Fanuc spindle alarm 55 means that in spindle switching or output (winding) switching control, the switching request signal (SPSL or RSL) and the power-lead state check signal (MCFN, MFNHG, RCH or RCHHG) disagree — while the motor is activated. It is the running-state sibling of AL-15: 15 says a commanded switch never completed within its one-second window; 55 says the contactor state changed — or disagreed with the request — while the motor was live. That is more serious: a winding contactor opening under a live motor draws arcs across its contacts and can take the amplifier's output stage with it. Treat repeated AL-55 as urgent.
What the mismatch means physically
On winding-switch machines (low-speed/high-speed motor windings selected by external contactors), the drive only allows power when the contactor state matches the requested winding. AL-55 during activation means one of three things: a contactor dropped out under load, the confirmation signal lied, or the ladder commanded a switch at a moment it must not.
Cause 1 — The switching magnetic contactor
- Dropping out under load: a weak coil, chattering interposing relay, or a voltage dip on the coil's control supply lets the contactor release momentarily mid-run — the classic AL-55. Watch/listen during a failing cycle; check the coil circuit for loose terminals and measure the control voltage during spindle accel (when the cabinet supply sags most).
- Welded or burnt contacts: a contactor that has been arcing gives unstable feedback — inspect the mains contacts; a contactor that has switched under load even a few times is due for replacement, not cleaning.
Cause 2 — The confirmation path (I/O and wiring)
The contactor is fine but its auxiliary-contact signal (MCFN/RCH) flickers: a worn auxiliary block, a loose wire in the confirmation loop, or a failing input on the I/O unit. Watch the confirmation signal on the PMC diagnosis screen while the machine runs — any flicker with the contactor visibly steady condemns the signal path. Replace the auxiliary block, wiring, or I/O unit as found.
Cause 3 — The ladder commands a switch at the wrong time
Fanuc's instruction: modify the sequence so switching is not performed during activation — winding changes belong at zero speed with the motor deactivated, per the signal timing in the αi Descriptions manual (B-65282EN). Suspect the ladder when AL-55 appeared after retrofit/sequence work, or when it trips at a consistent program point (a gear-range M-code issued while the spindle still runs).
Diagnosis checklist (in order)
- When does it trip — random under load, or at a fixed program point?
- Watch the contactor and its confirmation signal (PMC screen) through a failing cycle.
- Contactor drops → coil circuit and control-supply voltage under load → replace the contactor if contacts are burnt.
- Signal flickers, contactor steady → auxiliary block, wiring, I/O input.
- Fixed program point / after sequence work → correct the ladder per B-65282EN timing.
- Afterwards: confirm no follow-on amplifier symptoms.
Related guides
- Fanuc spindle alarm 15 (switch sequence not completed) — the stationary sibling
- Fanuc spindle alarm 12 (DC link overcurrent)
- Fanuc αi drive alarms — PSM, SVM & SPM codes explained