If you can only instrument one thing on an electric motor, measure its current. A clamp goes on without touching the machine, needs no downtime to install, and carries more information about mechanical condition than most people expect.
Why current is such a useful proxy
A motor is a transducer between electrical and mechanical energy. Anything that changes the mechanical load changes the electrical draw. A blocked impeller, a worn bearing, a misaligned coupling and a dry-running pump all leave a trace in the current waveform because they all change how hard the motor is working.
The start-up transient is worth capturing
Inrush current at start is often more diagnostic than steady-state running. The shape of the transient reflects the mechanical load the motor has to overcome to reach speed. Capturing that curve consistently gives you a repeatable fingerprint that is easier to compare across days than a running average.
This requires sampling fast enough to see the transient. If your acquisition is averaging over a one-second window, the most useful part of the signal is gone before you record it.
Load drift versus process change
The hard part is not detecting that the current changed. It is deciding whether the change came from the machine or from the process. A pump moving thicker fluid draws more current, and so does a pump with a failing bearing. This is exactly why a single threshold fails and why cross-referencing with vibration and temperature matters.
Where current alone runs out
Current is a summary signal. It tells you the load changed but not always where. Early bearing degradation, for instance, often shows in vibration well before it moves the current draw enough to notice. Treat current as your always-on primary channel and add vibration when you need to localise a fault.
Practical notes
Use a clamp rated for your expected range rather than the maximum the panel might see, since resolution at the low end matters more than headroom. Keep the clamp on the same conductor and orientation between visits. And record the supply voltage if you can, because a sagging supply looks like a load change if you only watch current.