DC shunt motor with open field circuit

If the field circuit of a running DC shunt motor opens, the flux falls suddenly to the small residual flux. Since $N \propto \dfrac{E_b}{\phi}$ and $T \propto \phi I_a$, both speed and current change drastically.

What happens

1. Flux collapses: $\phi$ drops to residual value, maybe 2-5% of normal.
2. Back emf drops: at that instant speed cannot change (inertia), so $E_b = k\phi N$ falls almost to zero.
3. Armature current rises sharply: $I_a = \dfrac{V - E_b}{R_a}$ approaches $V/R_a$, many times the rated value, like a start without a starter.
4. Torque and speed:
   • With a light load, the large $I_a$ times the small $\phi$ still gives enough torque, so the motor accelerates rapidly. To build enough back emf with tiny flux, speed must rise to a very high value. The motor runs away (over-speeds).
   • With a heavy load, the torque $k\phi_{res} I_a$ may be less than load torque. The motor slows or stalls while drawing a very large current.

Implications

• Mechanical damage: centrifugal forces at over-speed can throw out armature windings, damage the commutator and bearings, and burst the rotor.
• Electrical damage: very large armature current overheats the winding and insulation.
• Heavy sparking and flashover at the commutator due to large current and weak field (poor commutation).
• Supply disturbance: fuses/breakers trip; voltage dip affects other loads.
• Safety hazard to persons nearby and to the driven machine.
• Inductive kick: opening a highly inductive field can produce a high voltage arc at the break, damaging the field insulation.

Protection

• A three-point starter has its no-volt coil in series with the field, so a field break releases the handle and disconnects the motor.
• Field-failure relays, over-speed trips, and overload/over-current relays or circuit breakers.
• The field circuit should never contain a fuse or switch that can open it alone.

 N, Ia
  |          ___ speed (runaway)
  |        _/
  |      _/   ___ armature current
  |    _/   _/
  |___/___/__________________ time
      ^ field opens