What happens when the field of a shunt-wound DC motor opens while the machine is operating under no load?

What happens when the field of a shunt-wound DC motor opens while the machine is operating under no load?

• A. The motor would race, and if unchoked, might destroy itself
• B. The motor would quit running without damage
• C. The motor would burn out the field coils
• D. None of the above

Answer: (A)

When the field of a shunt-wound DC motor opens while it is operating under no load, the field winding is effectively removed from the circuit, which eliminates the magnetic field that is necessary for normal operation. In this scenario, because the armature winding remains energized, the motor would begin to operate as a no-load condition. With the field winding open, the motor loses its field strength, and as a result, it experiences an increase in speed, sometimes referred to as "racing." This happens because the armature is still being energized, but with no magnetic field to provide the necessary counter torque. If unrestricted, this can lead to destructive conditions for the motor, including excessive speeds that can cause mechanical damage or failure. In contrast, if the load were present or if there were a speed limitation device ("choke") in place, the negative impact on the motor would be mitigated. However, under light load or no load conditions, the risk of racing is significantly heightened, making it crucial to understand the operational limits of shunt-wound motors to prevent potential damage.

When the field of a shunt-wound DC motor opens while it is operating under no load, the field winding is effectively removed from the circuit, which eliminates the magnetic field that is necessary for normal operation. In this scenario, because the armature winding remains energized, the motor would begin to operate as a no-load condition.

With the field winding open, the motor loses its field strength, and as a result, it experiences an increase in speed, sometimes referred to as "racing." This happens because the armature is still being energized, but with no magnetic field to provide the necessary counter torque. If unrestricted, this can lead to destructive conditions for the motor, including excessive speeds that can cause mechanical damage or failure.

In contrast, if the load were present or if there were a speed limitation device ("choke") in place, the negative impact on the motor would be mitigated. However, under light load or no load conditions, the risk of racing is significantly heightened, making it crucial to understand the operational limits of shunt-wound motors to prevent potential damage.