Depending on the size of the motor used and the load it is attached to, motor current and the degree with which it changes under various load conditions can vary greatly. You would need to do some experimentation with the configuration you are working with. Place the motor and load under varying conditions and monitor current etc. Modern servo systems use a combination of current, voltage and speed measurement to determin the running condition of a motor. For breaking a motor under controlled conditions a number of methods are used. For AC motors, the most common methods are, resistive regeneration and DC injection.
On DC motors, resistive regeneration is commonly used.
If you inject DC to one or more of the windings of an AC motor, the end result is the cumulative magnetic field generated in the windings in which DC has been injected work against the AC field of the other windings.
Resistive regeneration is a system whereby you disconnect one of the windings of the motor from the supply, then connect this winding to a resistive load that acts as a current "dump". As the motor continues to spin
the winding connected to the resistive load is in effect a generating circuit. Again the resulting magnetic field associated with this winding works against the other fields and thus causes this breaking action.
I work with machinery that have 7Kw spindles running at upto 22,000 RPM.
Using the resistive regeneration techniques above, these can be brought to a complete halt within 1 to 2 seconds. All in a controlled manner.
Keep in mind these systems use high speed DSP processing in the control of the motors and are pretty much out of the question for the general hobbyist or low end user. As a matter of interest, the controllers use IGBT's for output switching, 3 modules, each costing around $2000. So $6000 just for transistor switching modules, you can see the cost of these units can be extremely high..
Jaba