Schaublin 125-CNC retrofit.

I vote encoder.

An encoder separate fan will allow:

• lower continuous motor speed without overheating
• higher continuous motor speed without the chance of a 45-year old fan leaving the chat
• you and LCNC not to guess what the motor speed is, adding confidence you aren't going to send the motor in to orbit

All giving you a wider safe frequency/speed range for your VFD/motor - and thus making each 'step' in the CVT more useful (wider range).

Actually, I vote for getting rid of the CVT.  Do what CNC mfgrs have been doing since the 80's & 90's and stick a big a$$ spindle-servo in there with an 8kRPM top end and appropriate-ratio single-speed belt drive.  Just horse the thing.  Simplify your control loop and save your sanity.

But I digress...

I have a few points.

First: you refer to the fan multiple times. If your machine is like mine, and I have no reason to believe otherwise, the motor has no fan where you think the fan is, the thing there is the brake.

Second: regarding your solution 2, I can make a simple op-amp for you. Scaling to 24 V is easy, as you already have 24 V, and it would have 2.5 times the resolution of the system that gives satisfactory results for me. Yes, it has way less resolution than an encoder, but you do not need it. This would be the fastest and least invasive route.

Marti