

http://en.wikipedia.org/wiki/Induction_motor

Most have a secondary internal device like a friction clutch to prevent free fall. It is a safety device. Has nothing to do with the electrical supply or frequency.

The only ones that will free fall are those that are installed where there is no possibility of person being under it. Which are very few areas of use.

Way back, Cutler-Hammer (now part of Eaton Corp.) had something called the Opotor system...an opposing torque system which would come into operation during lowering injecting an appropriate opposing torque to the motor to slow it down (slip-ring motor, one assumes). It could be fine-tuned for very soft lowering when used in hot-metal cranes. i am sure it has gone highly electronic by now, but some of the old products are alive still ! http://www.eaton.com/ecm/groups/public/@pub/@electrical/documents/content/ca05801001e.pdf

Yes, you are very near to answer. Actually rotor speed exceeds the Ns (Sync. Speed). And it will behave as induction generator and torque generated will act as brake.