Some DC powered vent and cooling fans can be annoyingly noisy, and slowing them down sometimes helps. Assuming that the fan still does enough cooling, you can run the fan on lower voltage. But beyond a certain point, the fan won't get started below some voltage. But if you gently finger the blades it will stay spinning. One way to "kick start" the fan is to have it see a higher DC supply voltage upon system power-up, and have it drop to a quieter operating voltage. My example is a small fan that is rated for 12V and half an amp. And I used a 200 ohm resistor in series of a 12V power source to drop the voltage and thus operating speed to a reasonably quiet and still effective cooling level. But it won't start like this. So I added a 1000μF cap across the resistor. This cap is initially discharged at power-up, and the fan will see for a short while a full 12V supply, then the cap charges up, and then the fan sees a lower supply voltage. This short period of higher DC voltage is enough to get the fan spinning, and once it is spinning, it keeps spinning at a lower rate at the lower voltage. And be less noisy.

Interesting idea, although I'd be concerned about maintaining adequate air flow.

One other concern is the health of the motor. Some types of motors are happy to spin at speeds lower than their full speed. Others don't like it at all, even when not under load. Very few motors like receiving fewer volts than they were designed for, which is why modern variable speed drives do not clip voltage but vary the supply frequency instead.

Another option could be to vary the fan speed depending on temperature.

Thermistor, a small signal transistor and a power transistor. Thermistor senses the temperature and adjusts small signal transistor bias ... which controls power transistor output ..... fan is power transistor load. There were several circuits on the web.

Cheers,
Ian

Depending on the motor type there is always the pulse width method. The 1920's AC ceiling fan in one shed, was made for a carbon pile regulator & it works well on an electronic type (not ballast).

So there are ways of slowing them.

The brushless DC motors often have an oscillator & are almost an AC motor; that may be able to be worked on & change the frequency, or add in the pulse.

Marc

It’s going to be more likely to stop if it gets dirty so I suppose you’ll watch that.