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  1. #1
    I'm currently replacing the motion controller in my router with a UC300ETH+UCBB board. Previous motion controller had an on-board relay for the VFD control which used a n/o contact to connect "For" to "DCM". DCM is the common ground for the digital inputs on the VFD. However, the UC300/UCBB combination does not have a relay. The UCBB digital outputs use a MOSFET to short the output to board ground, which is fine for things like the stepper drivers. I don't really want to have to install a separate relay board, so wonder if I can connect DCM to the UCBB ground and "For" on the VFD to a UCBB digital output. The main issue here is whether it is safe to connect VFD DCM to ground in the control box.

    The HYmanual I have is no help on this one!

    Anyone have experience of switching an HY VFD like this? I'm sure that there are plenty of UC300/HY VFD systems out there...

    Thanks in advance,

  2. #2
    Doddy's Avatar
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    That's from the manual. The UCBB should be more than able to drive the inputs to the VFD. Connecting the DCM to the main controller ground (/UCBB ground input) is perhaps something to be avoided if possible, but it's not the end of the world to connect them (arguably, less noise immunity). If there is a problem then you know you have a solution that is to provide true galvanic isolation with a relay. It might be worth examining your system - you *might* find you have a lightly coupled 0v reference in any case through any EMI capacitors in any SMPS you have, negating any advantage and the connecting of 0V to DCM may actually be beneficial.

    Exec summary: It'll work, go into this with eyes open for noise immunity.

  3. #3
    Thanks for the comments. My main concern was about the isolation of DCM inside the VFD but overnight it occurred to me that if I just check volts between FOR and DCM (expecting 12-24v or so), then both to control box ground, I can get some idea whether or not they are isolated. And if all else fails, I stick a relay in there. I've just found that I need to get a 16-pin IDC connector to pick up the analogue output on the UC300 as they don't provide that as part of the kit so things are going to be a little delayed anyway.

    I have a copy of the manual - it's good that they show a circuit diagram of the board's interfaces. Gives a bit more insight into what's going on under the covers!

  4. #4
    Quote Originally Posted by Neale View Post
    Thanks for the comments. My main concern was about the isolation of DCM inside the VFD but overnight it occurred to me that if I just check volts between FOR and DCM (expecting 12-24v or so), then both to control box ground, I can get some idea whether or not they are isolated. And if all else fails, I stick a relay in there. I've just found that I need to get a 16-pin IDC connector to pick up the analogue output on the UC300 as they don't provide that as part of the kit so things are going to be a little delayed anyway.

    I have a copy of the manual - it's good that they show a circuit diagram of the board's interfaces. Gives a bit more insight into what's going on under the covers!
    You can use Dupont 2.54mm wires/ connectors directly on the UC300eth IDC pins.
    VFD FOR - DCM is just a ground sinking line.
    External relay (or opto module) is needed to control it, COM/NC to make or break that single line (on/off).

    See my posts of this cncdrive thread:
    http://www.forum.cncdrive.com/viewto...=3780&start=30

    There are some wiring diagrams on page3 and images on page4

    Diagram here too:
    http://www.forum.cncdrive.com/viewtopic.php?f=19&t=3829

    Shows connecting relay via Bob and also dupont uc300 direct.
    Has AO1 wires too.

    (your vfd may differ slightly)
    Last edited by dazp1976; 09-05-2022 at 11:06 AM.

  5. #5
    I would definitely use an interposing relay.
    Spelling mistakes are not intentional, I only seem to see them some time after I've posted

  6. #6
    Quote Originally Posted by EddyCurrent View Post
    I would definitely use an interposing relay.
    You need to. Or any other kind of 'switch'.
    You can't directly connect a breakout board / controller output to vfd FWD / DCOM.
    It simply does not work (or will break them) .

    Put a single wire link between fwd an dcom and fwd will come on. Break that link and it will go off.
    Use the relay to control the make or break link.

  7. #7
    Doddy's Avatar
    Lives in Preston, United Kingdom. Last Activity: 15 Hours Ago Has been a member for 8-9 years. Has a total post count of 1,344. Received thanks 187 times, giving thanks to others 65 times. Referred 1 members to the community.
    Quote Originally Posted by dazp1976 View Post
    You need to. Or any other kind of 'switch'.
    You can't directly connect a breakout board / controller output to vfd FWD / DCOM.
    It simply does not work (or will break them) .
    Why?

  8. #8
    I was wondering the same thing! At the heart of my original question was, "is the digital input section of the HY VFD floating WRT ground?" My quick measurements suggest that it is - 24V between FOR and DCM (to be expected), but a few tenths of a volt between either connection and ground. Measurement with a DMM, so high impedance and that doesn't sound unreasonable. When I look at how I wired the analogue speed signal to the VFD, I see that ACM (analogue "ground") was connected to the control box ground as that was how the analogue output from the motion controller was referenced so floating digital input doesn't seem silly.

    I'll be testing this anyway, once I've resolved the little issue of why my stepper drivers don't seem to be responding to the pulse output from the UC300. Probably forgotten a wire somewhere...

  9. #9
    Quote Originally Posted by Doddy View Post
    Why?

    It's been done on another forum a while back.
    Directly connected it and broke the axbb-e. Had to replace it.
    May have done it wrong in first place.
    Relay is the way to go.
    24v coil spst.
    24v module type.
    24v ssr.

    Think of vfd like sticking a jumper between for and dcom.
    In = on, removed = off. Simples.

  10. #10
    Doddy's Avatar
    Lives in Preston, United Kingdom. Last Activity: 15 Hours Ago Has been a member for 8-9 years. Has a total post count of 1,344. Received thanks 187 times, giving thanks to others 65 times. Referred 1 members to the community.
    Final comment from me then I'm going back under the rock for 6 months. The control inputs to the HY VFD are fed each though onto-isolators on the control board internal to the VFD. You already have galvanic isolation... until the point that Neale asked about - providing a common 0v line between the VFD DCM and the UCBB 0v. I've not stripped my HY down for a few years, but there's descriptions on the net of all internal supplies (including the 24V sensor input bias supply, and presumably the logic supplies) from a single transformer (presumably independent secondaries) - so that helps to explain the isolated nature of the supplies that Neale has observed. Yes, you have to expect that you will be presented 24V (minus any Vf on the opto-isolator LEDs) and that could destroy any control board directly coupled to the VFD UNLESS that control board is designed with an open collector/open drain output current sink, as is the case with the UCBB - see first reply - or for the isolated outputs from the AXBB-E. Both of these controllers are intelligently designed around the intent to drive such 24V signalling control systems 9such as the VFD).

    Anyway, I'm off.

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