VFD DCM and COM:What do the terminals DCM and COM mean on a VFD?
Q: What do the terminals DCM and COM mean on a VFD?
A: On a VFD, DCM typically stands for Digital Common, which is the common return path for digital input signals. COM is a more general Common terminal, often used for analog or multi-function reference. Both serve as the reference point for control signals, but their exact function depends on the manufacturer. For example, on many Delta VFDs, DCM is the common for multi-function digital inputs, while COM may be shared with analog outputs or relay commons. It is critical not to connect DCM and COM together unless the manual explicitly permits it, as they may be isolated or at different potentials. Always consult the VFD's wiring diagram to identify which common is tied to which circuit. Miswiring can cause erratic operation, damage to the drive, or failure to respond to start/stop commands.
Q: How should DCM and COM be wired when using a 2-wire control for a VFD?
A: For 2-wire control, you typically use digital inputs like FWD and REV. One side of your switch or relay contact connects to the input terminal (e.g., FWD), and the other side connects to DCM (Digital Common). COM is often not used in this case unless your drive uses it as the common for digital inputs. Some VFDs label the digital common as COM instead of DCM. The key is to identify the common terminal associated with the digital input group. Never run external voltage into DCM; it is usually a low-voltage reference tied to the drive's internal supply. Use dry contacts only. If you also have analog speed reference, its common (often ACM or COM) should be separate from DCM to avoid noise coupling. Always verify with a multimeter that DCM is not shorted to earth or power terminals before wiring.
Q: Can I connect DCM and COM together on a VFD to simplify wiring?
A: Generally, no. DCM (Digital Common) and COM (Common) are often internally isolated or serve different signal groups. Connecting them can create ground loops, introduce noise, or damage the drive if they are at different potentials. For instance, on some VFDs, COM is tied to analog signal ground, while DCM is the return for digital inputs that may be optically isolated. Bridging them defeats isolation and may cause false triggering of inputs. Only connect them if the manufacturer's manual explicitly states they are the same net or can be tied. Even then, use a single point connection to avoid circulating currents. If you need a common reference for both analog and digital, look for a dedicated terminal like GND or ACM, or use separate isolated supplies. When in doubt, keep them separate.
Q: What are common troubleshooting steps if a VFD does not respond to digital inputs wired to DCM?
A: First, verify that the input terminal (e.g., FWD) is actually programmed for the desired function and that the control source is set to terminals, not keypad or communication. Measure voltage between the input and DCM: you should see the drive's internal control voltage (often 24Vdc) when the input is open, dropping to near zero when closed. If no voltage, check that DCM is properly connected and not accidentally tied to COM or earth. Ensure the switch or relay is dry contact, not sourcing voltage. Also check for blown internal fuses or opto-isolator failure by testing another input. If DCM is shared with other functions, disconnect them temporarily. Finally, confirm that the drive is not in a fault state or local lockout. Consult the manual for specific terminal functions, as DCM may be sink or source logic depending on the model.
Words of experience
Common scenarios of "VFD DCM and COM"
【Engineer】 Hi, I'm having trouble with a VFD. The display shows 'DCM' and 'COM' terminals, but I'm not sure what they mean.
【Technician】 DCM stands for Digital Common, and COM is also a common terminal. They are often used for digital input/output connections.
【Engineer】 So are they the same? Can I use them interchangeably?
【Technician】 Not always. It depends on the VFD model. DCM is typically the common for digital inputs, while COM might be for analog or relay outputs. Check your manual.
【Engineer】 I see. In my manual, DCM is for digital input common, and COM is for relay output common.
【Technician】 That's common. So if you're wiring a digital input, you connect the switch between the input terminal and DCM. For a relay output, you use COM as the common for the relay contacts.
【Engineer】 What about the voltage levels? Are they isolated?
【Technician】 Usually DCM is referenced to the VFD's internal 24V DC supply, while COM for relay outputs is isolated and can handle higher voltages. Always verify with a multimeter.
【Engineer】 So I shouldn't connect them together?
【Technician】 Correct, unless the manual explicitly says they are the same. Connecting them could cause a short circuit or damage.
【Engineer】 What if I need to use both a digital input and a relay output? Do I need separate commons?
【Technician】 Yes, typically you would use DCM for the digital input circuit and COM for the relay output circuit. They are separate to maintain isolation.
【Engineer】 OK, that makes sense. Now, for wiring a digital input, do I need a jumper between DCM and another terminal?
【Technician】 Sometimes there's a jumper to select between sink and source logic. For example, if you want sink logic, you connect DCM to the common of the switches, and the switches to the input terminals.
【Engineer】 What's sink vs source logic?
【Technician】 Sink logic means the input is activated when connected to common (0V). Source logic means the input is activated when connected to a positive voltage. The VFD manual will show how to set it up with DCM and possibly a jumper.
【Engineer】 Got it. So in sink logic, DCM is the 0V reference, and I connect the switch between the input and DCM.
【Technician】 Exactly. And for source logic, you might connect DCM to a positive voltage or use a different terminal. Always follow the manual.
【Engineer】 Thanks. I'll double-check the manual for my specific VFD model.
【Technician】 You're welcome. If you're still unsure, feel free to ask. Safety first!
