If a shunt motor running with a load has its shunt field opened, how would this affect the speed of the motor?

In a shunt motor, the field windings are connected in parallel (or shunt) with the armature windings. When the shunt field circuit is opened, the magnetic field that the motor relies on to operate effectively collapses. This situation leads to a significant decrease in the field current, effectively reducing the strength of the magnetic field.

As the magnetic field weakens, the motor's back electromotive force (back EMF) will also decrease. According to the relationship between torque, speed, and back EMF in a DC motor, a decrease in field strength will cause the motor to draw more armature current for the same load. However, the decreased field strength results in a reduction of the counter electromotive force, allowing the armature speed to increase significantly.

Consequently, the motor will begin to accelerate and may run at dangerously high speeds due to the lack of sufficient magnetic braking. This is why the correct answer is that the motor will speed up when the shunt field is opened. The uncontrolled increase in speed can potentially damage the motor if not addressed.