Fan Speed

http://www.freescale.com/files/microcontrollers/doc/app_note/AN3471.pdf

http://america.renesas.com/media/products/mpumcu/child_folder/Renesas_Jani_Seminar.pdf

speed controlled by voltage change or frequency change or many other finer methods...choose your flavour

The resistor/s are sized correctly to suit the the load. This is your cheapest form of speed control, after this you get a winding per speed, which again cost more. All you waste here is the electricity in the form of heat. I'm not sure whether anybody will bother to calculate this "waste" I'm don't believe that the cost of the waste heat will be near the capital outlay required for an inverter, or any other electronic speed control. Besides why would one want to slow down a fan more than its slowest speed? ( Given that it is a tablefan?)

"Besides why would one want to slow down a fan more than its slowest speed? ( Given that it is a tablefan?)"

To make it quieter! (less audio noise)

or to stop blowing your papers around on (and off!) your desk.

While you can design and build a speed controller for a 1 or 3-phase induction motor (you didn't specify which one), purchasing a Variable Frequency Drive for the 3-phase motor is usually a much easier solution.

guys, he is asking about a simple tabletop fan.

I believe you are right, or perhaps a floor or ceiling fan...

I have also wondered how they get three speeds; unfortunately, I've never taken one apart to find out. Can anyone say for sure?

Multiple windings I'm fairly sure. speed one is winding one. speed two is winding two and speed three is both windings.

Yes thats how they do it, Multiple windings

The typical electronic you can buy at your local hardware store is generally a triac, or back to back SCRs,, they can be noisy, both electrically, and acoustically. More expensive, quieter and more efficient are variable frequency/voltage controllers.

You are correct that ONE option to control an induction motor (fan or other use) is with a variable resister.

Yes- if one is used, it does waste electricity (turning it into heat) but it DOES control fan speed and does it "capital cost" cheaply. Variable winding motors with selector switches are BETTER, but maybe you NEED accurate flow control, not inexpensive control that does not meet the need. Another option for a FAN is to use a damper (inlet or outlet) which will control volume AND will result in lower energy usage due to fan law rules.

Latest speed controllers for fan speed are done using Thyristors......when we rotate the regulator the gate voltage is varied inturn the output of the thyristor is varied.....this output is fed to the fan which now rotates at variable speed.

i thought that SCR or Triac based Fan regulators are the norm nowadays ? No wasted heat in the resistors ?

If you are from India- a reputed company called CROMPTON was selling ballast type fan regulator. It was about 2 to 3 times the price of resistor type speed regulator. I have it fitted in my house for 2 decades. It is just a ballast (or call it choke) with 5 tapping's. It does not get hot and does not waste energy in heat like resistor. Unfortunately not many people know it and few traders stock it. Triac or thyristor based fan controllers are cheap and produce lot of harmonics. Again the potentiometer used there goes bad as wiper contact gets worn out. Many stop working after about 1 year depending on usage!!! Best solution is choke with tappings. German company was making small size variac of say 0.5 Amps rating- unfortunately cost conscious, illiterate common man looks for cheaper solution (recommended by electrical contractors/ traders, architects etc!!)

As far as I remember the induction motor has a little problem working properly if the voltage drops below 80 % or so.

If you have a universal motor- that is a brushed motor, than you can vary its speed by varying the voltage( from zero to rated voltage)-includes variation by potentiometer.

The guys that said that the speed is controlled by controlling the number of wintings being used at one particular time are right. This works for both brushed and brushless(induction) motor. The bruushed motor also works in dc voltage, the induction doesn't, but you probably knew this.

In a hand mixer used in kitchen I found out that the speed variation for the universal motor is done by varying the stator windings - that is the stator has taps.

The same kind of motor I have found in desk fans -some 200W fans. The speed variation was done by stator taps.

nikolay

Yes. Resistor definitely wastes considerable energy. It would be imprudent to neglect waste in today's world. Inverter is the solution. If one considers millions of inverters to be made for fan then the cost will drastically reduce. In India, we get solid state controllers for about 1.5 Euro.