Measuring the RPM (rotations per minute) of electric motors or the devices they drive can be a useful diagnostic tool. It can aid in the proper gear or belt drive system along with figuring out power application problems. An adjustable speed strobe light can become an invaluable tool for finding the exact RPM of almost any rotating machine. By following a basic process you measure the RPM of most any circular rotating device.
Use the white chalk or black marker and place a small white line or black dot on the end of the rotating shaft or pulley. The finer the line or dot, the more accurate will be the sighting for determining the shaft speed.
Plug in the adjustable speed strobe light and test the unit. You should be able to turn the adjustment dial on the rear of the strobe and change the "flashes" per second or minute. You will also note that the strobe light will have maker indications on the dial to identify speeds of the strobe flash. Depending on the manufacturer, the strobe light may come with a conversion table to convert the dial settings from flashes per second into minutes.
Start the electric motor or similar device and allow the unit to come up to full speed, this may take a few seconds depending on the application.
Shine the strobe light onto the area where you placed the mark. The white line or black dot should begin to illuminate. The goal is to slowly adjust the speed dial on the strobe light to make the line or dot stand still. As you adjust the speed of the strobe the line or dot may move to either the left of center (to slow of speed) or to the right of center (too fast of speed). You want the mark to remain in the center of the light flash and appear as a solid mark. Once you have found this image, read the speed on the dial indicator this is the speed at which the shaft is rotating.
Calculate the speed in RPM from a dial reading on the strobe light that is 60 flashes per second (FPS). Since there are 60 seconds in every minute you can multiply 60 seconds per minute times 60 flashes per second and the result will be 3600 RPM.
