Looks like you are on your way to a great project. But, consider your 3-phase motor will almost certainly be at speed within a fraction of a second after the relay turns it on. That will surely break the thread.
You have not allowed for a broken thread, but this can be added later if you have a way to detect it.
Where is the thread coming from? Another spool? Is that spool also motor driven, or is the equipment you are planning going to pull it from a spool? Will the thread get tangled while coming from the source spool?
The moving thread will generate static electricity and this may have an effect on your Arduino.
What will the thread be wound onto? If more than one layer, the thread speed will increase as more layers are added. Will this be a problem?
What material is the thread made from? If plant/animal fiber, may I suggest the wheel that measures the length should be rubber coated so the thread does not slip on the wheel.
Paul