How to measure motor RPM with a tachometer

The short answer

Contact: fit the right tip, touch lightly and square to the shaft, wait for the reading to settle. Non-contact: stick one strip of reflective tape on the shaft, hold the tachometer 75 to 300 mm away and read. Compare the result with the nameplate speed.

Reading RPM with a contact tip

Fit the tip: cone (male) for a shaft with a centre hole, cup (female) for a shaft end that protrudes, wheel for surface speed.
Make sure the shaft end is safe to reach and is guarded from belts and couplings.
Touch lightly and in line with the shaft. Pressing hard slows the shaft and gives a low reading.
Wait about a second for the display to settle, then read.
Use the stored minimum and maximum values if you need to pull the tool clear first.

Reading RPM with reflective tape

Clean the shaft and stick one strip of reflective tape, about 10 to 12 mm wide.
Hold the tachometer 75 to 300 mm from the tape and point the beam at it.
Start the machine and read the display once it settles.
Use one strip only. A second strip or a shiny keyway can double the count.

Safety. Never put a hand, cuff or loose clothing near a rotating shaft, and never measure on a machine whose guard has been removed.

What the reading tells you

Induction motors: speed well below the nameplate figure at the same load points to low supply voltage, overload or a rotor problem.
Belt drives: driven speed equals motor speed times the pulley diameter ratio, less a little creep. A larger gap means a slipping belt.
Fans and pumps: flow depends heavily on speed, so a few percent of lost RPM matters.
Conveyors and rollers: use surface-speed (m/min) mode with the wheel tip.

Tip. Take two or three readings and look for agreement. Note the load condition so readings from different days can be compared.