SP9073 (Alarm 73) Motor Sensor Disconnect (Signal) fault in Fanuc drive

SP9073 (Alarm 73) Motor Sensor Disconnect (Signal) fault in Fanuc drive

Description

SP9073 (Alarm 73) indicates a loss of the feedback signal specifically for the Spindle Motor's speed/position sensor. This is distinct from a power cable disconnect. The drive looks for the "PA/PB" (Phase A/B) signals coming from the spindle sensor (magnetic pickup or built-in encoder). If the amplitude of these signals drops to zero while the drive is active, Alarm 73 is generated.

Cause

• Connector JYA2/JYA3: The feedback connector on the spindle amplifier is loose.
• Sensor Gap: If using an external magnetic sensor (MZ sensor) on a gear, the gap is too wide, and the signal is too weak to be detected.
• Cable Break: The feedback cable running through the flexible conduit to the spindle head has fatigued and broken a wire.

Solution

1. Check Sensor Alignment: If external, use a feeler gauge to set the gap between the sensor and gear tooth (typically 0.15mm).
2. Continuity Test: Check continuity of the cable from the motor to the drive.
3. Oscilloscope: Connect a scope to the test pins on the drive (PA/PB). Spin the spindle by hand. You should see clear sine waves. Flatline means dead sensor or cable.