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RPM – rotation speed explained

RPM (revolutions per minute) is how fast a machine is rotating: an electric motor, a fan, a pump. A Treon Industrial Node X can report it alongside the vibration it measures, without a separate tachometer.

Why speed matters

Machine speed changes with load and process demand, and most of all on a variable-frequency drive (VFD). Vibration rises and falls with speed, so without the RPM you cannot tell whether a higher reading is a developing fault or simply a faster machine.

Speed is also what makes a spectrum readable. Imbalance, misalignment, looseness, gear faults and bearing faults appear as peaks at multiples of the rotation speed: 1×, 2×, 3× and so on. Knowing the RPM turns an anonymous peak at 48.6 Hz into "2× running speed", which is a diagnosis. See Spectra & waveforms explained.

How Treon measures it

The Treon Industrial Node X has a magnetometer. An electric motor's stator radiates a magnetic field at the frequency that drives it, and the node measures the strongest frequency in that field. Treon Connect converts it to a shaft speed:

RPM = magnetic field frequency × 60 ÷ pole pair count

A 50 Hz supply on a motor with 2 pole pairs (a four-pole motor) therefore gives 1500 rpm.

Two things follow from how the measurement works:

  • It needs an electric motor. The field being measured is the motor's own. A fan or pump gets its RPM from the motor driving it, not from its own rotation, so the node has to sit on the motor.
  • The figure is the motor's synchronous speed. The calculation uses the supply frequency and the pole pair count, and applies no slip correction, so an induction motor under load turns slightly slower than the reported value. Slip is typically 1 to 3 per cent at full load.

Treon Industrial Node X only

The Treon Industrial Node C has no magnetometer. It reports acceleration, velocity and temperature, and never RPM.

Setting it up

Three things have to be in place, and RPM appears only when all three are:

  1. Set the measurement point type to Motor. In Assets, open the asset, go to Measurement Points and set Type to Motor.
  2. Enter the pole pair count. The Motor Pole Pair Count (1-8 or N/A) field appears once the type is Motor. A two-pole motor is 1 pole pair, a four-pole motor is 2, and so on. The nameplate usually gives the pole count, which is twice the pole pair count.
  3. Give the node a measurement set that includes the magnetometer. Sets 1, 3, 4, 6 and 12 do; the default sets 0 and 5 do not. See Configuration.

RPM then appears as a trend in Condition, alongside the other magnetometer values, and is recorded with the same timestamp as the vibration measured in the same run. A set without the magnetometer produces vibration data with no speed attached to it.

Nominal RPM is informative only

The Nominal RPM field on a measurement point records the machine's rated speed for people reading the asset. Nothing calculates with it, and filling it in does not make a node report RPM.