Speed  of the Shunt Wound D.C. Motor increases as the field supply is weakened. field supply.
Theoretically motor runs at infinite speed if run without field supply.
This break opens the armature coils from the commutator
and totally damages the prime mover.
 The FIELD FAILURE RELAY, when used in series with the field circuit, ensures that no power is supplied
to the armature unless Field Circuit is complete. This is also used as safety guard in D.C. Clutches,
Solenoids and Brakes. Different ranges are available to suit the field current.