SV0436 Soft Thermal (OVC) fault in Fanuc drive

SV0436 Soft Thermal (OVC) fault in Fanuc drive

Description

SV0436 is the "Soft Thermal" or "Overcurrent" (OVC) alarm. Unlike SV0430 which uses a physical temperature sensor, SV0436 is a calculated digital thermal model. The servo drive software monitors the current being sent to the motor over time. It compares this against the motor's rated continuous current curve. If the motor draws high current for a sustained period (even if it's below the instantaneous trip point), the software calculates that the motor must be overheating and trips this alarm to protect it. It is essentially a "Duty Cycle Exceeded" alarm.

Cause

This alarm implies the motor is working harder than it was designed to for too long.

• Mechanical Binding: This is the #1 cause. If the ways are dry, the ball screw is worn, or the gibs are too tight, the motor draws 60-70% load just to hold position or move slowly. This constant high load fills the "thermal bucket" in the software.
• Counter-Balance Failure: On vertical axes (Z-axis), if the nitrogen counterbalance pressure is low or the counterweight chain is broken, the motor must lift the entire weight of the head, leading to constant high current.
• Aggressive Acceleration: Acceleration/Deceleration time constants set too aggressively for the mass of the table.
• Wrong Motor Initialization: If the wrong motor code is entered in the parameters, the drive might think it is controlling a smaller motor and trip the thermal protection too early.

Solution

Troubleshooting requires analyzing the load meter on the CNC screen:

1. Check Static Load: Go to the servo monitor screen. When the axis is stopped, what is the Load %? For a horizontal axis, it should be near 0-10%. For a vertical axis, it should be typically 30-50% depending on the counterbalance. If it is 80% while standing still, you have a mechanical bind or counterbalance issue.
2. Check Dynamics: Move the axis back and forth. If the load spikes to 150% during simple rapid moves, the friction is too high. Lubricate the ways and ball screw.
3. Brake Check: Ensure the motor brake is fully releasing. A dragging brake will cause SV0436 very quickly.
4. Review Cycle: If the mechanics are fine, the machining cycle might be too aggressive (too many rapid starts/stops per minute). You may need to add dwelling times or reduce acceleration parameters.