Centroid Community CNC Support Forum

I went out to my machine today to see if a cap would help but my vfd was rock solid from 5000 all the way to 24000. I tried about 8 different speed settings from 5000 rpm up to 24000 rpm and I was actually shocked the vfd never fluctuated on any of the speeds. Needless to say I didn’t add the cap. Just lucky I guess.

There is no problem with the analog output under normal circumstances. The output is an opamp. The most basic form of the circuit has worked well on hundreds of Centroid products over the years. It looks like this:
The problems occur from external non-resistive loads or electrical disturbances. This could be from wire routing, cable type, poor quality VFDs, or other sources of noise. Sometimes the noise sources are extreme enough to damage the ACORN's output. To solve this, ACORN revisions 180625 and newer have a zener diode added for protection.
Now, the external noise might be enough to cause some instability, but not damage. In this case it may be useful to add a capacitor to absorb external disturbances. Opamps are not designed to drive large capacitive loads, so the capacitor should be as small as possible to solve the original problem. If too much capacitance is added, the output may oscillate (like a sine wave).
A larger capacitor can be used if the opamp load is reduced with a resistor before the capacitor. The resistor improves stability, but also creates a voltage divider with the input impedance of the VFD. A higher value resistor will be more stable but create more error in the voltage level at the VFD.
Another approach is a snubber circuit. The resistor and capacitor added in this version prevent oscillation from the extra capacitance. There is no resistor in series, so the average voltage to the VFD is not reduced. This circuit requires 3 parts, so it is not as convenient to hand wire.

I considered creating a new thread about my issues and eventual solutions on this same topic ( RPM fluctuating due to noise in the Acorn analog output wire causing voltage fluctuation at the VFD ) but it made more sense to add to this thread . Most of the solutions are outlined already in this thread however I think these pictures and youtube video add some good "visual" detail.

Pictures/examples of proper shielded (VFD) wire termination, a youtube video of a good technique to terminate the ends of a VFD cable and a description of how to solve the issue of noisy analog inputs even if proper VFD shielding does not fully fix the issue, according to a VFD manual from Automation Direct ( for the GS4 line of VFD's ). It mentions the use of a cap but also ferrite cores "rings" and other details. ( see the picture that starts with "Analog inputs" )

Best, Jake

link to youtube on how to properly terminate the shield on a VFD cable :

I too was having the RPM fluctuation on my lathe using a Teco A510 for a 5hp spindle motor. Tried adjusting the VFD Time Filter and also had to adjust the Bias setting in VFD to -2. This did help a lot but RPM was not accurate ,sometimes would be stable then drop about 100 rpms and would be off about 150RPMs at higher RPMs 1000 and above. Did not have this issue with my previous controller using the same cable.
So i did what was suggested a few posts back and placed a .1uf ceramic cap across Acorns 0-10v output and fluctuating RPM issue went away. Was also able to remove the Bias and Time filter in the VFD.
Will also be adding a separate shielded cable with drain for the analog Acorn 0-10v output.
Thanks for the resources guys.
Troy

Glad to hear it helped you as well Troy. Either good grounding, the cap or both will always do the trick. Thanks for sharing.

Glad I found this. I have a similar problem on a newly retrofitted Tree J325. Yesterday I did an Autotune on the VFD (Unidrive) and it helped. I'll try the shielded cable next
UPDATE: The shielded cable worked great. The surging sound on my RPC stopped and the spindle speed is stable

The snubber with 2 capacitors and a resistor worked perfect for me to connect a Servo in speed mode.

Thanks for the infos.

Kind regards

Uwe