Fanuc spindle alarm 03 (AL-03) means the DC link fuse inside the spindle amplifier has blown — the amplifier sees insufficient DC link voltage. The check runs the moment the emergency stop is cancelled, so the alarm typically appears either mid-operation (the fuse blew while cutting) or immediately at E-stop reset / CNC start-up. The critical thing to understand before ordering parts: a DC link fuse almost never blows for no reason. The fuse is the symptom — the short that blew it is the disease, and Fanuc's manual lists exactly three suspects. Replace the amplifier without finding the short, and the replacement can blow on day one.
Case 1 — Alarm appeared during spindle rotation
The fuse blew under load — something in the power path shorted. Fanuc's three causes, and how to check each (power down, lock out, wait for the charge LED to go out):
- Power lead shorted to ground: disconnect the motor power cable at both ends and megger each conductor to earth (500 V DC). Below 1 MΩ is your fault — the usual finds are drag-chain crush damage and coolant pooling in conduit. Replace the cable; do not tape it.
- Motor winding shorted to ground: megger the motor windings to frame at its terminal box. A winding leg down to earth — often after coolant ingress or a genuine overheat history (see AL-01) — means rewind or replacement. Re-test warm if the cold reading is borderline; moisture-degraded insulation passes cold and fails hot.
- IGBT / IPM module failure inside the amplifier: a shorted output module dumps the DC link through itself and takes the fuse with it. Diode-test across the output legs — near-zero reads confirm it.
In all three cases the amplifier needs opening — the internal fuse is not an operator-serviceable part, and if the IPM shorted, the gate drives and current sensing must be verified too or the new module dies at first power-up. This is precisely the component-level repair we do daily; Fanuc's ‘replace the Spindle Amplifier’ is the fast-but-expensive path, our bench repair the economical one — but only after the cable and motor have been megger-cleared, or the repaired unit blows again on reconnection.
Case 2 — Alarm at E-stop cancel / CNC start (contactor pulls in, then drops)
The power supply's input magnetic contactor switches on and immediately back off with AL-03 (note: with two spindles connected, the contactor may stay on). Fanuc's two causes:
- DC link wiring not connected / wrong: the short bus bars linking PSM → SPM → SVM DC link terminals are missing, loose or mis-ordered — the classic aftermath of amplifier replacement or cabinet work. Check every DC link bar is fitted, torqued, and in the right order; a loose bar also arcs and burns its terminal face (inspect for blackening).
- The internal DC link fuse is already blown from a previous event — the amplifier drops the chain at the first E-stop reset. Same repair path as Case 1, including finding out why it blew.
Diagnosis checklist (in order)
- When did it trip — during rotation, or at E-stop cancel/start-up?
- During rotation → megger the power cable, then the motor windings, then diode-test the amplifier output.
- At start-up after recent work → inspect DC link bus bars (fitted, ordered, torqued, no arc marks).
- Amplifier confirmed → bench repair: fuse + IPM + gate drives + sensing verified under load.
- Before recommissioning: megger-clear cable and motor, then reconnect and test at low speed first.
Related guides
- Fanuc spindle alarm 12 (DC link overcurrent)
- Fanuc spindle alarm 01 (motor overheat)
- Fanuc αi PSM alarm 04 (single phase failure)
- Fanuc αi drive alarms — PSM, SVM & SPM codes explained