Is there a way to know rpm?

[amron]

Is there a way to know the rpm of an electric motor without using a rpm measuring meter? what I mean is I want to know if it's a 1400 or 2800rpm.
by looking at the shaft (the motor is not installed) I can't determin what the speed is and of course, I've not mentioned that there is not a plate on the motor, otherwise all I had to do is reading it!

 

[tttonyyy]

Can you stick a bit of tape on the shaft, have it tap past something on every revolution then record that sound using a laptop at 48kHz then measure the number of samples per revolution and work it out from there? That'd be a cheap way to do it.

Or you could buy a laser tachometer.
http://www.ebay.co.uk/itm/New-Digital-Laser-Photo-Tachometer-Non-Contact-RPM-Tach-/290537775535

When fiddling with a diesel stationary engine governor to get a particular RPM used the sound recording trick, because it was just aircooled you could really see the thunk from the combustion stroke

 

[Drains]

There's an engine RPM app for the iphone. If you had an iphone and did what tttonyyy suggests, that might work - although you'd have to watch for the 2:1 factor between 2 and 4 pole motors. It'b be very easy to get it wrong!

 

[rtbcomp]

It should be possible to time how long it takes to stop after switching it off and compare with a known motor of the same physical size.

 

[CRB]

?How about a bit of coloured paper?

http://www.jeffree.co.uk/pages/speedmeasurement.html

Really cheap,and the maths has been done for you.

 

[GrahamGKD]

Take a video clip and see how much the shaft moves between frames?

 

[brightspark]

just eyeball it. u can see a 1450 rpm shaft and keyway spinning no problem. a 2800rpm shaft u can hardly notice it as its that fast

 

[madkayaker]

just a daft idea but does it seam fast or F ing fast!

 

[Tiggy_dorset]

A simple bicycle speedo? This one measure rpm of wheel. Attach the pickup to a plate mounted to the shaft and take a reading.

http://www.ebay.co.uk/itm/BICYCLE-B...ds_Activity_Monitoring_MJ&hash=item45fccf35d2

 

[amron]

Thank you all. I think that I will get a motor that I know its rpm and try to compare the speed.
The tachometer suggested by Tony looks nice and the price is reasonable, the seller "ships worldwide" but then there is a list arm long of countries that he doesn't ship to.