PCNC1100 loosing RPM

Hello all,

Hoping for a bit of support with an issue I’ve been experiencing.

My (very old) 1100 spindle seems to be running at 1/2 speed. This occurs intermittently, and is not a result of the hi/lo improperly set. A few minutes into almost any job, I audibly notice the rpms drop and the machine will continue to run at reduced rpm regardless of the commanded rpm. Sounds like exactly half the rpm. If I stop the job and restart the machine, it will return to normal operation for at least a few minutes. Has anyone experienced this? This machine does not have the vfd upgrade, so I’m obviously weary of the drive given its age, but wanted to see if anyone had this happen before dropping spending the money. Thanks.

The only real troubleshooting to do on those Series 1 units, would be the “sweep the pots” on the “vfd board”

Thanks Sam, any details on that? I feel like I read something about a long time ago but some googling turned up nothing.

Went through the control cabinet looking for clues and found several loose terminal block screws, particularly on “C2”. Not sure if that was the issue or not but having trouble replicating.

Loose terminals on the C2 would certainly could cause some issues, as the C2 will latch and deliver power once the spindle is commanded. This old document makes note of “Wiping the Potentiometers.”
SB0009_Drive_Calibration_1113A.pdf (463.1 KB)

Take a picture of your potentiometers so you can get things reset to where they were. Then get a metal or plastic tip screwdriver with the machine powered down and spin the pots from end to end more times than you thing is needed - like 50x. Be very careful to slowly find the end of the potentiometer first so you don’t try to overspin and break it. The potentiometers oxidize and get dirty resulting in intermittent contacts. You might be able to put some corrosion inhibitor paper / foam to put in the cabinet with the VFD to reduce the oxidation growth.

The J1-1 (0-5 VDC) and J1- 2 (0V) should give you some idea if the spindle driver is getting the correct voltage in. This should be roughly 1 vdc / 1000 rpm programmed and is non proportional to the exact rpm given it’s an analog input and can vary slightly.

If the voltage is changing (going into the spindle driver board), then the main control board should be looked at.