Motor Rpm calculation

[Ario]

Hi there I have a project where I need to know the rpm value of a 40 cm floor fan. I know it can be calculated according to the number of motor poles. Do someone with experience know what type of ac motor is most commonly used in motors of this type. I do not want to bother opening it up. Here is the know info:
 40 cm logitek BF1601A floor fan
220 -240 volts
50 hz 66 watt max power.
 furthermore I how would like to know how the speed setting normally be facilitated?(capacitive voltage dropper ).
I would like to obtain a moderately accurate rpm value for each of the fan's 3 speed settings?

Any help would be appreciated. I have limited time and resources, and any help would be beter than none at all. I can measure the operating power if need be.

[richard.cs]

Often those fans have shaded pole induction motors, but the number of poles varies. Most of the ones I have seen have 2 or 3 speed settings accomplished by switching the windings around to alter the effective number of poles.

The least-effort approach is probably to measure the speed. Signal generator plus strobe light or photocell plus oscilloscope would be two simple electronic approaches.

[S. Petrukhin]

If you don't have any measuring equipment, make a simple circuit of two resistors, one transistor, and an LED to connect to the computer's audio output. You will get a strobe light.

[CaptDon]

Since the motor is not syncronis to the line frequency it will have a 'slip' factor which
will vary with load and input voltage. The change in speed is not usually done by pole
count switching but is done in a manner similar to making the motor a 300 volt motor
running on 240 for reduced speed. If you try the suggested strobe light idea you should
some how mark one or more of the blades to prove that when your strobe light appears
to make the blades stand still you are not strobing at a harmonic or subharmonic frequency
as either erroneous frequency will still make the blades appear to be motionless.

[Ario]

Hi all sorry for the late reply.
Thanks so much for you all. Ended up scraping the idea as a whole. My professor did not like it. Made a wind tunnel and used the pipe povler technique to determine wind-speed.  This forum is great. Thank you.

[Ario]

My wind tunnel

[S. Petrukhin]

My wind tunnel

This is some kind of nonsense that has nothing to do with the speed of the fan blades. 

[Siwastaja]

Classic cheap fan AC induction motors (well, even some less cheap) have a lot of slip, and the speed control (if available) is actually often voltage-based (triac dimmer, or classically variable transformer), just adding even more slip. They are designed to be inefficient over the whole slip curve.

2-pole design is quite typical so 3000rpm is often the synchronous speed at 50Hz, but due to the said slip, actual rotation speed can be much less.

The only way to know is to measure. Buy a tachometer. Or, carefully tape a small piece of cardboard so it hits the blade each rotation (carefully enough not to slow it down), record the audio and measure the interval in audio editing software.