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I would like to run a standard multi-speed desk fan at a lower speed than its minimum setting. On Amazon, one reviewer recommended a variac for fan control, saying that it could adjust the fan from barely-on up to full speed, without introducing the hum that sometimes occurs with triac-based controls. As I understand it, variacs are simply adjustable transformers, which reduce the voltage fed to the fan (and triacs working in phase-controlled manner have a similar effect).

However, most fans are induction motors, and I always thought these had a weak response to voltage. For example, Figure 27 here shows that peak torque occurs around 70% of synchronous speed regardless of the applied voltage. I believe the operating speed must always be somewhere above this speed. If this is true, then the only effect you could get by adjusting the voltage would be to reduce the operating speed from ~90% of synchronous speed to ~70% of synchronous speed, at which point the fan would stall. This doesn't sound like enough variation to be useful (and is at odds with the variac reviews on Amazon).

Does this sound correct? If this is the case, then it sounds like my best option is to find a fan speed controller that uses a triac to cut the voltage for whole AC cycles, along the lines described in an earlier question. But that is a challenge, since most fan controllers that I've found don't include descriptions of their mode of operation.

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    \$\begingroup\$ If the fan has an induction motor then speed control is difficult. \$\endgroup\$ – Andy aka Jun 9 '14 at 7:20
  • \$\begingroup\$ Yes, my impression was that this is difficult, based on the information I found about induction motors in general (referenced in my question). But then how do you explain people's positive experiences using variacs, e.g., link and link? Maybe voltage-based speed control is more effective in practice than in theory? \$\endgroup\$ – Matthias Fripp Jun 10 '14 at 20:26
  • \$\begingroup\$ I meant to elaborate on the links above, but then ran out of time to edit. The first is an Amazon review that says a variac allowed any speed from barely moving up to top speed. The second is a general discussion of fan speed controllers which lists a variac as the best option short of a variable frequency drive. \$\endgroup\$ – Matthias Fripp Jun 10 '14 at 20:34
  • \$\begingroup\$ After a little more research, it looks like stator voltage control is a legitimate way to control the speed of a motor, or at least a fan. See, e.g., shodhganga.inflibnet.ac.in/bitstream/10603/26425/10/… . Voltage control uses the fact that at a given slip rate, the torque is proportional to the square of the voltage. If a fan produces less torque, it will turn more slowly. (This method is probably not suitable for applications that can't tolerate reduced torque, in which case volts per hertz control is a better choice.) \$\endgroup\$ – Matthias Fripp May 25 '16 at 7:59
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    \$\begingroup\$ By the way, I think this probably means fans (at least under variable voltage control) often operate to the left of the maximum torque point. But they can't stall because drag drops along with speed. Instead, they find an equilibrium speed where drag equals torque. \$\endgroup\$ – Matthias Fripp May 25 '16 at 8:05
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A variac works just fine. I have a medium sized AC fan hooked directly up to a small, 1A variac which I made years ago as a quick and dirty solder fume extractor, but it worked so well that quick and dirty turned into 'what I've used for years'. The speed control works exactly like you'd expect.

What the amazon seller says is correct. You're also correct. The key difference is that virtually all AC fan motors are a special kind of induction motor called a shaded pole induction motor. These motors do not need any external capacitors to introduce phase lag, and instead rely on copper rings around each of the poles that act as single turn transformers, introducing a delay in the magnetic flux for that pole, and the net effect is a second phase.

This makes for a fairly inefficient, but extremely reliable motor. Due to their construction, they generate torque over their entire RPM range, including stopped, and their torque is mostly dependent on voltage, except for some nonlinearities at very slow speeds where reduced torque is generated.

You can see this in most fans - you turn them on but they take a while to get up to speed. The load from the fan blades and the air they move usually determines the final speed, which is whatever speed where the generated torque and the load torque match.

Since the load on a fan increases with rotation speed, this means that simply by varying the voltage, and thus the torque, this will also vary the speed the fan will spin, as less torque generated means the fan will reach that matching load torque at a lower speed.

Also, just to be clear, there is no magic going on with the TRIAC driver, it's nothing more than a dimmer circuit. It's chopping up the AC waveform to generate a lower Vrms. It's just a very noisy and harmonic filled way of reducing the average voltage, so using a TRIAC driver isn't any different from using a variac.

The variac would be much less noisy, and not add any harmonics, and ignoring cost, size, and weight, would be the preferred solution. The primary advantage of TRIAC drivers are cost. Either will work, but a small variac would be the more elegant and low-noise solution.

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