SP9051 (Alarm 51) Spindle DC Link Undervoltage fault in Fanuc drive

SP9051 (Alarm 51) Spindle DC Link Undervoltage fault in Fanuc drive

Description

SP9051 (Alarm 51) is the Spindle Amplifier's specific version of a DC Link Undervoltage. While the PSM usually manages the bus, the Spindle drive monitors the DC level locally. If the voltage drops below a critical threshold (e.g., 200V DC) while the spindle is idle or running, Alarm 51 triggers. It differs from Input Undervoltage (Alarm 10) as it focuses on the stored DC energy available for the inverter.

Cause

• Momentary Power Loss: A micro-cut in the facility power supply that wasn't long enough to shut off the CNC but was deep enough to drain the capacitors.
• Loose Bus Bars: High resistance on the DC link connection between PSM and Spindle Drive.
• Charging Circuit: The PSM failed to fully pre-charge the bus.

Solution

1. Tighten DC Link: As with all DC link alarms, the first step is ensuring the screws on the top terminals (P/N) are torqued to spec.
2. Check PSM: If the Spindle says "Low Voltage" but the PSM looks normal, the PSM might be failing to output full voltage under load. Measure P to N with a multimeter (expect ~300VDC).
3. Check Input: Verify the 3-phase AC input to the machine is stable.