Fan speed

Have you got a multimeter that measures AC frequency ? If so, you might be able to cobble together something with a battery, photocell and a light source. Or you could rig it so it just clips a piece of card or something and compare by ear the pitch with comparison tones from the internet. Simon.

Is it mains or battery?

Sorry, should have said it was standard mains. Two speed.

If the motor doesn't have any brushes, and there's no gears it'll be

Otherwise, the model aircraft bods use non-contact tachometers, and for a one-off measurement, you might be able to borrow one from a local model shop.

Light beam interruption and divide by three?

Brian

Put a thin white line on one blade. Photograph the fan whilst it rotates at various shutter speeds. Calculate the RPM from the angular movement of the line at each shutter speed. Average the result.

Bill

From the motion blur, I like that. Don't use flash though. B-)

I had a similar thought and, convinced someone must have written an app to do it, went looking in both the Android and iOS market. Was somewhat surprised no one has written one yet. Unless the camera hardware isn't up to it ;)

My programming skills certainly aren't! ;)

Lee

What sort of motor is it? Very small motors are difficult to measure as the measuring device can disturb it.

On a related topic a guy I knew in the 60s who repaired cameras used to adjust rollerblind shutters by photographing a TV screen and counting the TV lines on the picture.

Who remembers the white square in the corner of the neg frame caused by the Zenit mirror not returning quickly enough?

Bill

It works to a point but the beginning and the end of the blur are hard to determine. I get around 3,400 rpm at low speed which could well be

That's assuming it's synchronous or near synchronous. Lots of induction motors aren't, e.g. the shaded pole motors very often used for driving mains fans (they couldn't be speed switchable if they were synchronous).

... but not fully concentrating I've just realised it is coming from China!