Which of the following is NOT a common cause of motor overheating?

Motor overheating can result from various operational issues and environmental factors. Undervoltage, high ambient temperature, and poor ventilation are all scenarios that can lead to excessive heat production in electric motors.

Undervoltage results in the motor drawing higher current than normal to maintain its power output, which increases heat generation. High ambient temperatures can significantly affect motor performance since they might prevent efficient heat dissipation from the motor. Poor ventilation restricts airflow, limiting the ability to cool the motor while it's operating, thereby causing it to overheat.

On the other hand, low hum frequency, while it may be associated with certain operational characteristics of motors, does not directly contribute to overheating in the same way the other factors do. It’s more related to the quality of motor operation rather than an increase in temperature. Therefore, identifying low hum frequency as a cause of overheating is less accurate compared to the other options listed.