What would most likely cause a motor to shut down unexpectedly during operation?

The most likely cause for a motor to shut down unexpectedly during operation is excessive load on the motor. When a motor experiences more load than it was designed to handle, it can lead to overheating and strain on the internal components. This situation is generally monitored by an overload protection device, which is designed to interrupt power to the motor if it exceeds its rated current. If this protection mechanism is activated, the motor will shut down to prevent damage. Thus, excessive load is a common reason for a shutdown, as it can lead to the motor reaching its thermal limits or triggering protective devices.

In contrast, activating the holding contact circuit typically maintains power to the motor rather than causing it to shut down. A properly functioning overload reset would not lead to a shutdown but instead allows the motor to resume operation if conditions are safe. Continuous power supply maintains the motor's operation, making it unlikely to cause an unexpected shutdown. Therefore, the most plausible scenario for an unexpected motor shutdown is indeed an excessive load.