If a DC motor runs faster than designed, with all other conditions being normal, what could be the possible cause?

Correct Answer: Option B — open shunt field coil

An open shunt field coil eliminates the main field flux, severely reducing counter-electromotive force (CEMF). To maintain torque equilibrium, speed increases drastically. Per IEEE 45 and 46 CFR Subchapter J, this runaway overspeed condition poses a severe mechanical hazard requiring immediate trip-out.