Reduce voltage to fan


5 Aug 2008
Reaction score
United Kingdom

My wife has bought a fan which looks nice but is too powerful for its intended use. It's three speed 110W whereas she wants something more along the lines of a desk fan which is 30-40W.

What's the best way to reduce fan power? A resistor in line would be inefficient. I've read a capacitor can be used, but I have no idea how to size it.

For a ready made solution could I use a lighting dimmer? Does it matter whether leading or trailing edge? I'm guessing not as the motor will be an induction motor so will attempt to maintain the same RPM?

Also Numatic make a 230V to 110V board for the low power settings on their vacuums. Any idea how it works and how about putting it in line? The vacuums are reduced from 1200W to 600W so I don't know if they require a certain VA to work (in the way that a resistor value would change with motor power). Again I think reducing voltage to an induction motor can result in increased slip and therefore a current increase (which is what I'm trying to avoid).

Thanks in advance
Last edited:
Sponsored Links
I've read a capacitor can be used, but I have no idea how to size it.
I tried that once. It did reduce the speed slightly, but the motor didn't last long. An autopsy showed carbon tracking evidence of arcing around its internal wire terminations, which I think was a result of high voltages generated by the capacitance resonating with the coil inductance at mains frequency or a harmonic thereof. So choice of capacitor value may be important.
It's three speed 110W
Take the fan apart and see how the manufacturer implemented the speed control. Component values can be modified from what's there to obtain other speeds.

However it's probably a bust, as running a motor too slowly will cause overheating.
  • Thanks
Reactions: CBW
Sponsored Links
I have run motors slower than their design speed, but the inverter which alters the frequency and voltage is likely more expensive than buying a new fan.
Yeah not looking too effective to design and build a PLC and VFD for a £50 fan!

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Sponsored Links