rpm test

Ok i was trying this to calculate RPM for this fan:



its the fan with a switch on the side and something to hit the switch on every full turn. Then i connected it to the logic analyzer on pic and got this:



Now i measures about 19.8ms for each length which is 1 full turn. I then divided 1000mS / 19.8mS = 50.5050 turns in 1 second then times that by 60 for 1 minute: 50.5050 * 60 = 3030.30

so if im right this fan is only 3030 RPM right?

I could have 60 Seconds time 1000Ms = 60000mS / 19.8 = 3030.30

so am i right?

My guess is that the fan, without the straw (?) turns faster. The straw, hitting the switch arm each rotation, will reduce the speed. If it's a standard induction/shaded pole motor it is probably a little less than 3600 RPM, free running. If it's a brushless DC motor, the free-running speed could be anything higher than your measured RPM. A photo interrupter would be a more accurate way to get you output pulses.

Ken

You maths is correct but your values from the diagram don't seem right. Why isn't your time 98.8-52.8 = 46mS? Also, you have measured from the end of one pulse to the beginning of the second. The correct time appears to be 48mS which gives a speed of 1300RPM.

Mike.

wrong image sorry:

You still need to measure from the start of the pulses which would add about 1.5mS.

Mike.

ok thanks im rigging up a ir led and sensor to do a better test.

I wish i had a hall effect sensor

I bought a infrared "point and shoot" tacho, was only about $30 USD. Its a very handy little gadget.

Ebay is full of them, probably cheaper than what I paid which was a while back.