SP9002 (Alarm 02) Spindle Speed Deviation fault in Fanuc drive

Description

SP9002 (often displayed as "Alarm 02" on the spindle drive LED) is a "Speed Error" alarm. It indicates that the actual RPM of the spindle motor does not match the commanded RPM within the allowable tolerance window. During acceleration or steady cutting, the CNC constantly compares the feedback from the spindle encoder (or built-in motor sensor) against the speed command. If the motor is lagging significantly behind the command, or if it is fluctuating wildly, SP9002 is triggered to prevent poor surface finish or loss of synchronization during tapping cycles.

Cause

This is frequently a mechanical transmission issue or parameter gain problem.

Belt Slippage: The most common cause on older mills and lathes. If the drive belts are loose or worn, the motor spins up, but the spindle load (and encoder) lags behind.
Transistor Saturation: If the IGBTs in the drive cannot supply enough current to reach the requested speed due to a weak input phase.
Parameter Tuning: If the acceleration parameter is set too fast for the inertia of a heavy chuck, the motor physically cannot keep up with the theoretical ramp-up curve.
Speed Sensor Noise: The magnetic sensor reading the spindle gear teeth is dirty or too far from the gear, causing erratic RPM readings.

Solution

Check Belts: Listen for a "squeal" during acceleration. Tighten or replace the spindle drive belts. This fixes SP9002 in 70% of cases.
Review Load Meter: Does the load peg at 100%+ during acceleration? If so, the acceleration time (Time Constant) in the parameters might need to be increased to reduce the strain.
Check Sensor Gap: If the machine uses an external magnetic pickup, check the gap between the sensor and the gear. It should be usually 0.1mm to 0.15mm. Remove any metal shavings stuck to the magnet.
Input Power: Verify all three phases of 200V are present. A missing phase reduces motor torque by approx 50%.

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