Take it down a notch (Voltage edition)
February 19, 2021 11:03 AM   Subscribe

I have a fan whose lowest speed is too high for its intended use. Is there a step-down transformer that will take 120V AC and reduce it even more?

Searching turns up plenty of travel converters and step-downs that go from 240 to 120, but I'm looking to turn 120V into 100V or lower.

Two questions:
1) Do you have any leads on a step-down transformer like this?
2) Does this pose any safety or equipment risk?

Fan is a basic box fan with no fancy digital anything, just a 3-position switch. In Canada BTW.
posted by Hardcore Poser to Shopping (8 answers total) 1 user marked this as a favorite
If you start an induction motor at too low a voltage, it may fail to start: https://boards.straightdope.com/t/whats-up-with-the-power-knobs-on-electric-fans/280240
posted by Lanark at 11:13 AM on February 19

This is a common problem when working with vintage radios and amplfiers which were built to expect 115VAC or so. You can make your own bucking transformer, use a variac or regulated AC supply, or even a home-brew light bulb limiter. But, as noted above, the fan may not start. You may need to give it a spin to get it going. Fun little projects but probably cheaper to get a different fan.
posted by sudogeek at 11:18 AM on February 19 [1 favorite]

Fans change speeds a few different ways. One of the ways is to introduce a capacitor in series with the power line. Higher capacitance (say, 2 5 uF caps in parallel to make 10 uF) gives you “medium”, and lower capacitance (1 5uF cap) gives you “low”. Bypassing the capacitors gives you “high”.

You could see what kinds of capacitors you have, and replace them with ones of the same type but lower capacitance, or you could introduce an additional capacitor in series.

Don’t do this unless you know how to calculate impedance, and unless you have access to capacitors that are rated for like 300V AC and are a suitable type. Even so, there is some risk of explosion. If the impedance is too high and the fan stalls, that’s bad news for your fan’s longevity (plus they get hot).
posted by Huffy Puffy at 11:26 AM on February 19 [2 favorites]

A transformer that converts an input voltage of 240 V to 120 V will also convert an input voltage of 120 V to 60 V. (It divides the input voltage by 2.)

I don't think there are any safety issues, especially if the wattage of the fan is less than about 200 watts, assuming the motor always starts.
posted by monotreme at 11:29 AM on February 19

Back when we had a US voltage fridge while living in a 220V area; its step-down transformer had a pair of jumpers (roughly a two-pin plug that obviously did not match US or Euro sockets) to select input and output voltage. From what I remember it could be set to 100, 110, 115, 120, 127, 220 and 240, both for input and output, so you could select 220 in/110 out, but also 100 in/127 out or 120 in/220 out. If you can find such a transformer 127 in/100 out would probably make the fan run sufficiently slower.

The way Huffy Puffy describes is how I tamed the ceiling fan in our bedroom: replacing the two capacitors in the speed controller by ones 50% lower in value, and wiring the selector switch so that the highest speed also had a capacitor (about 30% larger than the largest of the other two) in series with the motor. If you go that route make TOTALLY sure the capacitors are rated 300V AC or better, and carry the designation X2.
posted by Stoneshop at 11:51 AM on February 19

Shoot, you could buy a VARIAC and have fun in the 0-130VAC range. The 5 Amp ones are pretty cheap, but adjusting them can be a little sparky. No guarantees that what you want to happen will actually happen when you drop the voltage.
posted by Dmenet at 2:35 PM on February 19

Try wiring a tungsten incandescent light bulb in series with the motor. The motor will have full voltage until the bulb filament heats up, then the voltage will drop. This is an old trick people who work on vacuum tube radios and amplifiers use to test their restoration work before bringing them up to full voltage.
posted by ZenMasterThis at 3:49 PM on February 19

You could buy a variac, or wire up your own step down transformer, or hack your way to a lower speed......

Or you could buy a fan controller designed for the job for about $18.00 and and have any speed you want.

posted by jmsta at 1:10 PM on February 20 [2 favorites]

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