Centroid Community CNC Support Forum

Anyone know if this:
https://www.phoenixcontact.com/online/p ... tegory=ALL
would be appropriate for isolating Acorn’s 0-10v output when connecting to a spindle motor speed control that has a floating ground? Apparently, a lot of magic smoke has been released from various electronics over the years due to the way the Chinese designed that speed control board. Floating grounds and such, I do not understand.

Scott,

What kind of VFD do you want to attach to the Acorn? You probably do not need an isolator unless you have a unique situation.

Russ

Hi Russ,
It’s not a VFD that I will be using. It’s some sort of speed control for a DC spindle motor. There’s a diagram and some cautions at this website:
http://tetralite.com/lathe/
He talks about the need to keep grounds separated. I’m a little lost with this.

Scott,

OK, looks like a DC speed controller. In their example they are using a CNC4PC C6 board which is taking PWM signal from Mach3 and turning into a voltage 0-10V. The C6 board provides opto isolation.

I assume you want to have Acorn control this DC speed controller for your lathe, correct?

Russ

Yes, exactly. Acorn already provides the function of the C6 board but does not have isolation. Can an optoisolator be added to the Acorn somehow?

I have looked at the schematic for the DC speed controller, they analog ground and analog out from the Acorn which are coming from an on board DAC go into terminals P2 and P4 on that controller. There should not be any high voltage in that part of the speed controller.

You can isolate the Acorn for the direction inputs by using the two on board relays in the Acorn. They could provide power to the relay on the DC speed controller to turn on the motor and change directions. That should isolate all the high voltage from the Acorn. They did not provide the full schematic for the DC controller so it is impossible to confirm this 100%.

Perhaps someone else has already connected this type speed controller to the Acorn which would confirm things, before you attempt this approach. I saw their warning, I think they just want to make sure you are very careful there is 120VAC on board and if you do not know what your are doing you can get hurt or killed.

Centroid team might have recommendations here.

Russ

I think the Sieg DC controllers are a copy of a KBIC-xxx SCR DC motor controller. As such it will need an signal isolation device between it and the analog out on the Acorn. I use the KBIC-240D myself. They are cheap & available easily on ebay, new & used. Has good schematics & is easily adjustable. Manual attached.

Hi Russ,

I’ll see if I can find a schematic that shows the speed control board in more detail. I am pretty certain some sort of isolation is needed based on other people’s experiences. It has something to do with the ground.

Here’s a quote from the link I posted earlier that sounds like the scary part:
“The ground from the C6 is connected to terminal P3 on the motor controller board. (Note that on the FC250J/110V controller board, P3 is the more negative side of the potentiometer and is therefore used as the "ground" reference for the analog input signal to P2. On other controllers, P1 is usually the more negative potentiometer connection. To be sure, measure the voltage across P1 and P3, noting which terminal is the negative side.) This negative reference "ground" must be isolated (or "floated") from the AC ground which is connected to the lathe chassis, the mains ground and the computer port ground because the P3 terminal is connected directly to the mains through the bridge rectifier on the motor controller board. Therefore, the power supply for the C6 board must be isolated from all other grounds and the jumpers must be set on the C6 board for optoisolation to keep the computer's ground isolated from the C6 ground.”

I’m confused about this:
“This negative reference "ground" must be isolated (or "floated") from the AC ground which is connected to the lathe chassis, the mains ground and the computer port ground because the P3 terminal is connected directly to the mains through the bridge rectifier on the motor controller board.”

So P3 is some sort of ground? P3 will be connected to the Acorn board, as will P2. In my case, I have a dual voltage power supply for 5vdc and 24vdc. Because Acorn is connected to the power supply and the PS is connected to AC ground (green wire), is it sufficient to remove the green ground wire going from the PS to the Acorn? Will that “isolate” the grounds?

Franco did this in his recent YouTube video, but he also did a few other things which would not work for me. So it was suggested by a viewer to install an isolator.

DICKEYBIRD wrote: ↑Wed Dec 27, 2017 12:50 pm I think the Sieg DC controllers are a copy of a KBIC-xxx SCR DC motor controller. As such it will need an signal isolation device between it and the analog out on the Acorn. I use the KBIC-240D myself. They are cheap & available easily on ebay, new & used. Has good schematics & is easily adjustable. Manual attached.

Hi Milton,
I’m not sure what it’s a copy of but I’m sure it’s a copy of someone else’s product. Thanks for the info. I’ll look into that isolator. Would you happen to have a drawing or something showing how it would be wired to the Acorn and your speed controller? I think I understand how it’s wired up but would be nice to be sure.

The part I linked to is, supposedly, something similar to the KBIC isolator...I think.

The DC motor speed controller was designed to handle high voltage motors over 180V. As someone else wrote this is why they want it totally isolated from Acorn. The one dickeybird used was only $20 on the link below, might be the best way to move forward.

Russ



https://pcmsurplusworld.com/product/kbs ... 02-a57141/