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FANUC Controller
Servo/Axis · Advanced · 30-90 minutes
Fanuc Alarm 440 - Motor Phase Detection Error
Alarm 440 CNV EX DECELERATION POWER indicates excessive regenerative energy during motor deceleration that exceeds the discharge capacity of the braking resistor circuit or signals a fault in the regenerative discharge system. This occurs on PSM or alpha series servo amplifiers when kinetic energy from rapid deceleration cannot be dissipated properly, causing DC bus voltage to rise beyond safe limits.
440
Fanuc Alarm 440 - Motor Phase Detection Error
Software Phase Alarm
Fanuc alarm 440: Software Phase Alarm. This alarm indicates a fault condition detected by the Fanuc control. Refer to the alarm number and name to identify the specific system or function involved.
Root Cause Summary
Alarm 440 CNV EX DECELERATION POWER indicates excessive regenerative energy during motor deceleration that exceeds the discharge capacity of the braking resistor circuit or signals a fault in the regenerative discharge system. This occurs on PSM or alpha series servo amplifiers when kinetic energy from rapid deceleration cannot be dissipated properly, causing DC bus voltage to rise beyond safe limits.
Safety
- Motor may move during initialization
Causes and Fixes
- 1
Power off the system and visually inspect the braking resistor unit for physical damage, burnt appearance, or loose/disconnected wiring at the CNV amplifier terminals.
Allow amplifier capacitors to discharge for minimum 5 minutes before touching any connections. Verify zero voltage with multimeter.
- 2
Measure braking resistor resistance with ohmmeter (disconnected from amplifier). Compare to nameplate specification (typically 20-80 ohms depending on amplifier model). Replace if open circuit or resistance exceeds +20% of nominal value.
- 3
Check amplifier LED codes on PSM/alpha SVU front panel. Red LED steady or specific blink pattern indicates regenerative circuit fault. Cross-reference LED pattern with amplifier hardware manual.
- 4
Navigate to DIAGNOSIS > SERVO screen and monitor diagnostic number 300 (DC link voltage) during normal operation. Voltage should be 280-320VDC nominal. If voltage spikes above 380VDC during deceleration, regenerative circuit is insufficient.
Call a technician if:
- ·Internal servo amplifier discharge circuit failure
- ·Repeated alarms after parameter adjustments
- ·DC bus voltage consistently above 400V
Prevention
- Regularly inspect braking resistor for thermal damage
- Monitor regenerative load levels during program optimization
- Maintain adequate cycle time spacing for thermal recovery
- Use gradual acceleration/deceleration profiles
Common Mistakes
- Ignoring braking resistor duty cycle ratings
- Setting deceleration parameters too aggressively
- Failing to account for total system inertia in calculations
- Not monitoring DC bus voltage during troubleshooting
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