439

Check LED indicators on PSM/PSMR: LED 'RDY' should be green, 'REGEN' LED should flash during deceleration. If REGEN LED never illuminates during rapid traverse deceleration, discharge circuit is not activating.

Measure braking resistor resistance at power supply terminals RB1-RB2 (or R1-R2 on older PSM). Typical values: 40-80 ohms depending on model. Open circuit or >200 ohms indicates resistor failure.

Disconnect AC input power and wait 5 minutes for DC link capacitors to discharge before measuring resistance. Verify 0VDC across P-N terminals with multimeter.

Check discharge transistor operation via DIAGNOSIS > SERVO > DC LINK screen. Monitor DC link voltage (typically parameter display shows 'Vdc') during controlled deceleration. Voltage should not exceed 750VDC; discharge should activate above 720VDC threshold.

Inspect power supply parameters: check parameter 1820 (regenerative load factor alarm threshold). If braking resistor was recently replaced, verify correct wattage rating (typically 200-400W continuous, review PSM model specification). Undersized resistor will overheat and fail open.

Review current NC program for rapid traverse deceleration (G00) immediately before alarm. Check DIAGNOSIS > SERVO screen during deceleration to monitor individual axis loads. Note which axes show high regenerative current (negative current values indicate regeneration).

Reduce servo deceleration time constants: increase parameter 1622 (rapid traverse time constant Tr) from typical 150ms to 200-250ms. Increase parameter 1624 (cutting feed time constant Tc) if alarm occurs during feedrate deceleration. Test with incremental 20% increases.

Increased time constants will reduce machine acceleration performance and may increase cycle time. Document original values before changes.

Check acceleration/deceleration bell curve settings: parameter 1620 (acceleration/deceleration mode). Set to 1 for bell-shaped acceleration (smoother energy profile). Parameter 1621 (bell-curve ratio) typical value 50-100, increase to reduce peak regenerative current.

If multiple high-inertia axes exist, enable stagger deceleration: review macro or ladder logic to sequence axis deceleration. Check parameter 1825 (regenerative alarm delay time) - increase from default 100ms to 200ms to allow temporary voltage spikes without triggering alarm.

Measure incoming 3-phase AC voltage at PSM input terminals L1-L2-L3 (R-S-T) under load during machine operation. Voltage should be 380-420VAC for 400V systems, 200-230VAC for 230V systems. Check for voltage spikes with oscilloscope if intermittent alarms occur.

Live voltage measurement - use appropriate PPE and insulated test equipment rated for 600VAC minimum.

Monitor DC link voltage in real-time: DIAGNOSIS > SERVO > DC LINK, observe 'Vdc' value at idle (should be 1.35-1.4x AC input voltage, typically 540VDC for 400V supply). If reading shows >600VDC at idle with normal AC input, suspect PSM voltage sensing circuit failure.