Electric fans typically have a motor that drives the rotation of the fan blades. When the fan is initially turned on, the motor starts at its maximum speed. As the motor runs and the fan blades rotate, the fan assembly starts to warm up due to the energy dissipated as heat.
The motor of an electric fan is designed with certain operating parameters, including temperature limits. As the fan assembly heats up, the internal components, such as the motor windings, also warm up. The resistance of the motor windings increases with temperature, causing a reduction in the overall electrical current flowing through the motor.
With a decrease in current, the motor's torque output decreases, resulting in a reduction in rotational speed. This reduction in speed is particularly noticeable in fans that have multiple speed settings. The highest speed setting may be designed to compensate for the expected decrease in speed as the fan warms up, aiming to maintain an acceptable level of airflow.
Furthermore, the decrease in speed as the fan heats up helps to prevent overheating of the motor and other components. It allows the fan to operate within its safe temperature range and avoids potential damage that may arise from excessive heat buildup.
Overall, the initial high speed and subsequent slowing down of an electric fan as it warms up is a result of the motor's design and the effect of temperature on its electrical characteristics.