If you're reading this, you've probably already been staring at a Danfoss VFD drive that's sitting idle with the green "ON" light glowing, the panel displaying status: ready—but the motor won't start in auto mode. It's a frustrating spot.
I handle troubleshooting calls for industrial drive systems. Over the past six years, I personally parked 30+ working hours into diagnosing "auto mode not working" calls, probably eight of which were my own misconfiguration. I now maintain a checklist for our team. This is that checklist, slightly cleaned up for sharing.
Who this is for: Engineers, maintenance techs, and electricians dealing with Danfoss VLT drives (FC 102, FC 202, FC 302) that won't start in auto mode.
What it covers: A 4-step diagnostic checklist that accounts for 90% of the auto mode failures I've seen.
Step 1: Verify the Terminal Configuration (Parameter 5-10 and 5-12)
This is the most common culprit, and honestly—the one I missed on my first three calls. The drive needs to know which terminals control start and run permissive when in auto mode.
Check parameter 5-10 (Terminal 18 Digital Input): This is typically the "start" command. In auto mode setups, this should be set to [8] Start or [9] Start latched, depending on whether you need latching or non-latching behavior.
Check parameter 5-12 (Terminal 27 Digital Input): This is often used as a "coast stop inverse" or safety chain input. Factory default is usually [2] Coast inverse. If this terminal is open (no 24 VDC present), the drive will refuse to run—even in auto mode. I can't count how many times I've found a safety device (like an e-stop or door switch) that was tripped, leaving terminal 27 floating.
Real talk: In 2022, I spent four hours on a site visit for a "dead drive." Turns out, the terminal 27 wire had been pulled out during a cleaning crew's vacuum session. No 24 VDC, no run. I felt like an idiot. But now it's on the checklist.
Check point: Measure 24 VDC between terminal 27 and terminal 39 (common). If you don't see ~24 VDC, trace the wire back to the safety relay or PLC output.
Step 2: Confirm the Reference Source (Parameter 3-15)
Even if the drive gets a start command, it won't spin if the reference source isn't properly configured. Parameter 3-15 defines where the drive looks for its speed reference in auto mode.
If using an analog signal (4-20 mA or 0-10 VDC):
- Parameter 3-15 should be set to [1] Analog input 53 or [2] Analog input 54, depending on which terminal you're using.
- Verify that terminal 53 is receiving 4-20 mA (or 0-10 VDC). I've seen cases where a broken wire gave a 0 mA signal, and the drive treated that as "0 reference"—so it readied up, but never turned on the output.
If using fieldbus (Profibus, Modbus, etc.):
- Parameter 8-xx series must be configured for the correct protocol. It's easy to accidentally leave it on a default setting that doesn't match your network.
- Check the fieldbus status LED. On most Danfoss drives, a steady green means communication is established. Blinking amber? You've got a configuration mismatch.
This was accurate as of Q1 2024. Danfoss released firmware updates for the FC 302 series, which may have altered default parameter values. Always verify with the latest version manual for your drive.
Step 3: Examine the Application Settings (Parameter 1-00)
This one is subtle. Parameter 1-00 (Configuration Mode) determines the drive's control structure:
[0] Speed closed loop — Requires an encoder feedback signal. If you're in this mode and no encoder is connected (or the encoder is faulty), the drive will not start. I learned this the hard way when I inherited a panel where someone had changed 1-00 from [1] to [0] thinking it was a "better" setting.
[1] Speed open loop — Standard V/Hz or sensorless vector. This is the correct mode for the vast majority of HVAC and pump applications.
[2] Torque — Rarely used for auto mode pump/fan applications.
Here's the thing: If you're 100% sure steps 1 and 2 are correct, look at 1-00. I've seen three cases where a technician accidentally switched it to closed loop mode. The drive didn't throw a clear fault—it just sat there, waiting for an encoder pulse that would never come.
Step 4: Check the Lockout / Interlock Circuit (Terminal 37)
Terminal 37 is the Safe Torque Off (STO) input on newer Danfoss VFD drives (FC 302, for example). If this terminal is not receiving 24 VDC, the drive is physically prevented from producing power to the motor. Even in auto mode.
Take this with a grain of salt: I only started checking terminal 37 after my second year in the field. It's not explicitly labeled as a "run" requirement in the basic manual, and it's easy to skip if you're following older documentation. But a missing 24 VDC on pin 37 = dead motor, every time.
How to test: Remove the terminal plug and measure for 24 VDC between 37 and 39. If present, reinsert the plug. If not, trace back to the safety relay or external STO switch.
Common Mistakes I've Documented
Over the last 18 months, our team caught 47 potential errors using this checklist. Here are the repeat offenders:
- Terminal 27 floating: Noted 12 times. Usually a loose wire or a disconnected safety device.
- Wrong reference source: Noted 8 times. Someone changed parameter 3-15 during a test and forgot to revert it.
- Encoder mode without encoder: Noted 4 times. Always double-check parameter 1-00 before calling for support.
- STO circuit open: Noted 7 times. Often caused by an external safety relay that wasn't reset after an emergency stop test.
My experience is based on about 150 drive troubleshooting sessions across HVAC, water treatment, and conveyor applications. If you're working with different drive series (like VLT AQUA or Micro Drive), terminal assignments and parameter names may vary. Check your specific manual.
Last-Minute Safety Reminder
Never open the drive enclosure while powered. The DC bus capacitors can hold lethal voltage for several minutes after power-off. I once saw a technician get a serious shock because he assumed a 30-second wait was enough. It wasn't. Wait at least 5 minutes, and always verify with a multimeter before touching any terminals.
Roughly speaking, this checklist solves 9 out of 10 auto mode issues I encounter. If you've gone through all four steps and the drive still won't start, it might be a hardware fault—but verify your parameter settings one more time before swapping the drive.
