Question

When and why field current control method is preferred, below rated speed or above rated speed?

Answer 1

speed of dc motor can be controlled by controlling flux per pole in the circuit. this controls the magnetic field associated with the current flowing in the circuit and hence known as field control method.
in this method current is decreased which decreases the flux linked with it. as the speed is inversely proportional to the flux, the speed of the motor is increased. thus motor can be operated above normal rated speed by using this method.