Fanuc spindle alarm 12 (AL-12) means excessive current in the DC link — the spindle amplifier's power stage has detected overcurrent and shut down instantly to protect its IGBT/IPM modules. You will see 12 on the spindle amplifier's 7-segment display (SPM / αi SP unit) and typically SP9012 / AL-12 ‘DC LINK OVERCURRENT’ (alarm 749/754 spindle alarm on older systems) on the CNC screen. It covers S-series, α (Alpha), αi and βi spindle amplifiers alike. Like any hard overcurrent trip, repeated resets into the fault do real damage — isolate the cause first.
What is happening
The amplifier switches the DC link (~300 V DC on 200 V class systems) onto the spindle motor windings. AL-12 trips when current rises beyond the module limit: a short in the power stage itself, in the motor power cable, in the motor windings, a wrong feedback/commutation condition, or occasionally the power supply module (PSM) side. The trip is instantaneous and latched — power off/on is needed to reset.
Step 1 — Split the system: amplifier, cable or motor?
Power down, lock out, and wait until the charge LED on the amplifiers is fully OFF (5+ minutes) before touching power terminals.
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Disconnect the spindle motor power cable at the amplifier (U/V/W). Power up and give a low-speed spindle command.
- AL-12 trips immediately even with the motor disconnected → the amplifier's power stage is shorted. Go to Step 2.
- No alarm with the cable off → the problem is downstream in the cable or motor. Continue.
- Insulation-test (megger) the cable and motor at 500 V DC with the amplifier disconnected at both ends: each phase to earth and phase to phase. Below ~10 MΩ to earth is suspect; below 1 MΩ is your fault. On spindle motors the usual killers are coolant ingress at the terminal box and carbonized dust inside older motors.
- Check winding balance: U-V, V-W, W-U resistance should match within a few percent. Imbalance or an open phase = motor rewind or replacement.
Step 2 — If the amplifier is shorted
- Bench confirmation is a diode test across the output legs — a shorted IGBT/IPM reads near zero. The repair is component-level: replace the module, verify the gate-drive circuits (a failed gate driver destroys the new module at first power), check snubbers and current sensors, then run under real load. This is a routine repair for us on SPM and αi SP units; exchange units are the fast path when the machine is down.
- Check the power supply module (PSM) too: a PSM with failing bus capacitors or a shorted leg can present as spindle-side overcurrent, especially if servo axes on the same PSM also misbehave. The PSM's own LED will often show its alarm.
Step 3 — If cable and motor test healthy
- Spindle sensor / feedback fault: a damaged motor sensor (MZi/BZi/CZi sensor or older pulse generator) gives wrong rotor position → wrong commutation → current spike. Often accompanied by feedback alarms in the history; check the sensor signal and gap where applicable.
- Mechanical load: seized spindle bearings, a jammed tool clamp/drawbar, or a locked gear stage forces maximum current at low speed. Rotate the spindle by hand (power off) — it should turn smoothly; listen for bearing roughness.
- Parameter mismatch after amplifier or motor replacement — wrong motor model code makes the drive push current at the wrong angle; verify spindle parameters (4133 motor model code and the auto-set parameters) against the motor nameplate.
- Belt/chuck side: on belt-driven spindles, an over-tensioned or misaligned belt shows as high load current at speed — check the load meter percentage against a known-good baseline.
Diagnosis checklist (in order)
- Note exactly when it trips: at spindle start, at speed, during tap/orientation, or randomly.
- Cable-off test at the amplifier → amplifier vs downstream in ten minutes.
- Megger cable and motor separately; check winding balance and the terminal box for coolant.
- Hand-rotate the spindle and check mechanics; review the load meter at low speed.
- All external checks pass → bench-test the amplifier (and PSM) under load; verify spindle motor parameters.
Can you keep running?
Not with a repeating AL-12. One clean trip after a coolant cleanup and a passing megger test — acceptable. Anything recurring means a genuine power-path fault, and every reset risks converting a module repair into a destroyed amplifier and possibly motor damage.
Related guides
- Fanuc αi drive alarms — PSM, SVM & SPM codes explained
- Fanuc servo drive alarm codes
- Identifying and replacing a faulty Fanuc encoder