If the field of a DC shunt motor gets opened while the motor is running, then the

• A. Armature current will drop
• B. The motor will attain the dangerously high speed
• C. Armature will oscillate about original speed as the mean speed
• D. Motor will become slow

Answer: (B)

In dc machine the field coils or field winding is excited by the current in order to produce the magnetic flux. In a DC shunt machine, the speed is Directly proportional to the back EMF and Inversely proportional to the flux.
N ∝ Eb/φ
Now if the field winding gets open than flux will become zero i.e φ = 0
∴ N ∝ Eb/0 
or
N = ∞
Hence the speed of the DC shunt Motor will attain the dangerous High Seed.
 
If the main field of a shunt motor or a compound motor is extremely weakened or if there is the complete loss of main field excitation, a serious damage to the motor can occur under certain conditions of operation.
Since the speed of a dc motor is inversely proportional to flux, its speed tends to rise rapidly when the flux is decreased. If the field failure occurs on a motor which is coupled to a load, which can neither be removed nor be reduced to a very low value, the residual flux due to the open field will develop a torque which will not be able to sustain rotation. Thus the motor will stall, heavy current will be drawn from the mains and the overload relay will trip.
On the other hand if field failure failure on an unloaded motor or if the application is such that it permits the motor to be unloaded or overhauled as in the ease of a hoist, the motor will not stall but instead its armature will accelerate quickly to a mechanically dangerous high speed or there will be destructive commutation. To prevent the above situation of over speeding, a field failure relay is used.