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I have seen recommendations for using a variable ac transformer to control the speed of a box fan. (like this: https://www.amazon.com/dp/B00BXJYO6I/?tag=stackoverfl08-20)

I thought that ac motors have to be in sync with the voltage frequency (ie 60hz) and the speed control switch on the fan changes which windings on the motor the current goes through. So changing the voltage with a variable transformer will cause the fan to overheat. (https://www.mjguide.com/tutorials/ElectricalSafety/1540.htm)

Am I right or is it really okay to use a variable transformer to control the speed of a box fan?

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Most portable fans use a type of motor called a "shaded pole AC induction motor". Here is a description of them from Wikipedia:

Shaded Pole Motor

You should not, however, try to control the speed using a "variac" device because the motor will attempt to continue to try to draw the same amount of POWER (Power = Voltage * Amperage) and you will get heating and lead to premature failure or even a fire hazard.

Here is a link to a discussion of low-voltage issues with AC motors:

AC Motor Low Voltage Issues

To quote the relevant section:

Effects of low voltage. When you subject a motor to voltages below the nameplate rating, some of the motor's characteristics will change slightly and others will change dramatically. To drive a fixed mechanical load connected to the shaft, a motor must draw a fixed amount of power from the line. The amount of power the motor draws has a rough correlation to the voltage 2current (amps). Thus, when voltage gets low, the current must increase to provide the same amount of power. An increase in current is a danger to the motor only if that current exceeds the motor's nameplate current rating. When amps go above the nameplate rating, heat begins to build up in the motor. Without a timely correction, this heat will damage the motor. The more heat and the longer the exposure to it, the more damage to the motor.

Fans with a built-in speed control generally have multiple windings on the motor that are switched in and out in order to control the motor speed. These will not suffer from the overheating effect.

Your best bet here would be to get a different fan that can be controlled to your liking.

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Go ahead and use a variac (variable transformer) to slow down a box fan to a reasonable speed. Just make sure it's moving enough air to cool its own motor.

jwh20 isn't wrong, but his explanation doesn't apply to your use case. He's right about not using voltage as a speed control for a motor with a fixed load. Since you don't need a certain amount of power to move air, it's not applicable here. (Using a variable frequency drive is the preferred way.)

The difference is that your box fan motor isn't connected to a fixed load. Air resistance increases as fan speed increases, and at low speeds the fan has less resistance.

I just tested it with two different fans on my variac and a Kill a Watt meter. (It's possible these aren't the same as a box fan though, both of my fans have run capacitors. I'm not sure if box fans have run capacitors or if that's the same as a shaded pole AC motor.)

As I lower the voltage, it also lowers the amperage it draws. The only problem would be if you lowered the fan speed too slow. A fan motor drawing 10 watts that isn't moving any air is going to get hot eventually.

Just keep it above 40% (about 50 VAC) and you'll definitely be fine. One of my fans takes 28 watts at 50 VAC, and 112 watts at 120 VAC. When it's running slow, it's moving fast enough to cool itself fine.

I'll leave it running at 15 watts for a while and let you know how hot it gets in an hour or so. It's barely audible at this slow of a speed, but it's too low of a voltage to measure directly with my Kill a Watt (and I'd rather not stick my multimeter in the socket just for the data.)

Edit: At 15 watts and an hour or so, the fan is 15 degrees F over ambient.

  • My box fan doesn't have a run capacitor. It just has wires going from the power cord to the 3 speed / off swtich and wires going from the switch to the motor. No other visible electronics. Does the lack of a run capacitor make a difference in your answer? Thanks – Morris Jul 4 at 1:52
  • Would a SCR High Power Dimmer Controller be okay too? amazon.com/Controller-DROK-Adjustable-Regulator-Temperature/dp/… It's a lot less expensive than a variac and I don't mind wiring it myself. – Morris Jul 4 at 2:01
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    VFD. Variacs are expensive, limited-run, copper and iron beasts, with boutique price tags to match (or, in the case of the linked ones, cheap rubbish that is not UL-Listed, so illegal to install, NEC 110.2). This is true of all things "Sold by XXX and shipped by amazon". An electronic VFD has gotta be cheaper. – Harper Jul 4 at 2:27
  • @Morris that's a triac dinner and will work fine too, but you risk more humming noise with that type of dimmer because it's clipping the waveform. I don't have a box fan to test with, but I'm 90% sure you'll be fine slowing it down slightly with a variac. Just don't slow it down too much without monitoring for excess heat. – Dotes Jul 5 at 1:25

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