V/Hz ratio In Electric Machines

V/Hz ratio is the ratio of voltage to Frequency in Electric Machines.  In Electric Machines such as motors, generators and transformers, the drop in frequency results in a decrease in the reactance of the stator winding and the core.
In a motor, when the frequency drops, the stator reactance reduces.  This causes a higher current to flow for the same voltage.  This can result in increased flux and increased torque. 
In the machine core, the reduction in reactance can cause more eddy currents to flow.  This can result in over heating and damage to the laminations in the core of the stator.  This can permanently damage to the machine. 
This is a condition known as overfluxing. 
Overfluxing protection is an important part of any protection scheme. 
V/Hz is a mode of control in AC drives.  For a motor to run at constant torque, its flux should be constant.  This is achieved by the V/Hz control.  Since the flux can change when the frequency drops, the voltage also reduces during reduced frequency.  This ensures that the flux in the stator field of the motor is always constant.