SP9031 (Alarm 31) Motor Lock / Constraint fault in Fanuc drive

SP9031 (Alarm 31) Motor Lock / Constraint fault in Fanuc drive

Description

SP9031 is the Spindle equivalent of a "Collision" or "Stall" alarm. It means the CNC commanded the spindle to rotate, but the position sensor (encoder) detected that the shaft did not move, or moved significantly slower than the physical physics model predicted. It effectively means the motor is seized or constrained. This is a protection mechanism to prevent the motor windings from burning out while trying to turn a stuck load.

Cause

• Mechanical Jam: The most obvious cause. The tool is welded into the part, the bearings have seized, or a mechanical brake (on a mill head) failed to release.
• U/V/W Cable Mix-up: If the power cables were recently disconnected and reconnected in the wrong order (e.g., U and V swapped), the motor tries to turn but fights itself magnetically, resulting in a lock alarm.
• Missing Phase (Output): The drive is firing only 2 of the 3 phases. The motor hums and vibrates but cannot generate rotation torque.
• Encoder Slip: The motor is spinning, but the belt driving the encoder is broken. The control sees 0 RPM while the motor spins, causing it to dump more current until it trips.

Solution

1. Hand Spin Test: With power off, try to rotate the spindle by hand. It should move freely. If it is stuck, investigate mechanical bearings, brakes, or gear shifters.
2. Check Cable Phase: If you recently did maintenance, double-check the U, V, W wiring order against the manual.
3. Encoder Check: Visually inspect the spindle encoder. Is the belt intact? Is the pulley tight on the shaft?
4. Megger Motor: Ensure the motor windings are not grounded or shorted.