As I understand it, this VFD will put out a certain frequency and amperage for a max load and has no idea if the load is stalling or not. I am not sure if bring a tach input back will help this or not.
A basic VFD (Scalar Drive in V/Hz mode) puts out a controlled voltage and
frequency, but amperage is a function of load. But you're right, it has no idea whether or not the motor is keeping up with the load, other than to monitor it for overload conditions.
This machine already has an encoder on the motor coupled before the transmission. It is used with the control to sync the motor and the Z axis for rigid tapping. However, it would seem that the actual drive does not know much of what is going on. It gets a reference voltage going back to it to half ass determine load. I am trying to find settings right now that will allow me to better boost the torque or "stall prevention". Seems anything over 50% on the load meter is destine to stall. I guess the way this one is setup is it drops frequency in a stall to reduce slip. I need to to boost everything and put put what it was made for...
What you need is indeed a "Vector Drive". Vector Drives are a higher tech variant of the original Scalar drives in that they have a MUCH more powerful microprocessor on them that can do the complex math of calculating voltage and current vectors to either maintain precise speed control or precise torque control (or both) within the limits of the motor regardless of the load applied to it. So it can essentially correct the error so to speak between what you want the motor to do and what the load is allowing it to do.
If your VFD is not currently a Vector Drive, then it cannot do anything with information from a tachometer or encoder etc. If it is, Closed Loop Vector Control (encoder feedback) is not something for the faint of heart, it takes a lot of tweaking capability to make use of that kind of functionality. But there are a lot of "Open Loop" or what are called Sensorless Vector Control (SVC) drives that use motor data and current measurements to accomplish nearly the same performance, probably all you need. Most SVC drives have what is called "autotuning" or "teach mode" where the VFD tests the motor it is connected to and creates a model in software so you don't have to. that makes them very simple to implement. The only difference now between Closed Loop Vector and SVC drive performance tends to be at Zero Speed meaning when you need full torque from a motor that is not moving, i.e. a hoist application.
Torque Boost is something entirely different an its purpose is to attempt to make up for a V/Hz drive's poor low end torque production, i.e. at roughly 25% speed and below. All it does it allow you to program a manual tweak to the V/Hz ratio at the selected speed and below, which would technically cause the motor to overheat, but by limiting it to low speeds, the motor power is already lower than it is rated for, so you can get away with it for a little while. Running in Torque Boost continuously however can be bad for your motor.
Stall prevention is something all together different again. Its purpose is to allow the VFD to keep the motor energized even if the motor stalls. It is exactly the OPPOSITE of what it sounds like you want. It drops the frequency output as a way to keep the motor from overloading on a large step change in load, essentially sacrificing HP and speed for continuity, assuming that it can re-accelerate it over time to recover.
Any idea if an inductive amp meter will read accurately on the motor leads from the VFD? Just real curious to see how much is actually getting to the motor. I have to account for some loss in the trans and such but 60%??!! I am hoping I can do something creative with this guy to just get a little more go out of it.
No, typical meters will not accurately read the output of a VFD, the signal is way to "dirty" with harmonics for most meters to accurately interpret it. The best way of seeing the output current is to read it from the VFD display, but if you have a transformer in between the VFD and the motor (why?) then that is not going to account for losses in the transformer.