8
\$\begingroup\$

I use this fan to cool my room, but sometimes it is too fast and I have to frequenctly turn it on and off to keep normal temperature. Maybe it is possible to reduce the speed by connecting a resistor in series or using a variac. The fan uses some form of shaded pole motor (the manufacturer calls it "Alveolate" and compares it to the standard shaded pole motor (pointing out that its motor is better). The "Alveolate" motor needs a starting capacitor.

I have searched the internet and this site but found contradicting answers. Some say that the speed can even be reduced by using a triac while others say that a resistor in series can destroy an induction motor. The fan motor is 60W.

\$\endgroup\$
  • 1
    \$\begingroup\$ possible duplicate of What is used to control AC motor speed? \$\endgroup\$ – W5VO Mar 25 '12 at 9:34
  • \$\begingroup\$ You could try a simple thermostat control. You are already turning the fan on and off to maintain the desired temperature. This would do the same, just automatically. Since the fan will be either full on or full off, you can switch it with a ordinary relay. Add a little bit of hysteresis or a time delay so it's not constantly turning on and off every few seconds. Maybe make it never less than 30 seconds on or something. \$\endgroup\$ – Olin Lathrop Mar 25 '12 at 12:13
  • \$\begingroup\$ @OlinLathrop I thought about it. It probably would be OK but not as good as slowing the fan down (previously I could put a smaller fan "after" the bigger fan if I wanted less airflow (turn the big fan off, turn the small fan on), but now I no longer have that option. Also, when I'm sleeping a fan turning on could wake me up (I'm OK if the fan is constantly on, just that I wake up when the noise changes). \$\endgroup\$ – Pentium100 Mar 25 '12 at 12:25
  • \$\begingroup\$ @Olin: I don't know if a thermostat would work, since fans don't really cool the room, they cool the person. Perhaps if the thermostat was measuring his forehead temperature. \$\endgroup\$ – Rocketmagnet Mar 27 '12 at 10:39
  • \$\begingroup\$ @Rocketmagnet, in my case the fan sucks air from outside, so it does cool the room (ideally down to the outside temperature). \$\endgroup\$ – Pentium100 Mar 27 '12 at 19:26
6
\$\begingroup\$

Connecting a series resistor or variac will reduce the applied voltage across the motor, but it will not change the applied frequency. The reduced voltage will increase the current flowing through the motor because motors draw a minimum current when the voltage and frequency are a fixed ratio. Changing either the frequency or the voltage while leaving the other unchanged can, depending on the motor design, rapidly increase the motor current. This increase in current can lead to motor damage due to the overheating of the winding in motors which do not have enough margin or thermal protection. This margin adds cost to the motor, so it often designed out as a cost savings measure.

Note that the reduced voltage will also result in a reduced torque, which will usually result in a lower speed. Thus a series resistor could achieve your desired goal of speed reduction, but at the potential cost of a reduced motor lifetime.

A triac circuit may or may not safely control the speed of a motor depending on how it is controlled:

  • If the triac circuit is operated in a phase controlled manner like a light dimmer and only conducts for a portion of every line cycle, then it effectively behaves like a resistor and can result in the same motor damage as a series resistor. In this phase controlled operation, the fundamental voltage amplitude will change based on the firing angle while the frequency is unchanged.
  • If the triac circuit is operated in a motor control mode where it conducts for full line cycles, but not every line cycle; then no motor damage will occur. When the motor is conducting current, its voltage and frequency are matched and the motor produces normal operating torque. When the triac doesn't fire, the motor doesn't conduct current and produces zero torque. The fan speed will depend on the average torque which is based on the ratio of conducting cycles (torque) to non-conducting cycles (zero torque). The inertia of the fan will keep it turning during the non-conducting cycles.

That's the background. In terms of your situation, your particular fan motor appears to have thermal protection and appears to be dual frequency rated. This suggests that your fan motor has extra margin and so it can handle a wider range of volts/Hz ratios. In addition, it has a protection circuit that should turn it off before damage would occur. Thus, you might be able to get away with using a series resistor or light dimmer to control the speed.

Getting a motor speed controller would be a better option since it won't damage the motor and you won't have the additional losses of the resistor/dimmer.

[Note: additional losses == a hotter room which seems counter-productive to the reason for the whole exercise of cooling your room!]

Alternate solutions:

  • you can mechanically control the flow as @rockmagnet suggests, but that isn't using electronics ( :) ) and it reduces the efficiency of the motor/fan (but probably not as much the resistor-motor-fan). Note that this could result in more noise depending on how the flow restriction is realized.

  • An alternative to flow restriction would be to have ducting that "recycles" outlet air back around to the inlet. This would allow the fan to operate at its normal unrestricted operating point, but you'll have a lower net flow into the room. Even more mechanically complicated, but...

  • you can get a full blown variable speed / frequency drive. This will likely be very expensive and is probably excessive overkill (but it would be tres cool ;) ).

  • Finally, I would personally just buy a multi-speed fan...

\$\endgroup\$
  • \$\begingroup\$ A variable speed fan is not available to me locally (and I do not want to buy it online because shipping is expensive), at least not one that is the correct size and speed. Thanks for the answer, it looks like the triac (motor mode) is probably the best for the cost (cheaper than variable frequency drive) and lack of damage to the motor (compared to resistor). \$\endgroup\$ – Pentium100 Apr 19 '12 at 9:21
  • \$\begingroup\$ Is there a VFD controller that could be put 'in front' of the fan to solve this problem? \$\endgroup\$ – Daniel Mošmondor Sep 11 '17 at 11:14
  • \$\begingroup\$ you are making stuff up - minumum current is when speed matches frequency. - but that's not useful. Also you are forgetting that for a fan torque is dependent on speed \$\endgroup\$ – Jasen Dec 13 '17 at 11:32
0
\$\begingroup\$

If you want to reduce the airflow of the fan, one easy way to do this is to simply partially block the intake with a piece of paper. If you want to get fancy, you could even make an adjustable system, so that you can easily vary the intake area.

\$\endgroup\$
  • \$\begingroup\$ I was hoping to slow the fan down so it does not make as much noise when I do not need the full 1400m^3/h airflow rate. But yea, partially closing the valve in the air duct is probably a better way to do it than using a thermostat. \$\endgroup\$ – Pentium100 Mar 27 '12 at 19:29
0
\$\begingroup\$

Reducing the voltage is a common practice used to control the speed of induction motors that drive fans.

It doesn't work well for ordinary mechanical losds, but because fan torque is proportional to speed squared (or some higher power) reducing voltage to allow the motor to slip more doesn't cause an overheat, because current also decreases..

for ceiling fans often a series capacitor is used, but electronic speed controls are also available

\$\endgroup\$

protected by Dave Tweed Nov 14 '15 at 12:44

Thank you for your interest in this question. Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).

Would you like to answer one of these unanswered questions instead?

Not the answer you're looking for? Browse other questions tagged or ask your own question.