You can use RPM to calculate load approximately but it’s not very accurate. The motor RPM at full load will be on the name plate. It is slightly less than synchronous which would be 3600, 1809, 1200, 900, 600…depending on the number of poles. Subtract both the actual speed measured and the full load number from synchronous then divide to get a percent load. It is reasonably linear up to about 200% of load but there are so many errors it’s just a guess. Using kw is more accurate and even better is looking at a frequency spectrum of the current and looking at the pole pass harmonic but that’s very advanced.
But beyond that if they are set up correctly you have two protection devices: overcurrent and overload. If it’s overloaded it’s mechanical. If overcurrent trips it’s electrical, in general. For such a simple device motors have a lot of ways they can fail.
A couple days ago we had a 7.5 HP tank agitator. FLA was 12.5 A. On startup it jumped right up to 96 A and held for 10 seconds until the overload relay tripped. The agitator had no build up on it. I did run a full blown LCR test on the motor (Megger, resistance, inductance) and found nothing wrong. Typically in a short situation you will be at 10x FLA or more. It pulls normal locked rotor until the motor gets above the peak torque point at around 75-80% if full speed before it drops down to normal. So 96 A is around 8x FLA. So no leaks to ground or anything like that so looks very mechanical. As a follow up the gearbox was full of water, pieces of gear teeth, etc.
It can be hard to definitively tell if you have a mechanical or electrical issue. There are things that can go wrong that can fool you into misdiagnosis. So check everything you can every time and let the data determine the troubleshooting path. Don’t just make assumptions. As an example if you repeatedly reset a eutectic overload (common with screw conveyors) it will gradually lose solder over time and eventually trip early. This one can be frustrating to find.
