ABB ACS550 AI1 LOSS - Causes & Fix

ABB ACS550 AI1 LOSS — What It Means

The AI1 LOSS fault on an ABB ACS550 drive means analog input 1 is either missing or has dropped below the minimum threshold set in the drive’s parameters. This is not a power-stage failure. The drive is telling you it cannot see a valid signal on AI1, or the signal voltage or current is lower than the fault limit configured in parameter 3021. The drive monitors AI1 for a valid process signal (like a 4–20 mA transmitter or 0–10 V controller output), and when that signal disappears or goes out of range, it throws this fault to protect the process.

The fault is tied directly to parameter 3021 (AI1 FAULT LIMIT) and parameter 3001 (AI<MIN FUNCTION). If your analog input is below the limit or the wiring is open, the drive stops and logs AI1 LOSS. This is a signal-chain problem, not a drive electronics failure.

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Common Causes

• No analog signal source or source is powered off The transmitter, PLC, or controller feeding AI1 is not energized or not providing an output.
• Open, loose, or broken wiring on AI1 circuit The cable between the field device and the drive terminal is damaged, disconnected, or landed on the wrong terminal.
• AI1 FAULT LIMIT (parameter 3021) set too high The configured fault threshold is above the normal operating range of your analog signal, so even valid signals trip the fault.
• Signal type mismatch at AI1 input The drive is configured for 0–10 V but the field device is sending 4–20 mA, or the input scaling does not match the physical signal type.
• Failed or disconnected loop power supply If the analog transmitter requires external loop power (common with 4–20 mA sensors), that power source may be dead or wired incorrectly.
• Faulty analog transmitter or sensor The upstream device itself has failed and is no longer producing a valid output signal.

Step-by-Step Fix {#fix}

1. Confirm the fault number on the drive display and verify it reads AI1 LOSS, not AI2 LOSS or another code, then note whether the drive was running or attempting to start when the fault occurred.
2. Inspect the physical wiring at AI1 terminals on the drive and at the field device for loose screws, broken conductors, wrong terminals, or damaged insulation.
3. Measure the analog signal with a multimeter at the AI1 input terminals (voltage or current depending on your signal type) and compare the reading to the expected operating range of your transmitter or controller.
4. Check parameter 3021 (AI1 FAULT LIMIT) in the drive menu and confirm it is set below the normal minimum operating value of your analog input so valid signals do not trip the fault.
5. Check parameter 3001 (AI<MIN FUNCTION) to verify the drive’s response to under-minimum signals is appropriate for your application and not accidentally forcing a fault when it should ignore or clamp the value.
6. Restore the signal source by repairing or replacing the damaged cable, reconnecting loop power if used, fixing the transmitter or controller, or correcting any wiring errors at either end of the circuit.
7. Clear the fault from the drive using the panel reset button or the fieldbus command, then monitor AI1 in real time on the drive display to confirm the signal is stable and within range before restarting.

Parts Often Needed

When to Call a Pro

Call a qualified electrician or controls technician if you have restored wiring and verified loop power but the AI1 signal still reads zero or out of range, if you are unfamiliar with analog signal troubleshooting and do not have a multimeter, or if the drive parameters were customized by an integrator and you are unsure which settings are safe to change. Also get help if the fault returns immediately after clearing even when the analog input measures correctly, since that may indicate a failed AI1 input circuit on the drive itself or a grounding or noise issue that requires specialized diagnostic tools.

See Also

• ABB ACS580 A7AB Fault - Causes & Fix
• ABB ACS580 A2A1 - Causes & Fix
• ABB ACS880 Fault 2310 - Overcurrent Diagnosis and Fix
• ABB ACS550 Complete Fault Code Guide — All Faults and Fixes