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How to Connect a VFD to a PLC: Hardwired, Analog and Network Control
There are three ways for a PLC to control a VFD, and many systems combine them:
- Discrete (hardwired) I/O for start, stop, direction and status.
- An analog signal (0–10 V or 4–20 mA) for the speed reference.
- A network (Modbus RTU, Modbus TCP, EtherNet/IP and others) for commands, speed and diagnostics over one cable.
Method 1: Discrete I/O
| PLC side | Drive side | Purpose |
|---|---|---|
| Digital output | Run forward input | Start/stop |
| Digital output | Reverse input | Direction |
| Digital outputs | Preset speed inputs | Select fixed speeds |
| Digital output | Fault reset input | Remote reset |
| Digital input | Drive relay output (fault, running, at speed) | Status feedback |
Check whether the drive's digital inputs are sinking or sourcing (often selectable by a switch or parameter) and match them to the PLC outputs. See sinking vs. sourcing. Many drives provide their own 24 V supply for their inputs, which a PLC relay output can switch directly.
Method 2: Analog speed reference
Wire a PLC analog output to the drive's analog input. Configure both for the same signal type, and set the drive's minimum and maximum frequency to match the signal range: for example 0 V = 0 Hz and 10 V = 60 Hz. You can also bring a drive analog output (output frequency or current) back to a PLC analog input.
- Use shielded twisted-pair cable, shield grounded at one end.
- Prefer 4–20 mA for longer runs or noisy areas; see 4–20 mA vs. 0–10 V.
- Keep the analog common separate from power commons, as the drive manual shows.
Method 3: Network control
A single communication cable replaces most of the hardwiring and gives far more information: speed command and feedback, motor current, fault codes, run hours and parameter access.
- Modbus RTU over RS-485 is supported by most drives, including many low-cost ones. See Modbus RTU vs. TCP.
- Industrial Ethernet such as EtherNet/IP, PROFINET or Modbus TCP, built in or by option card. See industrial Ethernet compared.
Typical steps: set the drive's communication address and settings, set the start/command source and speed reference source to "network," map the drive's control word, status word, speed reference and feedback registers in the PLC, and set a communication-loss action (for example, stop) so a broken cable cannot leave the motor running uncontrolled.
The drive parameters that matter most
- Start/command source: keypad, terminals or network.
- Speed reference source: keypad, analog input, preset speeds or network.
- Digital input functions: what each terminal does.
- Analog input type and scaling.
- Relay output functions: fault, run, at speed.
- Motor nameplate data, accel and decel times.
If a drive ignores the PLC, the start or speed source is usually still set to keypad.
Safety and good practice
- Do not rely on a PLC run signal for safety stopping. Use a proper safety circuit, such as the drive's Safe Torque Off inputs.
- Run motor cables separately from control and communication cables, and use shielded motor cable.
- Bring the drive fault output into the PLC so the program can alarm and stop the process.
Frequently asked questions
Which method is best?
Networking gives the most information with the least wiring. Hardwired I/O plus analog is simplest to troubleshoot and works with any PLC.
Can one PLC control several drives?
Yes. On a network, each drive just needs a unique address.
Build the system
Unitronics PLCs and UMI drives pair naturally; see how to choose a UMI VFD and what is a PLC. Browse Unitronics, call 1-800-724-4209, or request a quote.