Danfoss FC302 AL-71 Fault — What It Means
Alarm 71 (PTC 1 safe stop) on a Danfoss FC302 VFD indicates that the drive has received a Safe Torque Off (STO) signal from an external PTC (Positive Temperature Coefficient) temperature sensor or a safety circuit connected to the drive’s STO input. The drive immediately stops and locks out motor output to prevent unsafe operation. This is a protection logic stop, not a thermal overload or damage fault. The drive will not restart until the STO condition is cleared and power is reset or the input is re-enabled.
The STO input is typically connected to terminals 19 and 20 or configured in parameter group 16-xx for safety interlocks. The fault is distinct from overtemperature alarms and is designed to enforce a safe, non-restartable stop when the PTC sensor detects motor winding overheating or when an external safety circuit (emergency stop, selector switch, or interlock) opens the STO loop.
Before You Replace Anything
Technicians sometimes replace the control card first, but most AL-71 faults are caused by loose wiring, tripped PTC sensors, or open safety interlocks. Check all external wiring and interlocks before ordering a new I/O PCB.
Common Causes
- Loose or broken STO wiring (~35%) A disconnected, corroded, or broken wire at terminals 19/20 (or configured STO terminals) breaks the Safe Torque Off loop and triggers the alarm.
- External PTC sensor activated (~30%) A motor winding temperature sensor (PTC type) detected overheating and opened the circuit, signaling STO to the drive.
- Safety interlock or emergency stop open (~20%) A safety interlock, emergency stop, or selector switch in the STO loop is not closed, triggering the safe stop.
- Incorrect parameter configuration (~10%) Safety interlock or STO input not properly enabled or mapped in parameters (group 16-xx) causes false activation.
- Faulty control card or STO input circuit (~5%) Internal failure of the control PCB’s STO detection circuit, even when external wiring is intact.
Quick Diagnosis
Answer these to narrow it down fast.
Are all emergency stops and selector switches in the closed (normal) position?
No: Close all safety switches and reset the drive. If the alarm clears, the interlock was the cause. If it remains, check wiring.
With power off, does a continuity test show a closed circuit between terminals 19 and 20?
No: Open circuit detected. Inspect and repair wiring, tighten connections, or replace corroded wires.
Is the motor temperature above its rated limit or are motor winding readings below 2 megohms?
No: Motor is fine. The fault is likely incorrect parameter settings or a faulty control card.
Step-by-Step Fix
- Power off the drive and lock out the disconnect to work safely.
- Check all emergency stops and selector switches in the STO loop to confirm they are in the closed (normal) position.
- Inspect STO wiring at terminals 19 and 20 (or configured STO terminals). Use a multimeter to verify continuity. Tighten all connections and replace corroded or broken wires.
- Test motor temperature and PTC sensor if present. Perform a megohm test on motor windings: readings below 2 megohms indicate insulation failure or overheating. Allow motor to cool if above rated temperature.
- Verify parameters in group 16-xx (such as 16-70 STO function) to confirm the STO input is correctly enabled and mapped. Check motor data in parameters 1-20 through 1-25, especially nominal motor current (1-24).
- Place a jumper between terminal 12 (24 VDC) and the digital input programmed as STO to test if the alarm clears. If it does, the external circuit is the issue.
- Cycle power to the drive after making corrections. If Alarm 71 persists despite correct wiring, interlocks, and parameters, replace the control card (I/O PCB).
Parts Often Needed
| Part | Notes |
|---|---|
| Danfoss FC302 Control Card (I/O PCB) | Amazon | Only if wiring, interlocks, and parameters are verified correct and the STO input circuit has failed internally. |
| PTC Temperature Sensor | Amazon | If motor winding sensor is confirmed faulty or damaged. |
When to Call a Pro
Call a qualified industrial electrician or VFD technician if you are not trained in variable frequency drive troubleshooting, motor thermal testing, or high-voltage DC bus work. This fault involves safety interlocks and Safe Torque Off circuits that are part of machinery safety systems. Incorrect diagnosis can lead to unsafe restarts or machinery damage. A pro has the proper test equipment (megohm testers, programming software like Danfoss MCT) and experience with parameter configuration in group 16-xx. If the control card needs replacement or motor insulation has failed, a technician should perform the repair and validate the drive operates safely under load.
Rough cost: A pro service call runs about $150-400.