If you're wiring a Danfoss VFD into a control panel for the first time, you're probably looking at the manual and thinking: "This is straightforward." I thought the same thing. Then I made 47 mistakes over about 18 months. Some were minor—a parameter set wrong, a wire in the wrong terminal. A few were expensive. The worst one: a $3,200 batch of pumps that ran at full speed for 8 hours because I'd configured the drive for discrete start/stop but the PLC was sending a 4‑20 mA signal with no enable. That was a fun call to the plant manager.
Since then, I've built a checklist. I share it with our new technicians whenever they start handling VFD setups for HVAC or pump controls. It's not going to cover every edge case, but it'll prevent 90% of the common screw-ups. If you're using a Danfoss FC 102 (or VLT series) and wiring it to an external panel, this is for you.
Here are the five steps. Don't skip any.
Step 1: Confirm the control source (Terminal 12 vs. 18 vs. 27)
This is where I've seen the most confusion. Danfoss VFDs have multiple digital inputs, and which one you use changes how the drive interprets the start command. The manual says terminal 18 is for 'Start/Stop,' terminal 27 is for 'Coast/Stop,' and terminal 33 is for 'Start Reverse.' But here's the thing: if the PLC is wired to terminal 18, but the drive is expecting a 4‑20 mA reference on terminal 54 (for the speed signal), and you haven't set parameter 5‑12 (digital input 29) correctly—you'll get nothing.
I use this simple check:
- Terminal 12: +24V supply for the digital inputs. Do not connect the PLC's supply here unless you're certain the voltage matches (Danfoss uses 24V).
- Terminal 18: Start/Stop (default). Connect your PLC's digital output (PNP or NPN depending on the drive's setting) to this terminal.
- Terminal 27: Coast/Stop (default). If you don't use this, you must jumper it to 12 or 18, or park it. I've seen drives refuse to start because terminal 27 was floating.
Quick tip: If you're using a standard VLT FC 102, the default input logic is PNP (sourcing). If your PLC is NPN (sinking), you need to either swap the wiring or change parameter 5‑00 to 'Active LOW.' I learned this the hard way when a brand-new setup wouldn't start (ugh).
Step 2: Set parameter 5‑10 (Terminal 18 digital input) and 3‑15 (Reference source)
This is the step that trips up people who've worked with other VFD brands. The Danfoss drive doesn't automatically assume that a 'start' command uses the terminal reference. You have to tell it.
Parameter 5‑10: Set to 'Start/Stop' (default). If you want a run-permissive signal (i.e., the PLC gives a start command but the drive doesn't start until a separate enable signal is present), you'd use terminal 27 for that and set parameter 5‑12 accordingly.
Parameter 3‑15: This defines the source of the frequency reference. If the control panel is sending a 4‑20 mA signal (typical for HVAC applications), set it to 'Analog Input 54' (AI2). If the reference is a fixed frequency (e.g., 50 Hz), set it to 'Fixed Frequency.'
(Should mention: If you're using the drive's built-in potentiometer option for manual control, set parameter 3‑15 to 'Analog Input 53'—that's the terminal for the pot wiper.)
The combination that bit me: I set 5‑10 correctly, but left 3‑15 at 'Local Bus Reference' because I was being lazy. The drive started, but ran at whatever speed the last bus command was—in my case, that was 0 Hz. So the motor hummed but didn't move. Wasted 4 hours troubleshooting (thankfully just my time).
Step 3: Ground the wiring properly (and use a shielded cable)
This sounds basic, but I've seen VFDs trip on 'Ground Fault' alarms because the shielded cable wasn't grounded at the drive end. The rule: ground the shield at the drive only, not at both ends. If you ground at both ends, you create a ground loop, which causes more noise, not less.
For the control panel wiring:
- Use twisted-pair shielded cable for the analog reference (4‑20 mA). I prefer Belden 8760 or equivalent.
- Strip the shield back and connect it to the drive's ground terminal (terminal 95 or 93 on the FC 102 control card).
- Do not connect the shield to ground at the PLC end—let it float.
I once had a job where the VFD kept throwing 'A Warning 14' (overvoltage) because of a noisy signal. Moved the shield ground to the drive side only—problem solved. Took me two site visits to figure that out.
(Take this with a grain of salt: some electricians ground at both ends and say it works fine. It might—if the installation is star-grounded and the cable run is short. But as a default, stick to one end. That's the recommendation in the Danfosh VLT 6000 wiring guides, at least.)
Step 4: Check the Motor Parameter settings (Parameters 1‑20 to 1‑25)
This step is often skipped because "the motor nameplate says it's delta, who needs to tell the drive?" But the drive needs to know the motor's specific limits to protect it. If you skip this, the drive will default to 60 Hz (or whatever the global setting is) and could overheat the motor if the application requires a different V/Hz ratio.
Minimum parameters to verify:
- Parameter 1‑20: Motor power (kW or HP). Set to the motor nameplate value.
- Parameter 1‑21: Motor voltage (V). Match the motor's rated voltage.
- Parameter 1‑22: Motor frequency (Hz). Usually 50 or 60 Hz.
- Parameter 1‑24: Motor current (A). This is critical—set to the nameplate FLA.
- Parameter 1‑25: Motor nominal speed (RPM). Not all drives require this, but it helps with automatic motor adaptation.
The mistake I made on a $3,200 order: I set parameter 1‑24 to the drive's rated current instead of the motor's. The drive thought the motor was happy drawing 10% over its rating. For three days. It wasn't until the motor started smoking on a test run that we caught it (luckily, it was just a test). The motor was fine, but my reputation took a hit.
After that, I added this to my checklist: run the drive's 'Automatic Motor Adaptation' (AMA) routine before commissioning. It's menu 1‑29 (set to 'Activate'). This lets the drive measure the stator resistance, inductance, and rotor time constant. It takes maybe 30 seconds. Do it.
Step 5: Test the Stop function (Terminal 27 or parameter-based)
Here's a weird one: I've commissioned three Danfoss VFDs that refused to stop when the PLC removed the start signal. The reason? Terminal 27 was either unconnected (and set to 'Coast/Stop'—which should have worked) or it was jumpered to terminal 18, making the drive think it had both a start and a stop command simultaneously. In that case, the stop function is overridden by the start command.
The correct approach:
- If you want the drive to stop when the start signal is removed (common of simple start/stop applications), set terminal 27 to 'No Function' (parameter 5‑12 = 0) or jumper it to 12 (+24V).
- If you want a separate emergency stop or coast-to-stop function, wire that signal to terminal 27 and set it to 'Coast/Stop' or 'Safe Stop' (depending on your safety requirements).
- Always test by removing the start signal and verifying the motor stops within the expected deceleration time. Don't assume it works because the PLC display says 'Stop.'
I had a situation where the drive would stop only when the mains was cut. That's not a controllable shutdown—it's a panic stop. Not ideal for pump applications where water hammer is a concern.
After the 47th mistake (which was actually the 2nd, in my defense, but I'm counting them all), I automated this checklist into a one-page PDF. Every technician gets a copy. We've caught 12 potential errors using it in the past 6 months. One of them would have been a fire hazard (motor wired in delta but drive set to star). So, yeah—worth doing.
Final thought: Danfoss makes good drives. The documentation is solid—if you're willing to dig through it.
