Sharp 1118H Lathe - High Gear Tensiometer Spindle Over Speeds

Good morning, at our universities machine shop we have a Sharp 1118H lathe that has a high gear and low gear spindle setting, each interacts with a tensiometer to increase or decrease speed. When in low gear the tensiometer works, when in high gear the spindle immediately revs to the highest speed and the tensiometer no longer can adjust the speed for the high gear.

I was hoping for some general information and if anyone has come across a similar issue. I can send pictures and more info if needed as well.

Thank you.

Is tensiometer the correct term? What are you trying to do with this lathe other than standard metal turning? Does the machine have a VFD controller?

Tensiometer may not be the best term, your right. It does have a VFD controller with a turn dial for interaction. That works fine on the low gear setting, but the high setting it's as if the VFD controller doesn't exist and the spindle speed just goes to full speed automatically. I have to turn the lathe to the low gear setting or the spindle would just continue on at the highest speed possible.

Also, I am just using it for standard metal turning, we have a 3 jaw chuck, nothing fancy.

I've seen some VFD's that have an auto-tuning feature when set to constant torque mode (maybe someone will correct me if I am giving wrong information). However, a lathe is not a constant torque device, as you have found out. So auto-tuning doesn't really work properly, because you can only tune the drive for one ratio at a time. As soon as you switch ratios, then a different set of parameters must be switched into use. Hopefully, there is some convenient way to do this with the VFD, such as a toggle switch that needs to be switched to invoke a different set of parameters which the motor has been tuned to, between low range and high range.

That might give you something to look for. You'll probably have to have the manual for the VFD and try to make sense of it.

Encoder feedback from the motor to the VFD is a far superior method of getting the motor to perform on a lathe. That requires a vector drive with provision to install encoder connections into it for feedback. Then the drive will be able to provide current (up to it's current limit) to the motor at any speed, to try to maintain that speed. A vectorless drive won't have much jam below about 50% of nameplate rpm for the motor.

Thank you, yeah it seems the issue is definitely within the VFD. I'm getting the wiring diagram now to troubleshoot. Thank you again.